Researcher Profile and Settings

Master

Affiliation (Master)

• Faculty of Science　Earth and Planetary Sciences　Earth and Planetary System Science

Affiliation (Master)

• Faculty of Science　Earth and Planetary Sciences　Earth and Planetary System Science

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Profile and Settings

Profile and Settings

• Name (Japanese)

  Kuritani

• Name (Kana)

  Takeshi

• Name

  201501096924326428

Alternate Names

https://www.sci.hokudai.ac.jp/grp/pv/pv/kuritani/index.html

Achievement

Research Interests

• マグマ   火山   マントル   時間スケール   マグマ溜まり   熱進化   地球化学   マグマ溜り   利尻火山   モデル化   U-Th放射非平衡   地殻・マントル   水循環   地殻   岩石学   マグマ進化   地質学   噴火様式   鉱物   岩石化学   

Research Areas

• Natural sciences / Solid earth science

Research Experience

• 2021/04 - Today Hokkaido University Faculty of Science
• 2014/04 - 2021/03 北海道大学 大学院理学研究院 准教授
• 2011/04 - 2014/03 大阪市立大学 大学院理学研究科 准教授
• 2010/04 - 2011/03 東北大学 大学院理学研究科地学専攻 准教授（GCOE）
• 2007/02 - 2010/03 東北大学 大学院理学研究科地学専攻 助教
• 2006/04 - 2007/01 京都大学 地球熱学研究施設 研究機関研究員
• 2003/04 - 2006/03 岡山大学 地球物質科学研究センター 研究機関研究員
• 2000/04 - 2003/03 日本学術振興会特別研究員 ＰＤ
• 1997/04 - 2000/03 日本学術振興会特別研究員 ＤＣ１

Education

• 1997/04 - 2000/03  The University of Tokyo  Graduate School of Science
• 1995/04 - 1997/03  The University of Tokyo  Graduate School of Science
• 1993/04 - 1995/03  The University of Tokyo  Faculty of Science  Department of Geological Sciences
• 1991/04 - 1993/03  The University of Tokyo

Committee Memberships

• 2019 - Today   Scientific Reports   Editorial Board Member
• 2016 - Today   Frontiers in Earth Science   Review Editor

Awards

• 2006 日本岩石鉱物鉱床学会研究奨励賞
  
• 2005 日本火山学会研究奨励賞
  

Published Papers

• Hydrous mantle plume promoted the generation of continental flood basalts in the Tarim large igneous province

  Yao Bi, Huan Chen, Eero Hanski, Takeshi Kuritani, Hong Xiang Wu, Feng Qi Zhang, Jia Liu, Xiao Yan Gu, Qun Ke Xia

  Scientific Reports 14 (1) 2024/12 [Refereed]
   

  Recent research on the water content of large igneous provinces (LIPs) has revealed that water has a significant impact on the formation of LIPs. However, most studies focus on the water content of mafic–ultramafic rocks, while relatively little attention has been paid to the water content of continental flood basalts (CFB), which form the major part of LIPs and are characterized by huge volumes (> 1 × 105 km3) and short eruption times. Here, we determined water contents of clinopyroxene crystals from the Akesu diabase, which is co-genetic with flood basalts of the Tarim LIP in China. Based on these measurements, we obtained a water content of higher than 1.23 ± 0.49 wt.% for the parental magma to the Tarim CFB and a minimum water content of 1230 ± 490 ppm for the mantle source, thus indicating the presence of a hydrous mantle plume. Combined with previous studies, our results suggest that water plays a key role in the formation of the Tarim LIP. Additionally, the whole-rock compositions of the Akesu diabase indicate a contribution of pyroxenite in the mantle source. This is consistent with a model, in which water was brought into the Tarim mantle plume by a subducted oceanic plate that entered the deep mantle.
• Machine learning reveals the influence of the Changbaishan mantle plume sourced from the mantle transition zone on Cenozoic intraplate magmatism in NE China

  Yuning Qi, Huan Chen, Sensen Wu, Takeshi Kuritani, Zhenhong Du, Qunke Xia, Renyi Liu

  Chemical Geology 663 0009-2541 2024/09/20 [Refereed]
   

  This study investigates the influence of the Changbaishan mantle plume, formed by dehydration of stagnant slab, on Cenozoic intraplate magmatism in Northeast China, focusing on the plume's dominant influence zone. Compared to prior research, this paper includes a more extensive published dataset of major and trace element compositions of basalts and adopts machine learning algorithms with better generalization capabilities for the data being analyzed. This approach measures the geochemical similarity of basalts in Northeast China to training data from different geological backgrounds, determining their degree of influence by subduction-derived fluids. Global ocean island basalts (OIBs) and East African continental rift basalts (CRBs) serve as training data for subduction-distal basalts, while global island arc basalts (IABs) represent subduction-proximal basalts. A binary classification model is constructed through ridge logistic regression and support vector machine to establish a hyperplane in high-dimensional space. The greater the positive distance of the basalt data from the hyperplane, the closer it is to subduction-proximal basalts, and vice versa. Model feature importances correlate with their enrichment capacity in fluids, and the application of well-trained models to basalts with known water content shows a positive correlation between model results and H2O/Ce ratios, collectively demonstrating the models' ability to capture fluid signals. Model results of all Cenozoic basalts in Northeast China reveal that within a 600-km range centered on Changbaishan, fluid signals attenuate radially outward, rather than a continuous decrease along the subduction direction of the Pacific plate. This indicates a fluid-rich upwelling centered around Changbaishan, with fluid distribution decreasing from the center to the periphery. This upwelling plays a primary role in controlling the generation of Cenozoic basalts within a specific geographic range.
• Eocene mafic alkaline volcanism in north Central Iran; Signatures of development and subsequent melting of a pyroxenite mantle source

  Parham Ahmadi, Mohammad Reza Ghorbani, Khadijeh Ghorbani, James Lawford Anderson, Takeshi Kuritani, Meysam Akbari

  Lithos 478-479 0024-4937 2024/08 [Refereed]
   

  Volcanic rocks from the Miami-Davarzan area are basalts to trachyandesites interlayered with fossiliferous beds and bear significant implications on the Eocene geodynamic evolution of northern central Iran. The rocks constitute two distinct groups, a low-Nb and a high-Nb suite. The least evolved low-Nb suite includes clinopyroxene-olivine phyric samples and is characterized by higher CaO and lower Al2O3 as compared to peridotite partial melts. The high-Nb suite includes more evolved plagioclase-clinopyroxene phyric samples. Fractional crystallization modeling using least-squares mass balance calculations for major elements suggests that the high-Nb suite was formed from a low-Nb parental melt by differentiation of a clinopyroxene-dominated mineral assemblage. The Miami-Davarzan volcanic rocks are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment and high field strength elements (HFSE) depletion, which are typical signatures of subduction-related settings. Immobile, highly to moderately incompatible elements such as Nb[sbnd]Ta, La[sbnd]Ce, Pr[sbnd]Nd are higher in the Miami-Davarzan volcanic rocks as compared to the typical magmatic arcs, which is an indication of slab melt involvement and metasomatism of the mantle source. The Sr-Nd-Pb isotopic data also confirm a common mantle source involving the addition of subducted sediments into a depleted mantle source for both the low-Nb and high-Nb suites. The geochemical characteristics such as high CaO, low Al2O3, and the isotopic data can be best explained by derivation of the low-Nb suite parental melt from a pyroxenite mantle source. The pyroxenite mantle source was probably developed as a result of interaction between shallow depleted mantle and slab plus sediment-derived melts. It is suggested that the extensional phase that dominated the northern central Iran during the Eocene prompted partial melting of a pyroxenite mantle source.
• Current situation of lead (Pb) exposure in raptors and waterfowl in Japan and difference in sensitivity to in vitro lead exposure among avian species.

  Mizuki Maruyama, Nana Ushine, Yukiko Watanabe, Chihiro Ishii, Keisuke Saito, Hidefumi Sakai, Takeshi Kuritani, Rio Doya, Kohei Ogasawara, Yoshinori Ikenaka, Yared Beyene Yohannes, Mayumi Ishizuka, Shouta M M Nakayama

  Environmental pollution (Barking, Essex : 1987) 123907 - 123907 2024/04/04 [Refereed]
   

  Although lead (Pb) poisoning in wild birds has been considered a serious problem in Japan for over 30 years, there is little information about Pb exposure and its sources throughout Japan except for Hokkaido. Furthermore, to identify and effectively prioritize the conservation needs of highly vulnerable species, differences in sensitivity to Pb exposure among avian species need to be determined. Therefore, we investigated the current situation of Pb exposure in raptors (13 species, N = 82), waterfowl (eight species, N = 44) and crows (one species, N = 6) using concentration and isotope analysis. We employed blood or tissue samples collected in various Japanese facilities mainly in 2022 or 2023. We also carried out a comparative study of blood δ-ALAD sensitivity to in vitro Pb exposure using blood of nine avian species. Pb concentrations in the blood or tissues displayed increased levels (>0.1 μg/g blood) in two raptors (2.4%), ten waterfowl (23%) and one crow (17%). Among them, poisoning levels (>0.6 μg/g blood) were found in one black kite and one common teal. The sources of Pb isotope ratios in ten blood samples with high Pb levels were determined as deriving from shot pellets (N = 9) or rifle bullets (N = 1). In the δ-ALAD study, red-crowned crane showed the highest sensitivity among the nine tested avian species and was followed in order by five Accipitriformes species (including white-tailed and Steller's sea eagle), Blakiston's fish owl, Muscovy duck and chicken, suggesting a genetically driven variance in susceptibility. Further studies on contamination conditions and exposure sources are urgently needed to inform strict regulations on the usage of Pb ammunition. Furthermore, detailed examinations of δ-ALAD sensitivity, interspecific differences, and other factors involved in the variability in sensitivity to Pb are required to identify and prioritize highly sensitive species.
• Olivine-Hosted Melt Inclusions Track Progressive Dehydration Reactions in Subducting Slabs Across Volcanic Arcs

  Raimundo Brahm, Daniel Coulthard Jr, Georg Zellmer, Takeshi Kuritani, Naoya Sakamoto, Hajime Taniuchi, Hisayoshi Yurimoto, Mitsuhiro Nakagawa, Eiichi Sato

  Journal of Petrology 0022-3530 2024/02/29 [Refereed]
   

  Abstract The stability and breakdown of mineral phases in subducting slabs control the cycling of trace elements through subduction zones. Stability of key minerals and the partitioning of trace elements between these minerals and liquid phases of interests have been charted by natural sample analysis and experimental constraints. However, systematic study from arc front to far back-arc has rarely shown that the expected geochemical variations of the slab liquid are actually recorded by natural samples. Complexities arise by uncertainties on the nature of the slab component (melts, fluids and supercritical liquids), source heterogeneities and transport processes. Using data from olivine-hosted melt inclusions sampled along and across the NE Japan and southern Kurile arcs, we demonstrate that experimentally and thermodynamically constrained phase stabilities in subducted materials indeed control the trace element signatures as predicted by these models and experiments. The main reactions that can be traced across arc are progressive breakdown of light rare earth element-rich accessory phases (e.g., allanite), enhanced dehydration of the lithospheric mantle (serpentine breakdown) and changes in the nature of the slab component. This work elucidates subduction zone elemental cycling in a well-characterized petrogenetic setting and provides important constraints on the interpretation of trace element ratios in arc magmas in terms of the prograde metamorphic reactions within the subducting slab.
• Molybdenum isotopic compositions in volcanic rocks from Northern Izu arc (Izu-Oshima, Niijima, and Kozushima)

  Tatsuya Tamura, Tetsuya Yokoyama, Takeshi Kuritani, Akira Ishikawa

  Geochemical Journal 58 (2) 51 - 70 0016-7002 2024 [Refereed]
   

  Variations in Mo isotopic ratios (δ98/95Mo values) in volcanic arc lavas reflect various processes occurring beneath island arcs, including slab dehydration, mantle melting, and crystal fractionation, making this tracer important in recent geochemistry. Previous studies of Mo isotopes have focused primarily on samples from the volcanic front (VF) of oceanic island arcs, and only a limited number of studies have been conducted on volcanoes in the rear arc (RA) regions. To better understand the behavior of Mo isotopes in subduction zones, we focused on the Northern Izu (N-Izu) arc and determined major and trace element abundances and δ98/95Mo values in basaltic samples from one VF (Izu-Oshima) and two RA (Niijima and Kozushima) islands. The N-Izu basalts are characterized by relatively uniform Zr/Hf ratios with systematic decrease in Ba/Th and increase in Ce/Mo ratios with increasing depth of the Wadati-Benioff zone, indicating a gradual decrease in the contribution of aqueous fluids from the subducting slab to a chemically homogeneous mantle wedge. Similarly, the δ98/95Mo values in the N-Izu basalts decrease with increasing slab depth from Izu Oshima (+0.11 ± 0.09％) to Niijima (–0.14 ± 0.05％) and Kozushima (–0.20 ± 0.21％). The most straightforward interpretation for the elevated δ98/95Mo values in the N-Izu VF basalts is the contribution of slab-derived aqueous fluid enriched in heavy Mo isotopes to the source mantle of the VF volcanoes, due to the presence of residual minerals in the slab in which light Mo isotopes are preferentially distributed. In contrast, the low δ98/95Mo values in the RA basalts, which are as low as the mean MORB value (–0.18％), suggest that the slab-derived fluid in the RA region is depleted in heavy Mo isotopes as a result of the continuous loss of heavy Mo isotopes via slab dehydration in the VF region, while other possibilities, including the incorporation of slab melts containing sedimentary components, cannot be ruled out.
• Pre-eruption magmatic processes and magma plumbing system at Hachijo-Nishiyama volcano, Izu–Bonin arc, Japan

  Kazuki Oiwa, Takeshi Kuritani, Mitsuhiro Nakagawa, Shumpei Yoshimura

  Earth, Planets and Space 75 (1) 1343-8832 2023/12 [Refereed][Not invited]
   

  Nishiyama volcano on Hachijojima Island is an active basaltic volcano located in the Izu–Bonin arc. In this study, petrological and geochemical analyses were conducted on mafic lavas and pyroclastics to understand the magma plumbing system and pre-eruption magmatic processes. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and the samples contain variable amounts of plagioclase phenocrysts (1–40 vol.%). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (< 0.7 ka) are homogeneous, whereas some samples from the older stage (3–1 ka) have relatively low Pb isotopic ratios. This observation suggests that the younger magmas were derived from a single parental magma, but another parental magma with distinct geochemical features was involved in the magmatic system before 1 ka. The temporal variation in the FeO*/MgO ratios of the volcanic products is complex and is considered to reflect the intermittent injection of primitive magmas into the main magma chamber in which fractional crystallization occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt. The estimated depth of 9–12 km for the main magma chamber coincides with the depth range over which earthquake swarms occurred in 2002, suggesting that the magma chamber is still active, and that the earthquake swarms may reflect the injection of primitive magma into the magma chamber. Graphical Abstract: [Figure not available: see fulltext.]
• Geochemical constraints on the evolution of the magmatic system leading to catastrophic eruptions at Aira Caldera, Japan

  Takeshi Kuritani

  Lithos 450-451 0024-4937 2023/08 [Refereed][Not invited]
   

  Caldera-forming eruptions of Aira volcano at 30 ka were among the most explosive eruptions in Japan during the late Pleistocene. In this study, new data on whole-rock major and trace elements, Sr–Nd–Pb isotopes, and 238U-230Th disequilibrium were obtained for representative volcanic products of caldera-forming and pre-caldera eruptions (31–90 ka) to acquire insights on the evolution of the felsic magma system leading to catastrophic eruptions. Voluminous rhyolitic pumices from the caldera-forming eruptions at 30 ka and rhyolitic lavas and pumices of the younger pre-caldera eruptions (31–36 ka) have similar Sr, Nd, and Pb isotopic compositions, and differ from those of dacitic and rhyolitic pumices produced by eruptions at 90 and 60 ka, respectively. These three types of felsic magmas are considered to have formed by partial melting of hornblende gabbroic rocks with distinct isotopic compositions in the middle-lower crust. Most felsic products from the pre-caldera and caldera-forming eruptions are in secular 238U-230Th equilibrium, suggesting that the original magmas were generated > ∼ 400 ka and existed as long-lived crystalline mush in the deep hot crust. Felsic magmas originating from the deep crustal mush zones ascended to form the shallow felsic magma system, which was replaced by a new one after the explosive eruptions at 90 and 60 ka. The voluminous rhyolitic magmas for the caldera-forming eruptions accumulated in a shallow magma reservoir between 60 and 30 ka. Rhyolitic magmas with compositions similar to those of the voluminous magmas erupted catastrophically at 30 ka began to erupt effusively at 36 ka and 33 ka as lava flows with significant 238U-230Th disequilibrium, suggesting that these magmas were generated in the deep crust much later than the other main rhyolitic magmas. The effusive magma eruptions presumably resulted from the relatively low content of dissolved H2O in the melt due to the low crystallinity because of the shorter residence time in the shallow felsic magma system.
• A Machine Learning Based-Approach to Predict the Water Content of Mid-Ocean Ridge Basalts

  Jingjun Zhou, Jia Liu, Qunke Xia, Cheng Su, Takeshi Kuritani, Eero Hanski

  GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS 24 (7) 2023/07 [Refereed][Not invited]
   

  Water is critical in the evolution of the mantle due to its strong influence on the physicochemical properties of mantle rocks. Mid-ocean ridge basalts (MORBs) are commonly used to study the compositional characteristics of the convecting upper mantle. However, there remains abundant samples in the global MORB data sets without direct measurements of water contents. The commonly observed good correlation between H2O and other incompatible trace components, such as Ce, has been applied to quantify water contents of MORBs. However, this approach assumes constant H2O/Ce in the target samples, which is not always true as the H2O/Ce ratios of MORBs could be rather heterogeneous even in some short ridge segments. Utilizing the present compositional data of global MORB glasses with measured water contents (n = 1,467), we construct a Random Forest Regression model based on machine learning, which can predict water concentrations of samples based on selected major and trace element data, without assuming a ratio between H2O and other trace elements. This model allows us to precisely recover water contents for MORBs with comparable accuracy with traditional analytical methods. The predicted results of MORB glasses from this model (n = 1,931) expand the water content database of global MORBs and indicate a broad existence of high-H2O MORBs. This new approach allows us to investigate the water content of MORBs from some ridges lacking previous water content measurements (e.g., the Chile Ridge and the Pacific-Antarctic Ridge) and infer changes in the water content of MORB sources through applying the model to transform fault samples.
• Petrological and Geochemical Study of Sundoro Volcano, Central Java, Indonesia: Temporal Variations in Differentiation and Source Processes During the Growth of an Individual Volcano

  Haryo Edi Wibowo, Mitsuhiro Nakagawa, Takeshi Kuritani, Ryuta Furukawa, Oktory Prambada, Agung Harijoko

  JOURNAL OF PETROLOGY 63 (9) 0022-3530 2022/09 [Refereed]
   

  Volcanic rocks of the Java sector of Sunda arc have a wide range of isotopic compositions that indicate significant addition of subjected sediment. What processes control these geochemical characteristics is a topic of long-standing debate. Here we report Sr-Nd-Pb radiogenic isotope ratios and geochemical data from stratigraphically well-constrained rocks of Sundoro volcano in central Java that represent the volcano's activity since 34 ka. The rocks range from basalt (51 wt % SiO2) to andesite (63 wt % SiO2) and are dominated by basaltic andesite. We divide them into magma types A, B and C, having low, medium and high Sr-87/Sr-86 and Pb isotopic ratios, respectively. According to various differentiation indices, the three magma types have separate, parallel Sr-87/Sr-86, Ba/Zr and La/Yb trends and disparate Pb isotopic trends. The dominant process of intracrustal differentiation appears to be magma mixing, in which each of the three magma types represents the mixing of a distinct mafic end-member and a distinct felsic end-member. The distinct geochemical profiles of these magma types indicate that the three mafic end-members are genetically unrelated and that their differences may represent characteristics of their magma sources. On the basis of trace element ratios (Ba/Yb and La/Yb) and Sr-Nd-Pb isotopic compositions, we estimate that magma types A, B and C represent mantle wedge materials fluxed by similar to 1%, similar to 1.5% and similar to 2% slab-derived materials containing 50%, 55% and 65% sediment component, respectively, reflecting increasing proportions of sediments and increasing slab flux. Geochemical data from Merapi volcano, interpreted using the same approach, reveal a similar increase in the slab-derived flux to the magma source, raising the possibility that such short-lived variations in magma genesis, perhaps related to the subduction of bathymetric relief features, characterize the unusual magmatism beneath the volcanic front of the central Java sector of the Sunda arc.
• Water enrichment in the mid-ocean ridge by recycling of mantle wedge residue

  Jia Liu, Chunhui Tao, Jianping Zhou, Kenji Shimizu, Wei Li, Jin Liang, Shili Liao, Takeshi Kuritani, Etienne Deloule, Takayuki Ushikubo, Mitsuhiro Nakagawa, Weifang Yang, Guoyin Zhang, Yunlong Liu, Chuanwei Zhu, Hao Sun, Jingjun Zhou

  Earth and Planetary Science Letters 584 0012-821X 2022/04 [Refereed]
   

  Trace amounts of water in the sub-oceanic mantle play crucial role in the vigor of mantle convection and the production of oceanic crust, and other many geodynamic processes. Consequently, the cycling of H2O between the mantle and the exosphere in the mantle is one of the critical processes governing Earth's geodynamical and geochemical evolution. While the deep cycling of altered oceanic lithosphere was considered as the main way to replenish the water in oceanic upper mantle, the significance of the arc mantle wedge after the genesis of arc magmatism dragged down by the subducting slab concomitantly has been not well constrained. Here, we report that fresh depleted basaltic glasses from the ultraslow-spreading Southwestern Indian Ridge (SWIR), located far from any recent subduction zones, show unusually high H2O/Ce ratios (>600), water contents and heavy hydrogen isotopic compositions. These results could be best explained by recycling of water through melting of a residual hydrous mantle wedge after early melt extraction. Considering that such mantle wedges residue dragged down to the deeper mantle could occupy a volume one order of magnitude larger than that of the subducted lithosphere in the earth history, we suggest that the potential role of such shallow recycling should be considered in studies of global water recycling and the origin of water in the upper asthenosphere.
• Structure of large silicic magma system and its eruption process: the case study of the 45 ka Shikotsu caldera-forming eruption, southwestern Hokkaido, Japan

  Matsumoto Akiko, Nakagawa Mitsuhiro, Miyasaka Mizuho, Kuritani Takeshi, Yoshimura Shumpei

  PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN 特定非営利活動法人 日本火山学会 2022 27 - 27 2433-5320 2022
• Modification for the matrix effect in SIMS-derived water contents of silicate glasses

  Kenji Shimizu, Takayuki Ushikubo, Takeshi Kuritani, Naoto Hirano, Shigeru Yamashita

  GEOCHEMICAL JOURNAL 56 (6) 223 - 230 0016-7002 2022 [Refereed][Not invited]
   

  Analyses of elemental abundances by secondary ion mass spectrometry (SIMS) require matrix-matched standard samples to account for the matrix effect on correction factors. This requirement makes it difficult to obtain accurate results for geological samples of variable chemistry. In this study, we prepared 39 volcanic glasses of foiditic, basaltic, basaltic andesitic, rhyolitic and pure SiO2 compositions, including synthetic samples and natural samples collected from the deep seafloor. The measured H2O contents of these samples were in the range 0.02-4.8 wt%. We showed that calibration curves (H2O content vs. (OH-)-O-16/Si-30(SIMS)- ratio) differed according to the composition of the volcanic glasses. Our results demonstrated that for a particular (OH)-O-16/Si-30(SIMS) ratio, water content could differ by up to a factor of five, depending on the composition of the volcanic glass. Although the correction factor (the slope of the calibration curve for water [H2O/((OH-)-O-16/Si-30(-))(SIMS)]) was weakly correlated with SiO2 content, we identified a stronger correlation with the molar weight (g mol(-1), on a one-oxygen mole basis) of the silicate glasses. Our results suggest that modification of the correction factor for the matrix effect on SIMS-based H2O content of volcanic glasses according to their molar weights provides more accurate water contents of silicate glasses, regardless of their chemical composition and water content and without the need for a series of standard glasses of known water contents.
• Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

  Asobo Nkengmatia Elvis Asaah, Tetsuya Yokoyama, Hikaru Iwamori, Festus Tongwa Aka, Jules Tamen, Takeshi Kuritani, Tomohiro Usui, Takeshi Hasegawa, Eric Martial Fozing

  Geochemistry 125865 - 125865 0009-2819 2022/01 [Refereed][Not invited]
  
• Compositional Variation of Picrites in the Emeishan Large Igneous Province Modulated by Water in the Mantle Plume

  J. Liu, Q. K. Xia, H. Sun, E. Hanski, T. Kuritani, X. Y. Gu, H. Chen

  Journal of Geophysical Research: Solid Earth 127 (1) 2169-9313 2022/01 [Refereed]
   

  Initiation of large igneous provinces (LIPs) and temporal and spatial variations in their chemical compositions provide unique opportunities to understand mantle dynamics. It has been recently revealed that water-rich reservoirs in the mantle sources have played a significant role in the production of LIPs. However, the dominant causes for the chemical variations in primitive magmas of LIPs remain debated. In the Emeishan LIP (ELIP), there are several places where picrites accompany more abundant flood basalts. These picrites span a large range of Ti/Y, Sm/Yb, and TiO2/Al2O3. In this work, we have determined the water contents of picrites with Ti/Y > 500 from the ELIP, based on the water content of clinopyroxene phenocrysts and water partitioning coefficients. We have also calculated the water content of the mantle sources by applying batch and fractional partial melting models. We show that the chemical compositions of picrites in the ELIP closely correlate with the calculated water content in their mantle sources. Our model suggests that the variation in the chemical composition of the primary magmas of the ELIP both on a local and regional scale can be mainly explained by the complementary roles of relatively dry recycled pyroxenite and hydrated components in the mantle plume, without requiring catastrophic thinning of the lithospheric mantle. Our model thus implies that both the initiation and chemical variation of the ELIP are affected by water from the deep mantle.
• Highly variable H2O/Ce ratios in the Hainan mantle plume

  Piao Yi Wang, Xiao Yan Gu, Takeshi Kuritani, Eero Hanski, Qun Ke Xia

  Lithos 406-407 0024-4937 2021/12/15 [Refereed]
   

  Mantle plume-generated basalts have long been utilized to assess the large-scale heterogeneity of the deep mantle. Their chemical and isotopic enrichment has been thought to result from the incorporation of recycled sediments or oceanic crust in their sources. Variable extents of dehydration or rehydration of recycled materials during subduction can induce different H2O/Ce ratios in plume-related basalts. The Hainan plume has been regarded as the cause of the massive Cenozoic basaltic volcanism in southeastern Asia. In this study, we measured bulk-rock major and trace element compositions of basalts from a volcano at Heishanling Hill, northern Hainan Island, and made FTIR analyses of water in clinopyroxene phenocrysts to determine magmatic water contents. In addition, we determined Sr-Nd isotope compositions of basalts at Heishanling Hill and other three localities. Water contents of Cpx phenocrysts vary from 20 to 254 ppm, which were used to calculate the water contents in the corresponding equilibrated melts, yielding values from 0.22 to 2.42 wt% with an average of 1.07 ± 0.47 wt%. Considering the effect of magma differentiation on water contents, the primary melts of Heishanling basalts were estimated to have a range of 0.67–1.40 wt% for individual Heishanling basalts, with the average of 1.03 ± 0.33 wt%. The calculated water contents in the mantle source of the Heishanling basalts vary from 279 to 582 ppm, falling in the range of global OIBs. Combined with the results previously reported, H2O/Ce ratios of Hainan basalts were estimated to vary from 50 to 425, covering the reported range from EM-type OIBs to NMORBs. The enriched Sr-Nd isotopic compositions of the studied samples resemble those of other Hainan plume-related basalts, and their 143Nd/144Nd ratios are well correlated with incompatible trace element ratios (e.g., Th/La, Ba/Nb). Bulk-rock compositions are consistent with a contribution of pyroxenite in the mantle source of the Hainan basalts and mixing modeling suggests coupled contribution of recycled sediment and oceanic crust. H2O/Ce displays a negative correlation with 143Nd/144Nd and a positive correlation with (Rb/Nb)n and (Th/La)n, meaning that an increasing amount of recycled oceanic crust and sediment in the source increases its H2O/Ce. The recycled material incorporated in the Hainan plume source may have undergone variable degrees of dehydration or deep rehydration by fluids released from subcrustal hydrous minerals at grater depths during subduction.
• Na/k diversity of primary basaltic magmas induced by the separation of slab-derived supercritical liquid: Implications from alkali basaltic lavas from rishiri volcano, southern kuril arc

  Hajime Taniuchi, Takeshi Kuritani, Mitsuhiro Nakagawa

  Journal of Petrology 62 (12) 0022-3530 2021/12/01 [Refereed]
   

  Primary basaltic magmas in subduction zones exhibit chemical diversity even in individual volcanoes. In this study, we aim to elucidate the dominant factors that determine the chemical composition, especially the Na/K, of primary basaltic magmas by conducting a comparative petrological study of the high-Na/K Horyosawa lava and low-Na/K Nozuka lava from Rishiri Volcano in the southern Kuril Arc. Both lava flows are basaltic and are suitable for estimating the magma generation conditions. The whole-rock SiO2 content of the high-Na/K Horyosawa lava, the main target of this study, ranges from 51 9 to 53 4 wt%. The petrological and geochemical features of this lava indicate that the effects of magma mixing and assimilation of crustal materials were limited, and the low-SiO2 magmas were essentially derived via olivine fractionation from the primary magma. The primary magma is estimated to have been generated by 2 4 % partial melting of the source mantle with 0 10 wt% water at 1300 ?C and 2 4 GPa. By comparing the results of the high-Na/K Horyosawa lavas with those published for the low-Na/K basalts (Nozuka lava), we find that (1) the slab-derived fluids involved in their formation were supercritical liquids with similar compositions, including Na/K, when released from the subducting slab, and (2) the Horyosawa primary magma was generated at a shallower level in the mantle than the Nozuka primary magma. From these observations, it is inferred that the primary magma of the Horyosawa lava was formed by the influx melting of aqueous fluid separated from the supercritical liquid at depths shallower than the critical point, whereas the Nozuka lavawas generated by supercritical liquid-fluxed melting at depths deeper than the critical point. These findings show that elemental partitioning during the separation of slab-derived supercritical liquid into aqueous fluid and hydrous melt can induce chemical diversity within the primary basaltic magmas of a single volcanic system.
• Tracing the subducting Pacific slab to the mantle transition zone with hydrogen isotopes

  Takeshi Kuritani, Kenji Shimizu, Takayuki Ushikubo, Qun Ke Xia, Jia Liu, Mitsuhiro Nakagawa, Hajime Taniuchi, Eiichi Sato, Nobuo Doi

  Scientific Reports 11 (1) 2021/12 [Refereed]
   

  Hydrogen isotopes have been widely used as powerful tracers to understand the origin of terrestrial water and the water circulation between the surface and the deep interior of the Earth. However, further quantitative understanding is hindered due to a lack of observations about the changes in D/H ratios of a slab during subduction. Here, we report hydrogen isotope data of olivine-hosted melt inclusions from active volcanoes with variable depths (90‒550 km) to the subducting Pacific slab. The results show that the D/H ratio of the slab fluid at the volcanic front is lower than that of the slab fluid just behind the volcanic front. This demonstrates that fluids with different D/H ratios were released from the crust and the underlying peridotite portions of the slab around the volcanic front. The results also show that the D/H ratios of slab fluids do not change significantly with slab depths from 300 to 550 km, which demonstrates that slab dehydration did not occur significantly beyond the arc. Our estimated δD‰ value for the slab materials that accumulated in the mantle transition zone is > − 90‰, a value which is significantly higher than previous estimates.
• High–μ signature in lavas of Mt. Oku: Implications for lithospheric and asthenospheric contributions to the magmatism of the Cameroon Volcanic Line (West Africa)

  A. N.E. Asaah, T. Yokoyama, H. Iwamori, F. T. Aka, T. Kuritani, T. Usui, J. Tamen, M. Gountié Dedzo, B. Chako-Tchamabé, T. Hasegawa, L. A. Nche, T. Ohba

  Lithos 400-401 0024-4937 2021/11 [Refereed]
   

  This study presents geochemical data for mafic lavas from Mt. Oku to investigate the mantle source for Mt. Oku lavas and regional variations in isotopic ratios along the Cameroon Volcanic Line (CVL). The investigated mafic lavas present trace element and isotope signatures akin to ocean island basalts (OIB) and cover the entire range of the CVL. Major and trace element compositions of the studied lavas indicates that they have undergone fractionation of olivine + clinopyroxene + Fe-Ti oxide ± Cr–spinel with insignificant crustal contamination. The lavas were generated by <2% partial melting from a heterogeneous source containing less than 4% garnet. Major and trace elements abundances indicate the presence of hydrous minerals indicative of modal metasomatism. The highest value of radiogenic 206Pb/204Pb (20.7) so far measured along the CVL is reported in this study. The isotopic features of the lavas suggest that the HIMU signatures are dominantly inherited from the asthenosphere (primary source) and melting of enriched components hosted in the subcontinental lithospheric mantle (SCLM; secondary source). Lavas from Mt. Oku present more comprehensive ranges of Sr–Nd–Pb isotopic composition than those from the continent-ocean-boundary (COB). The isotopic composition of Mt. Oku lavas with 206Pb/204Pb > 19.5 differs from those of the COB but are similar to FOZO. The isotopic composition of Mt. Oku lavas (87Sr/86Sr = 0.7030–0.7036, 143Nd/144Nd = 0.5127–0.5129, 206Pb/204Pb = 17.9–20.7) suggest that the lavas are the result of contributions from the asthenosphere and the SCLM. Using a combination of Sr–Nd–Pb isotope display on a 3D plot and Sm/Yb, we identify four essential components denoted; A, B, C and D involved in the petrogenesis of Mt. Oku lavas and the approximate melting depth. While A and B are from the asthenosphere with HIMU mantle flavour, C and D owe their origin from the SCLM with EM1 and DMM characteristics, respectively. Although depleted in Sr-Nd isotopes, D is radiogenic in 206Pb/204Pb, indicating the influence of metasomatism. Therefore, the petrogenesis of Mt. Oku lavas involves the mixing of at least three mantle end-members: HIMU–DMM–EM1. The “fanning” of samples from”A" towards the other components (B, C and D) indicates a HIMU–like end–member dominance in Mt. Oku magmatism. Considering the geophysical studies on mantle convection and plumes, we have developed a schematic model that explains the petrogenesis of the CVL lavas. The continuous spectrum from relatively depleted to HIMU characteristics of CVL lavas is associated with the progressive change in source components (i) SCLM with ± pockets of enriched metasomes to (ii) SCLM + asthenosphere sources and finally to (iii) typically asthenospheric source material ± pockets of enriched metasomes. Lavas derived from stage (i) exhibit geochemical variability involving DMM–like and EM1–like signatures. The magma generated at Stage (iii) is dominated by the HIMU mantle component and accounts for over 85% of CVL lavas.
• Lead concentrations and isotope ratios in blood, breastmilk and feces: contribution of both lactation and soil/dust exposure to infants in a lead mining area, Kabwe, Zambia

  Haruya Toyomaki, John Yabe, Shouta M.M. Nakayama, Yared B. Yohannes, Kaampwe Muzandu, Tiza Mufune, Hokuto Nakata, Yoshinori Ikenaka, Takeshi Kuritani, Mitsuhiro Nakagawa, Kennedy Choongo, Mayumi Ishizuka

  Environmental Pollution 286 0269-7491 2021/10/01 [Refereed]
   

  Lead (Pb) poses a serious public health concern. Breastmilk may be a possible source of Pb exposure in infants, as Pb can be transferred from the maternal blood to breastmilk. The present study was undertaken to determine the Pb exposure and the contribution of lactation as one of the exposure pathways to infants in a Pb mining area, Kabwe, Zambia. Blood, breastmilk and infants' feces were collected from 418 pairs of infants and mothers. The Pb concentrations, isotope ratios in the samples, and biochemistry in mothers' plasma were analyzed. The overall mean of blood lead levels (BLLs) in infants and mothers were 18.0 and 11.3 μg/dL, respectively. High Pb concentration in breastmilk (range: 0.4–51.9, mean: 5.3 μg/L) above the WHO acceptable level between 2 and 5 μg/L were found and could be one of the sources of Pb exposure in infants. The Pb isotope ratios in infants’ feces were the most similar to Pb ratios in the soil samples. The results suggest that infants are also exposed to Pb from the environment. Pb exposure in infants through breastfeeding and soil ingestion could potentially exceed daily intake of Pb which causes neurodevelopmental toxicity. In contrast to the high BLLs in mothers, the plasma biochemical profiles of most analyzed parameters were interestingly within, or close to, the standard reference values. Our data suggest that environmental remediation is urgently needed to reduce the Pb exposure in infants and mothers from the environment in Kabwe in parallel with chelation therapy.
• Conditions of magma generation at the Me-akan volcano, northern Japan

  Takeshi Kuritani, Eiichi Sato, Keiji Wada, Akiko Matsumoto, Mitsuhiro Nakagawa, Dapeng Zhao, Kenji Shimizu, Takayuki Ushikubo

  Journal of Volcanology and Geothermal Research 417 0377-0273 2021/09 [Refereed]
   

  The Me-akan volcano is one of the most active volcanoes in Japan. In this study, the conditions of magma generation were estimated for basaltic products derived from Akan-fuji, the youngest mafic volcanic edifice at Me-akan, to elucidate the magmatic system. The volcanic products show significant variations in their whole-rock compositions (50.1 to 52.6 wt% SiO2), and can be divided into a low-SiO2 group (<50.4 wt% SiO2) and a main group (>50.7 wt% SiO2) based on Pb isotopic variations. The rocks exhibit the following two groups of phenocryst assemblages: one group contains high-Fo [100 × Mg/(Mg + Fe)] olivine and high-An [100 × Ca/(Ca + Na)] plagioclase phenocrysts (A group), and the other consists of low-Fo olivine, pyroxene, and low-An plagioclase phenocrysts (B group). Our study demonstrates that both the low-SiO2 group and the main group magmas were produced by assimilation and fractional crystallization of distinct basaltic magmas and via subsequent mixing with more primitive magmas in lower crustal magma chambers. It is suggested that the A-group phenocrysts were derived from the primitive magmas, whereas the B-group phenocrysts crystallized in situ before and/or after the occurrence of magma mixing. Multi-component thermodynamic analyses suggest that the primary magmas were segregated from the mantle at 1210–1230 °C and 0.9–1.0 GPa. The observations on estimated melt segregation depths of ~30 km are consistent with seismological observations that low-frequency micro-earthquakes (M 0.0–2.5) occur actively in the uppermost mantle. The generation of primary magmas involved slab-derived fluids of similar composition for both groups. The primary magma for the main group was generated by a greater contribution of the slab-derived fluid relative to the mantle peridotite than that of the low-SiO2 group.
• Mush, Melts and Metasediments: A History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase

  May Sas, Phil Shane, Takeshi Kuritani, Georg F Zellmer, Adam J. R Kent, Mitsuhiro Nakagawa

  Journal of Petrology 62 (8) 1460-2415 2021/08/01 

  The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr-Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042-0·7065 ± 0·0004 average 2SE) however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512-0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.
• Reconstruction of the exhumation history of the eastern Nepal Himalaya based on provenance changes

  Lalit Kumar Rai, Kohki Yoshida, Takeshi Kuritani

  Sedimentary Geology 420 0037-0738 2021/07 [Refereed]
   

  The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan denudation history. In this study, we present sandstone petrography, heavy mineral assemblages, Nd isotopes and chemical composition of detrital garnet from the Siwalik succession along the Muksar Khola section. This study reveals the Tibetan Tethys Himalaya, Higher Himalayan Crystalline and Lesser Himalaya Sequence as the source of sediments and records a provenance change from the middle Miocene to Pliocene/Pleistocene. This was observed as a shift from shallow to deeper parts of the Higher Himalayan Crystalline as a source area around 10.6 Ma. The Lesser Himalaya Sequence supplied a significant amount of sediments after 7.5 Ma, but the deeper part was only exposed after the late Pliocene to Pleistocene. Around 4.0 Ma an increase in the sediments from the Higher Himalayan Crystalline was observed. This provenance study reveals two stages of the exhumation of the eastern Nepal Himalaya. The first occurred between 11.0 and 5.5 Ma and the second after 4.0 Ma, due to the activation of an out-of-sequence thrust known as the Sun Koshi Thrust, and formation of the Lesser Himalayan duplex respectively.
• MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts

  Raimundo Brahm, Georg F. Zellmer, Takeshi Kuritani, Daniel Coulthard, Mitsuhiro Nakagawa, Naoya Sakamoto, Hisayoshi Yurimoto, Eiichi Sato

  Frontiers in Earth Science 8 2021/01/29 [Refereed]
   

  Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo ) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c. 125 – 150 MPa) at undersaturated H O contents of c. 1 – 2 wt% H O). ® 77–78 2 2
• Thermal state of the upper mantle and the origin of the Cambrian-Ordovician ophiolite pulse: Constraints from ultramafic dikes of the Hayachine-Miyamori ophiolite

  Takafumi Kimura, Kazuhito Ozawa, Takeshi Kuritani, Tsuyoshi Iizuka, Mitsuhiro Nakagawa

  American Mineralogist 105 (12) 1778 - 1801 0003-004X 2020/12/16 [Refereed]
   

  Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ∼500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (∼0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ∼1450 °C, ∼5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ∼1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ∼500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.
• A new concept for the genesis of felsic magma: the separation of slab-derived supercritical liquid

  Hajime Taniuchi, Takeshi Kuritani, Tetsuya Yokoyama, Eizo Nakamura, Mitsuhiro Nakagawa

  Scientific Reports 10 (1) 2020/12/01 [Refereed]
   

  Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth’s surface, and that this process may have contributed to the growth of the continental crust.
• Slow ascent of unusually hot intermediate magmas triggering strombolian to sub-plinian eruptions

  C. Lormand, G. F. Zellmer, G. N. Kilgour, K. Németh, A. S. Palmer, N. Sakamoto, H. Yurimoto, T. Kuritani, Y. Iizuka, A. Moebis

  Journal of Petrology 61 (10) 0022-3530 2020/10/01 [Refereed]
   

  To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years bp to ad 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (>30 μm) display complex growth zoning, and only the smallest crystals (<30 μm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16·5 km) from hot andesitic magmas (1010-1130 °C) with low H2O content (0-1·5 wt%). Size distributions of a total of >60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of <5 cm s-1 prior to H2O exsolution, which occurs only at <33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 and 12 m s-1; that is, at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s-1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke changing to cylindrical conduit).
• Magma plumbing system for the Millennium Eruption at Changbaishan volcano, China: Constraints from whole-rock U–Th disequilibrium

  Takeshi Kuritani, Mitsuhiro Nakagawa, Jumpei Nishimoto, Tetsuya Yokoyama, Tsuyoshi Miyamoto

  Lithos 366-367 0024-4937 2020/08 [Refereed][Not invited]
   

  A caldera-forming eruption in 946 CE, known as the Millennium Eruption (ME), at Changbaishan (or Baekdusan) volcano, located on the border between China and North Korea, was one of the largest explosive eruptions on Earth in the last 2000 years. In this study, we obtained new whole-rock Sr, Nd, and Pb isotopic data and U– Th disequilibrium data of representative volcanic products of the ME, as well as those of a pre-ME (4–5 ka) eruption, to understand the evolution of the magma plumbing system related to the caldera-forming eruption. The volcanic products of the ME have SiO contents ranging from ~54 to ~75 wt%, and the whole-rock ( Th/ Th) ratios tend to decrease systematically from basaltic trachy-andesite through trachyte to comendite. The relatively high ( Th/ Th) ratios of the felsic (trachytic and comenditic) volcanic products suggest that they were formed primarily from fractional crystallization of mafic magmas, rather than from partial melting of crustal materials. The basaltic trachy-andesite sample has a distinctly high ( Th/ Th) ratio and the least radiogenic Pb isotopic ratios among ME volcanic products, suggesting that the mafic magma was recently introduced into the main felsic magma system and is hypothesized to have triggered the eruption. The lower ( Th/ Th) ratios of the comendite samples than those of the trachyte samples, as well as published U-series disequilibrium data, suggest that the comendite magmas were produced by mixing old (> ~ 23 ka) and young (<~6 ka) comenditic magmas. The old comendite magma was the main component of ME volcanic products. Pre-ME pantelleritic magmas with distinctly low ( Th/ Th) and high Sr/ Sr ratios from those of ME magmas may have formed a short-lived magma chamber until ~4–5 ka, which was isolated from the long-lived, main comendite magma chamber. 238 230 230 232 230 232 230 232 230 232 230 232 87 86 2
• Factors associated with lead (Pb) exposure on dogs around a Pb mining area, Kabwe, Zambia

  Haruya Toyomaki, John Yabe, Shouta M.M. Nakayama, Yared B. Yohannes, Kaampwe Muzandu, Allan Liazambi, Yoshinori Ikenaka, Takeshi Kuritani, Mitsuhiro Nakagawa, Mayumi Ishizuka

  Chemosphere 247 0045-6535 2020/05 [Refereed][Not invited]
   

  Lead (Pb)-poisoning is a serious public health concern and dogs have been useful as a sentinel-animal for Pb exposure of humans. In the present study, the blood Pb concentrations (BLC), isotope ratios (208 Pb/206 Pb and 207 Pb/206 Pb), and biochemistry of 120 domestically owned dogs living around a Pb mining area, in Kabwe, Zambia were analyzed to determine factors associated with Pb exposure. The overall mean value of Pb in dog blood in the present study was 271.6 μg/L. The BLC in the dogs from sites near the mine were significantly higher than those in the dogs from a site 4 km from the mine (352.9 ± 205.1 μg/L versus 28.0 ± 13.9 μg/L). BLC significantly decreased with both increasing age of the dogs and distance from the mine. The Pb isotope ratios in the dog that resided near the mine showed values similar to those reported at the galena mine in Kabwe, which is considered to be the source of Pb exposure. In contrast to the high metal exposure that was determined in these dogs, the mean values of most analyzed parameters in the blood biochemical analysis were surprisingly within or close to the standard reference values. Moreover, none of the dogs showed overt signs of Pb-poisoning or other clinical symptoms. The results of analysis of Pb exposure of the dogs obtained in the present study, which are similar to the previously reported results in human in this location, suggest that dogs could be useful as a sentinel animal for Pb exposure of humans in Kabwe.
• Generation of calc-alkaline andesite magma through crustal melting induced by emplacement of mantle-derived water-rich primary magma: Evidence from Rishiri Volcano, southern Kuril Arc

  Hajime Taniuchi, Takeshi Kuritani, Mitsuhiro Nakagawa

  Lithos 354-355 0024-4937 2020/02 [Refereed][Not invited]
   

  Calc-alkaline andesite is a typical rock series in many island and continental arcs, and it commonly coexists with tholeiitic series rocks in individual volcanoes. In this study, the main controlling factors on the occurrence of calc-alkaline magmatism are examined for andesitic lavas from Rishiri Volcano, northern Japan. The whole-rock SiO contents of lavas range from 57.9 wt% to 61.9 wt%, and they are classified as calc-alkaline series. The lavas have petrographical and mineralogical features that show evidence of magma mixing. The whole-rock compositional variations of the lavas indicate that three end-member magmas were involved in their generation, a primitive mafic end-member magma, an evolved mafic end-member magma, and a felsic end-member magma. We suggest that the primitive mafic end-member magma represents a near primary magma generated in the upper mantle, the evolved mafic end-member magma was derived from the primitive magma by olivine fractionation, and the felsic end-member magma was produced primarily by partial melting of the crust. The primitive end-member magma is estimated to contain ~5 wt% water, which is much higher than those of the primary basaltic magmas of other volcanic stages of Rishiri that do not include calc-alkaline magmas. Based on this observation, we suggest that the water content of the primary magma was the main factor controlling the genesis of calc-alkaline magmatism at Rishiri. The more hydrous the primary magma and its derivative magma, the more extensive the melting of the crust due to the higher water flux from the solidifying magma chamber. 2
• Current situation regarding lead exposure in birds in Japan (2015–2018); lead exposure is still occurring

  Chihiro Ishii, Yoshinori Ikenaka, Shouta M.M. Nakayama, Takeshi Kuritani, Mitsuhiro Nakagawa, Keisuke Saito, Yukiko Watanabe, Kohei Ogasawara, Manabu Onuma, Atsushi Haga, Mayumi Ishizuka

  Journal of Veterinary Medical Science 82 (8) 1118 - 1123 0916-7250 2020 [Refereed][Not invited]
   

  Birds of a number of species have died as a result of lead (Pb) poisoning, including many Steller’s sea eagles (Haliaeetus pelagicus) and white-tailed sea eagles (Haliaeetus albicilla) in Hokkaido, the northernmost island of Japan. To address this issue, the use of any type of Pb ammunition for hunting of large animals was prohibited in Hokkaido in 2004. However, Pb poisoning is still being reported in this area, and there are few regulations regarding the use of Pb ammunition in other parts of Japan, where it has been reported that eagles and water birds have been exposed to Pb. This study was performed to accurately determine the current level of Pb exposure of birds found dead in the field or dead in the wild bird centers in Japan (June 2015–May 2018) and to identify the sources of Pb. Pb exposure was found to still be occurring in raptors and water birds in various parts of Japan. Twenty-six point five % and 5.9% of the recorded deaths of Steller’s sea eagles and white-tailed sea eagles, respectively, were found to have been poisoned by Pb. In addition, Pb isotope ratio analysis showed that both Pb rifle bullets and Pb shot pellets cause Pb exposure in birds, and these endangered eagles are also exposed to Pb in Hokkaido due to the illegal use of Pb ammunition. Changing to Pb-free ammunition, such as copper (Cu) rifle bullets, steel shot pellets, or bismuth shot pellets, will be essential for the conservation of avian species in Japan.
• Major/trace elements and Sr–Nd–Pb isotope systematics of lavas from lakes Barombi Mbo and Barombi Koto in the Kumba graben, Cameroon volcanic line: Constraints on petrogenesis

  A. N.E. Asaah, T. Yokoyama, F. T. Aka, H. Iwamori, T. Kuritani, T. Usui, M. Gountie Dedzo, J. Tamen, T. Hassegawa, E. M. Fozing, M. J. Wirmvem, A. L. Nche

  Journal of African Earth Sciences 161 1464-343X 2020/01 [Refereed][Not invited]
   

  This study presents major elements, trace elements and Sr–Nd–Pb isotope data for fifteen (15) representative volcanic rock samples from volcanic edifices of Lakes Barombi Mbo (n = 9) and Barombi Koto (n = 6), located in the Kumba graben along the Cameroon Volcanic Line (CVL). The samples are dominantly basalts and trachy-basalts with MgO ranging from 7 to 9 wt.%. The lavas from the edifice of Lake Barombi Mbo are slightly richer in TiO (mean 3.3 wt%) and lower in CaO (mean 9.2 wt%) relative to those from the edifice of Lake Barombi Koto (2.9 wt% and 10.3 wt%, respectively). Trace element characteristics of the lavas suggest an enrichment of the source by carbonatitic metasomatism and melting of a heterogeneous source at varying depth in the garnet-spinel transition zone. Trace elements and isotopic composition of the lavas suggest insignificant crustal contamination. Modelling of the degree of partial melting suggest that the magmas were formed at varying depth (60–80 km) and by ~2% partial melting of a source containing less than 4% garnet. Samples from Lake Barombi Mbo formed at a higher degree of partial melting (2%–0.5%) and at a shallower depth relative to those from the volcanic edifice of Lake Barombi Koto (mostly <0.5%) and Mt. Cameroon. Interestingly, the relationship of Zr/Hf and Pb/ Pb suggests that the magma was affected by carbonatitic metasomatism whose effect decreased from Mt. Cameroon to Lake Barombi Mbo. Our results suggest that Lakes Barombi Mbo and Barombi Koto lavas are typical of lavas with HIMU-like signature and involved contributions from a dominantly sublithospheric mantle (SLM) and minor contributions from the sub-continental lithospheric mantle (SCLM). The SLM material rose and was hosted in the SCLM prior to melting. 2 206 204
• Buoyant hydrous mantle plume from the mantle transition zone

  Takeshi Kuritani, Qun Ke Xia, Jun Ichi Kimura, Jia Liu, Kenji Shimizu, Takayuki Ushikubo, Dapeng Zhao, Mitsuhiro Nakagawa, Shumpei Yoshimura

  Scientific Reports 9 (1) 2019/12/01 [Refereed][Not invited]
   

  Magmatism at some intraplate volcanoes and large igneous provinces (LIPs) in continental areas may originate from hydrous mantle upwelling (i.e. a plume) from the mantle transition zone (MTZ) at 410–660 km depths in the Earth’s deep interior. However, the ultimate origin of the magmatism, i.e. why mantle plumes could have been generated at the MTZ, remains unclear. Here, we study the buoyancy of a plume by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology. Based on carefully determined water contents of the basalts, the potential temperature of the source mantle is estimated to be 1310–1400 °C, which is within the range of the normal upper mantle temperature. This observation suggests that the mantle plume did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ. Such a hydrous mantle plume can account for the formation of extremely hydrous LIP magmatism. The water was originally sourced from a stagnant slab and stored in the MTZ, and then upwelled irrespective of the presence or absence of a deep thermal plume.
• Fingerprint of silicic magma degassing visualised through chlorine microscopy

  Shumpei Yoshimura, Takeshi Kuritani, Akiko Matsumoto, Mitsuhiro Nakagawa

  Scientific Reports 9 (1) 2019/12/01 [Refereed][Not invited]
   

  Volatile-rich silicic magma erupts either explosively as a jet of a mixture of pyroclasts and high-temperature gas, or non-explosively to effuse lava. The bifurcation of the eruption style is widely recognised as being controlled by the efficiency of open-system gas loss from vesiculated magma during ascent. However, the fundamental question of how the gas escapes from highly viscous magma still remains unsolved because the pathways of gas flow are rarely preserved in dense lava. Here we show that such pathways are visualised in groundmass glass using high-resolution chlorine (Cl) mapping analysis on the rhyolitic lava of the Mukaiyama volcano, Japan. The results showed that the glass was highly heterogeneous in Cl content. A spatial distribution of the Cl content in the groundmass glass showed that volatiles diffused towards most bubbles, but the bubbles collapsed into the dense melt rather than growing. All observations, in combination with melt inclusion analysis, indicate that vesiculation, the formation of interconnected bubble channels, open-system gas loss via the channels, and channel collapse repeated within the period of a few days to two weeks during ascent. This cycle repeated individually in centimetre-sized portions of magma with different timing.
• Magma generation conditions at the Akita-Komagatake volcano, Northeast Japan arc: Implications of across-arc variations in mantle melting parameters

  Takeshi Kuritani, Chizuko Kanai, Shigeru Yamashita, Mitsuhiro Nakagawa

  Lithos 348-349 0024-4937 2019/12/01 [Refereed][Not invited]
   

  Akita-Komagatake is an active volcano in the Northeast Japan arc with a slab depth (H) of ~110 km. In this study, magma generation conditions were estimated for basaltic products from the volcano to examine the thermal structure of the sub-arc mantle and the across-arc variations in the parameters related to magma genesis. The studied basalts have the highest MgO content (~6 wt%) among the products from the volcano and contain ~15% plagioclase phenocrysts, ~5% olivine phenocrysts, and <1% pyroxene phenocrysts. The water content of the melt was estimated to be ~4 wt% by melt inclusions in the phenocrysts using plagioclase–melt hygrometry. Multi-component thermodynamic analyses suggest that the primary magma was generated by ~14% melting of the source mantle with ~0.5 wt% H O at ~1275 °C and ~1.3 GPa. The estimated pressure condition of ~1.3 GPa coincides well with the low-velocity anomaly observed in the mantle at ~40 km depth beneath the volcano. By comparing the results from Akita-Komagatake to the published results of the frontal-arc Iwate volcano (H = ~90 km) and the rear-arc Sannome-gata volcano (H = ~150 km), which are arranged along a cross-arc line in the Northeast Japan arc, we found that the pressure conditions of the magma generation are ~1.3 GPa at both Iwate and Akita-Komagatake, which are significantly lower than the ~1.8 GPa at the rear-arc Sannome-gata volcano. The source mantle temperature of ~1275 °C at ~1.3 GPa beneath Akita-Komagatake is higher than the ~1250 °C at ~1.3 GPa beneath Iwate, which suggests that the ascending flow of the mantle wedge corner flow may reach the shallowest levels beneath Akita-Komagatake. It was also demonstrated that the depth at which fluid was released from the subducting slab tends to increase for the volcanoes with increasing H. 2
• One year exposure to Cd- and Pb-contaminated soil causes metal accumulation and alteration of global DNA methylation in rats

  Shouta M.M. Nakayama, Hokuto Nakata, Yoshinori Ikenaka, John Yabe, Balazs Oroszlany, Yared B. Yohannes, Nesta Bortey-Sam, Kaampwe Muzandu, Kennedy Choongo, Takeshi Kuritani, Mitsuhiro Nakagawa, Mayumi Ishizuka

  Environmental Pollution 252 1267 - 1276 0269-7491 2019/09 [Refereed][Not invited]
   

  Metal pollution has been associated with anthropogenic activities, such as effluents and emissions from mines. Soil could be exposure route of wild rats to metals, especially in mining areas. The aim of this study was to verify whether soil exposure under environmentally relevant circumstances results in metal accumulation and epigenetic modifications. Wistar rats were divided to three groups: 1) control without soil exposure, 2) low-metal exposure group exposed to soil containing low metal levels (Pb: 75 mg/kg; Cd: 0.4), and 3) high-metal exposure group exposed to soil (Pb: 3750; Cd: 6). After 1 year of exposure, the metal levels, Pb isotopic values, and molecular indicators were measured. Rats in the high-group showed significantly greater concentrations of Pb and Cd in tissues. Higher accumulation factors (tissue/soil) of Cd than Pb were observed in the liver, kidney, brain, and lung, while the factor of Pb was higher in the tibia. The obtained results of metal accumulation ratios (lung/liver) and stable Pb isotope ratios in the tissues indicated that the respiratory exposure would account for an important share of metal absorption into the body. Genome-wide methylation status and DNA methyltransferase (Dnmt 3a/3b) mRNA expressions in testis were higher in the high-group, suggesting that exposure to soil caused metal accumulation and epigenetic alterations in rats. Soil exposure caused metal accumulation and DNA hypermethylation in rats.
• Low water content in the mantle source of the Hainan plume as a factor inhibiting the formation of a large igneous province

  Xiao Yan Gu, Piao Yi Wang, Takeshi Kuritani, Eero Hanski, Qun Ke Xia, Qin Yan Wang

  Earth and Planetary Science Letters 515 221 - 230 0012-821X 2019/06/01 [Refereed][Not invited]
   

  Large igneous provinces (LIPs) are generally considered to be related to mantle plumes and to have a great importance to the supercontinent break-up events, climate change and biological evolution over Earth's history. The high melt production rates of LIPs can be achieved under rigorous melting conditions: an abnormally high temperature, substantial decompression, addition of fusible components, or remarkable enrichment in water. Although it has been repeatedly noticed that water enrichment has occurred in many Phanerozoic LIPs, the significance of water enrichment in the mantle source for the generation of LIPs has not been explicitly highlighted. The southeastern Asian basalt province (SABP), which is thought to have formed by the Hainan mantle plume from a pyroxenite-bearing source, was emplaced over a small area of about 0.037 Mkm over a long period of time from 28.5 Ma to Holocene, thus differing from a typical LIP (>0.1 Mkm in volume emplaced in one or multiple pulses of less than 5 Ma). In this work, we measured H O contents of high-Mg# clinopyroxene phenocrysts (12–179 ppm H O) from Cenozoic basalts in the northern Hainan Island, which is part of the SABP. These data were utilized to estimate the water content of the mantle source of these basalts, yielding values in the range of 84–360 ppm H O, which are significantly lower than those obtained for many Phanerozoic LIPs (thousands of ppm). After calculating the mantle potential temperature for the Hainan basalts about 170 °C higher than that for the MORB source mantle, we consider that the paucity of source water likely depressed the melt productivity and the velocity of plume upwelling, and induced the Hainan plume to stall at depths of 350–500 km, inhibiting the formation of a LIP by the Hainan plume, despite the occurrence of other favorable conditions. Hence, our results from the SABP provide evidence for the significance of water enrichment in the mantle source in the formation of LIPs. The low water content in the Hainan plume can be attributed to the incorporation of subducted slabs that had experienced significant dehydration. 2 3 2 2 2
• High-Nb hawaiite–mugearite and high-Mg calc-alkaline lavas from northeastern Iran: Oligo-Miocene melts from modified mantle wedge

  Parham Ahmadi, Mohammad Reza Ghorbani, Massimo Coltorti, Takeshi Kuritani, Yue Cai, Anna Maria Fioretti, Eleonora Braschi, Pier Paolo Giacomoni, Farzaneh Aghabazaz, Shahrouz Babazadeh, Sandro Conticelli

  International Geology Review 61 (2) 150 - 174 0020-6814 2019/01/22 [Refereed][Not invited]
   

  Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit), constitute vast outcrops (up to 250 km ) of high-Mg basaltic andesites to dacites that are associated with high-Nb hawaiites and mugearites. Whole-rock Ar/ Ar ages show a restricted range of 24.1 ± 0.4–22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of Sr/ Sr and Nd/ Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in the high-Mg basaltic andesites–dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27–100) and the isotopic composition of the subalkaline high-Mg basaltic andesites–dacites indicate derivation from a mantle modified by slab and sediment partial melts. Evidence such as reverse zoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate that magma mixing with mafic melts and concurrent fractional crystallization lead to the compositional evolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodic alkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and a wide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr–Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from a mantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggested to have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatized by slab-derived melts. 2 40 39 87 86 143 144
• Magma plumbing system at Izu-Oshima Volcano, Japan: Constraints from petrological and geochemical analyses

  Takeshi Kuritani, Azusa Yamaguchi, Sayuki Fukumitsu, Mitsuhiro Nakagawa, Akiko Matsumoto, Tetsuya Yokoyama

  Frontiers in Earth Science 6 178  2018/11/13 [Refereed][Invited]
   

  © 2018 Kuritani, Yamaguchi, Fukumitsu, Nakagawa, Matsumoto and Yokoyama. The Izu-Oshima volcano is one of the most active volcanoes in Japan, and has generated relatively large-scale eruptions every 30–40 years for the past 200 years. As more than 30 years have passed since the last eruptions in 1986–87, volcanic activity is expected to resume in the near future. To help elucidate the current and future state of the volcano’s magma system, the temporal evolution of the recent magma plumbing system was investigated through a petrological and geochemical analysis of its basaltic lavas and pyroclastics (<∼53 wt.% of SiO2) that were erupted during the last ∼1.5 kyr. The basaltic products have variable phenocryst contents, ranging from ∼0 to ∼20 vol.%, and phenocryst-bearing samples commonly contain plagioclase, orthopyroxene, and clinopyroxene phenocrysts. The whole-rock compositions are significantly scattered in the Harker variation diagrams, suggesting that the compositional diversity was established by at least two independent magmatic processes. The application of principal component analysis on the whole-rock major element data suggests that one magmatic process was crystal fractionation of crystal-poor magmas, and the other process was either plagioclase accumulation or mixing of plagioclase-rich magmas. Based on this observation, and combined with the petrological analysis and previous geophysical studies, we propose that aphyric magmas, stored in an 8–10 km-deep magma chamber, progressively differentiated over time from the 7th to 20th century. Furthermore, the compositional variations in basalts resulted from the mixing of the differentiating aphyric magmas with variable proportions of porphyritic magmas derived from a 13–18 km-deep magma chamber. Because recent eruptions have been triggered by the ascent of porphyritic magma from the 13–18 km-deep magma chamber, and its injection into the 8–10 km-deep magma chamber, it is important to monitor the deeper magma chamber to predict future volcanic activity.
• Differentiation of a Hydrous Arc Magma Recorded in Melt Inclusions in Deep Crustal Cumulate Xenoliths from Ichinomegata Maar, NE Japan

  Y. Yanagida, M. Nakamura, A. Yasuda, T. Kuritani, M. Nakagawa, T. Yoshida

  Geochemistry, Geophysics, Geosystems 19 (3) 838 - 864 2018/03 [Refereed][Not invited]
   

  © 2018. American Geophysical Union. All Rights Reserved. The Ichinomegata maar, located in the back-arc side of the northeastern Japan arc, erupted calc-alkaline andesitic magma with abundant deep-crustal and mantle xenoliths at about 60–80 ka. We investigated the relationship between fractionated solids and differentiated silicic melts at middle to lower crust conditions through petrologic analyses of hornblende-bearing cumulate xenoliths and melt inclusions. The Sr and Nd isotope compositions are similar to those of the host magmas, suggesting their cognate origin. The crystallization sequence is determined to be olivine, spinel → clinopyroxene → hornblende, magnetite → plagioclase → apatite, based on the observation of texture and solid solution compositions. Of the five types of xenolith studied, the leuco-hornblende gabbro preserved interstitial glass and melt inclusions with a silica content (SiO2) of 63.9–74.0 wt % and high water content (up to 8.1 wt %). Hornblende geobarometry indicates an equilibrium pressure of 0.39–0.64 GPa, which corresponds to a depth of 15–24 km. The delay of plagioclase crystallization due to high water content characterizes differentiation of the hydrous arc magma. We successfully constructed an internally consistent differentiation model of the corundum-normative calc-alkaline trend to the silicic melt inclusions, starting from primitive basalt via successive fractionation of a calculated wherlite and cumulate xenoliths. In middle to late stage differentiation, hornblende gabbros are the dominantly fractionated assemblages. Our results are consistent with published differentiation experiments of hydrous arc magmas and the lithology of the deep crust beneath the back arc of the NE Japan arc as estimated from seismic tomography.
• Mantle hydration and the role of water in the generation of large igneous provinces

  Jia Liu, Qun Ke Xia, Takeshi Kuritani, Eero Hanski, Hao Ran Yu

  Nature Communications 8 (1) 2041-1723 2017/12/01 [Refereed][Not invited]
   

  © 2017 The Author(s). The genesis of large igneous provinces (LIP) is controlled by multiple factors including anomalous mantle temperatures, the presence of fusible fertile components and volatiles in the mantle source, and the extent of decompression. The lack of a comprehensive examination of all these factors in one specific LIP makes the mantle plume model debatable. Here, we report estimates of the water content in picrites from the Emeishan LIP in southwestern China. Although these picrites display an island arc-like H2O content (up to 3.4 by weight percent), the trace element characteristics do not support a subduction zone setting but point to a hydrous reservoir in the deep mantle. Combining with previous studies, we propose that hydrous and hot plumes occasionally appeared in the Phanerozoic era to produce continental LIPs (e.g., Tarim, Siberian Trap, Karoo). The wide sampling of hydrous reservoirs in the deep mantle by mantle plumes thus indicates that the Earth's interior is largely hydrated.
• Reflectance spectra of Asteroids and Meteorites: their classifications and statistical comparisons

  Hideaki Miyamoto, Peng K Hong, Takafumi Niihara, Takeshi Kuritani, Kenji Fukumizu, Hideitsu Hino, Kenji Nagata, Shotaro Akaho, J Alexis P Rodriguez, Hemmi Ryodo, Seiji Sugita, Masato Okada

  Journal of Physics: Conference Series 1036 1742 - 6596 2017/09 [Refereed][Not invited]
  
• Fluid-fluxed melting of mantle versus decompression melting of hydrous mantle plume as the cause of intraplate magmatism over a stagnant slab: Implications from Fukue Volcano Group, SW Japan

  Takeshi Kuritani, Tetsuya Sakuyama, Natsumi Kamada, Tetsuya Yokoyama, Mitsuhiro Nakagawa

  Lithos 282-283 98 - 110 0024-4937 2017/06/01 [Refereed][Not invited]
   

  © 2017 Elsevier B.V. The Pacific Plate subducting from the Japan Trench has accumulated in the mantle transition zone beneath NE Asia, and intraplate magmatism has been active above the stagnant Pacific slab. To understand the origin of the intraplate magmatism in relation to slab stagnation, a petrological and geochemical study was carried out on basaltic samples from a monogenetic volcano of the Fukue Volcano Group, southwest Japan. The eruption products consist of low-Si and high-Si groups, and the two magmas are hypothesized to originate from different mantle source material based on radiogenic isotopic compositions. The H2O contents of the primary magmas were estimated as ~ 2 wt.% for both the low-Si and high-Si groups. Analyses using multicomponent thermodynamics suggested that the low-Si and high-Si primary magmas were generated at ~ 2.5 GPa and 1345 °C and at ~ 1.8 GPa and 1285 °C, respectively. These results, and the geochemical characteristics of the products, indicated that the low-Si magma was generated in the asthenospheric mantle whereas the high-Si magma was produced by interaction of the low-Si magma with the sub-continental lithospheric mantle. The low mantle potential temperature of ~ 1300 °C and hydrous nature (H2O/Ce = ~ 650) of the low-Si magma suggested that the magma was generated by fluid-fluxed melting of the asthenospheric mantle. Based on these results and those obtained in previous studies, intraplate magmatism over the stagnant Pacific slab can be summarized as having been caused by either melting of the asthenospheric mantle through an influx of fluids derived from the mantle transition zone or decompression melting of a hydrous mantle plume derived from the mantle transition zone. We infer that the fluids for the flux melting have been released from the mantle transition zone where water was locally saturated. Meanwhile, hydrous mantle plumes have been generated at the mantle transition zone where a return flow of sub-lithospheric mantle material entrained beneath the subducting Pacific slab has intruded from below.
• A petrological and geochemical study on time-series samples from Klyuchevskoy volcano, Kamchatka arc

  Olga Bergal-Kuvikas, Mitsuhiro Nakagawa, Takeshi Kuritani, Yaroslav Muravyev, Nataliya Malik, Elena Klimenko, Mizuho Amma-Miyasaka, Akiko Matsumoto, Shunjiro Shimada

  Contributions to Mineralogy and Petrology 172 (5) 0010-7999 2017/05/01 [Refereed][Not invited]
   

  © 2017, Springer-Verlag Berlin Heidelberg. To understand the generation and evolution of mafic magmas from Klyuchevskoy volcano in the Kamchatka arc, which is one of the most active arc volcanoes on Earth, a petrological and geochemical study was carried out on time-series samples from the volcano. The eruptive products show significant variations in their whole-rock compositions (52.0–55.5 wt.% SiO2), and they have been divided into high-Mg basalts and high-Al andesites. In the high-Mg basalts, lower-K and higher-K primitive samples (>9 wt.% MgO) are present, and their petrological features indicate that they may represent primary or near-primary magmas. Slab-derived fluids that induced generation of the lower-K basaltic magmas were less enriched in melt component than those associated with the higher-K basaltic magmas, and the fluids are likely to have been released from the subducting slab at shallower levels for the lower-K basaltic magmas than for higher-K basaltic magmas. Analyses using multicomponent thermodynamics indicates that the lower-K primary magma was generated by ~13% melting of a source mantle with ~0.7 wt.% H2O at 1245–1260 °C and ~1.9 GPa. During most of the evolution of the volcano, the lower-K basaltic magmas were dominant; the higher-K primitive magma first appeared in AD 1932. In AD 1937–1938, both the lower-K and higher-K primitive magmas erupted, which implies that the two types of primary magmas were present simultaneously and independently beneath the volcano. The higher-K basaltic magmas evolved progressively into high-Al andesite magmas in a magma chamber in the middle crust from AD 1932 to ~AD 1960. Since then, relatively primitive magma has been injected continuously into the magma chamber, which has resulted in the systematic increase of the MgO contents of erupted materials with ages from ~AD 1960 to present.
• Origin of ultra rear-arc magmatism at Rishiri Volcano, Kuril Arc

  Takeshi Kuritani, Mitsuhiro Nakagawa

  Geochemistry, Geophysics, Geosystems 17 (10) 4032 - 4050 1525-2027 2016/10/01 [Refereed][Not invited]
   

  © 2016. American Geophysical Union. All Rights Reserved. The Rishiri Volcano is located at the very rear of the Kuril Arc at its junction with the NE Japan Arc, and its 300 km depth to the slab surface is one of the deepest among the active arc volcanoes in the world. In this study, the origin of this ultra rear-arc magmatism was investigated by analyzing the basaltic lavas from the volcano. The lavas consist of low-K and high-K groups, with the low-K lavas predating the high-K lavas. Since it is unlikely that the high-K magmas are derivatives of the low-K magmas, the two magmas are thought to be derived from different source mantle materials. Analyses using multicomponent thermodynamics suggest that these magmas were both generated through the ∼2% melting of a source mantle with 0.04–0.11 wt.% H2O at 1280–1340°C and ∼2.3 GPa. The temperatures at the surface of the subducting Pacific slab, from which the slab fluids were released, were estimated to be 860–960°C for the low-K magmas and 930–1040°C for the high-K magmas. These temperatures of the slab surface are remarkably higher than those predicted by thermal models. The estimated high temperatures of the slab surface and the latest detailed seismic tomography results suggest that the low-K and high-K magmatism resulted from the progressive production of fluids at the slab surface due to heating by the injection of hot mantle materials into a relatively large-scale fracture in the distorted portion of the subducting Pacific plate.
• Intensive hydration of the wedge mantle at the Kuril arc-NE Japan arc junction: Implications from mafic lavas from Usu volcano, northern Japan

  Takeshi Kuritani, Mayumi Tanaka, Tetsuya Yokoyama, Mitsuhiro Nakagawa, Akiko Matsumoto

  Journal of Petrology 57 (6) 1223 - 1240 0022-3530 2016/06/01 [Refereed][Not invited]
   

  © The Author 2016. Published by Oxford University Press. All rights reserved. The generation and evolution of basaltic magmas at Usu volcano, located at the junction between the NE Japan arc and the Kuril arc, have been investigated. The mafic products, which form the somma edifice of the volcano, consist of basalt (49.6-51.3wt % SiO2) and basaltic andesite (52.0-54.9wt % SiO2) lavas. The basaltic lavas show relatively tight compositional trends, and 87Sr/86Sr ratios tend to decrease with increasing whole-rock SiO2 content. The water content of the basaltic magmas was determined to be ~4.8wt % based on plagioclase-melt thermodynamic equilibrium. Using this information and an olivine maximum fractionation model, the water content of the primary Usu magma was estimated to be 3.9wt %. Multi-component thermodynamic calculations suggest that the primary magma was generated by ~23% melting of the source mantle with ~0.94 wt % H2O at ~1300°C and ~1.4GPa. The 0.94wt % water content of the source mantle is significantly higher than that beneath volcanoes in the main NE Japan arc (generally <0.7wt% H2O); this implies that the wedge mantle at the arc-arc junction is intensively hydrated. The temperature of the wedge mantle of ~1300°C at ~1.4GPa is also significantly higher than that of the mantle in the main NE Japan arc. Unlike the basaltic lavas, the whole-rock compositions of the basaltic andesite lavas are scattered in Harker variation diagrams. This observation suggests that the compositional diversity was produced by at least two independent processes. To elucidate the processes responsible for this compositional diversity, principal component analysis was applied to the major element compositions of the samples. This suggests that 47% of the diversity of the whole-rock compositions can be explained by mixing with partial melts of lower crustal materials, 25% is explained by redistribution of plagioclase phenocrysts, and 16% is explained by fractionation of accessory minerals.
• Cluster analysis on the bulk elemental compositions of Antarctic stony meteorites

  Hideaki Miyamoto, Takafumi Niihara, Takeshi Kuritani, Peng K. Hong, James M. Dohm, Seiji Sugita

  Meteoritics and Planetary Science 51 (5) 906 - 919 1086-9379 2016/05/01 [Refereed][Not invited]
   

  © 2016 The Meteoritical Society. Remote sensing observations by recent successful missions to small bodies have revealed the difficulty in classifying the materials which cover their surfaces into a conventional classification of meteorites. Although reflectance spectroscopy is a powerful tool for this purpose, it is influenced by many factors, such as space weathering, lighting conditions, and surface physical conditions (e.g., particle size and style of mixing). Thus, complementary information, such as elemental compositions, which can be obtained by X-ray fluorescence (XRF) and gamma-ray spectrometers (GRS), have been considered very important. However, classifying planetary materials solely based on elemental compositions has not been investigated extensively. In this study, we perform principal component and cluster analyses on 12 major and minor elements of the bulk compositions of 500 meteorites reported in the National Institute of Polar Research (NIPR), Japan database. Our unique approach, which includes using hierarchical cluster analysis, indicates that meteorites can be classified into about 10 groups purely by their bulk elemental compositions. We suggest that Si, Fe, Mg, Ca, and Na are the optimal set of elements, as this set has been used successfully to classify meteorites of the NIPR database with more than 94% accuracy. Principal components analysis indicates that elemental compositions of meteorites form eight clusters in the three-dimensional space of the components. The three major principal components (PC1, PC2, and PC3) are interpreted as (1) degree of differentiations of the source body (i.e., primitive versus differentiated), (2) degree of thermal effects, and (3) degree of chemical fractionation, respectively.
• Classification of Meteorites based Purely on Bulk Elemental compositions for Analysis of Data Obtained Through Space Missions

  H. Miyamoto, T. Niihara, T. Kuritani, P.K. Hong, J.M. Dohm, S. Sugita

  Lunar and Planetary Science Conference 46. #1802 2015 [Refereed][Not invited]
  
• Geochemistry of lavas from maar-bearing volcanoes in the Oku Volcanic Group of the Cameroon Volcanic Line

  Asobo N.E. Asaah, Tetsuya Yokoyama, Festus T. Aka, Tomohiro Usui, Takeshi Kuritani, Mengnjo J. Wirmvem, Hikaru Iwamori, Eric M. Fozing, Jules Tamen, Gilbert Z. Mofor, Takeshi Ohba, Gregory Tanyileke, J. V. Hell

  Chemical Geology 406 55 - 69 0009-2541 2015 [Refereed][Not invited]
   

  © 2015 Elsevier B.V. Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr-Nd-Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr-Nd-Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source (<6% garnet) and modelled melting results indicate <2% partial melting of the source material. Isotope data plot within the focal zone, extending towards enriched mantle 1 (EM1; e.g. Lakes Oku and Nyos samples). This indicates the involvement of at least three mantle components: depleted mid-ocean ridge basalt mantle, high-μ and EM1 components in the magmatism of the lavas studied. The contributions of these components in different proportions, originating from asthenospheric and subcontinental lithospheric mantle sources, can account for the observed variations in geochemical characteristics. The geochemical characteristics of the studied lavas indicate that the magma source need not necessarily have an abnormal CO2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO2 stability.
• Primary melt from Sannome-gata volcano, NE Japan arc: constraints on generation conditions of rear-arc magmas

  Takeshi Kuritani, Takeyoshi Yoshida, Jun-Ichi Kimura, Toshiro Takahashi, Yuka Hirahara, Takashi Miyazaki, Ryoko Senda, Qing Chang, Yoshinori Ito

  CONTRIBUTIONS TO MINERALOGY AND PETROLOGY 167 (2) 1 - 18 0010-7999 2014/02 [Refereed][Not invited]
   

  The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9-11.1 wt% MgO). High-MgO scoriae (MgO > similar to 9.5 wt%) have mostly homogeneous Sr-87/(86) Sr ratios (0.70318-0.70320), whereas low-MgO scoriae (MgO < similar to 9 wt%) have higher Sr-87/(86) Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing similar to 5 vol% olivine microphenocrysts with Mg# [100 x Mg/(Mg + Fe2+)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160-1,220 degrees C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by similar to 7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannomegata magma was generated in the source mantle with 0.5-0.6 wt% H2O at 1,220-1,230 degrees C and at similar to 1.8 GPa, and that the H2O content of the primary magma was 6-7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by similar to 15 % melting of the source mantle with 0.6-0.7 wt% H2O at similar to 1,250 degrees C and at similar to 1.3 GPa.
• Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano, NE Japan arc: Implications for differentiation mechanism of frontal-arc basalt magmas

  Takeshi Kuritani, Takeyoshi Yoshida, Jun Ichi Kimura, Yuka Hirahara, Toshiro Takahashi

  Mineralogy and Petrology 108 (1) 1 - 11 0930-0708 2014 [Refereed][Not invited]
   

  The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78-85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85-92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4-5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa. © 2013 Springer-Verlag Wien.
• Transition zone origin of potassic basalts from Wudalianchi volcano, northeast China

  Takeshi Kuritani, Jun-Ichi Kimura, Eiji Ohtani, Hideaki Miyamoto, Katsuhiko Furuyama

  LITHOS 156 1 - 12 0024-4937 2013/01 [Refereed][Not invited]
   

  The origin of historic potassic basalts (from the 1719 to 1721 AD eruption) from the Wudalianchi volcanic field, northeast China, is investigated. The samples examined in this study are aphyric, and contain similar to 5 vol.% olivine microphenocrysts. Geochemical analyses of the samples, along with published data for material from the 1719 to 1721 AD eruption, show that the basalts exhibit linear compositional trends in whole-rock variation diagrams, and are characterized by K2O enrichment (>4 wt.%) and an EM1-like isotopic signature. Using thermodynamic calculations and the observed petrological features of the basalts, the temperature of the magmas shortly before eruption is estimated to have been similar to 1250 degrees C, and the water content of the magma at depth is estimated to have been >similar to 1.1 wt.%. Because the estimated temperatures are significantly higher than the projected maximum temperature of the lithospheric mantle beneath the Wudalianchi volcanic field, the magmas were likely derived from the asthenospheric mantle. We suggest that both the potassic- and EM1-like natures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids similar to 1.5 Ga ago through a stagnation of an ancient slab. The linear whole-rock compositional trends of the basalts primarily reflect the geochemical heterogeneity in the mantle transition zone, with variable but coupled contributions of similar to 1.5 Ga sediments in the ancient stagnant slab, and recent sediments and peridotites in the stagnant Pacific slab. We infer that the Wudalianchi magmatism was caused by an upwelling of a hydrous mantle plume from the mantle transition zone, which was hydrated through the stagnation of the ancient subducted slab and the recent Pacific slab. (C) 2012 Elsevier B.V. All rights reserved.
• Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation

  Takeshi Kuritani, Eiji Ohtani, Jun-Ichi Kimura

  NATURE GEOSCIENCE 4 (10) 713 - 716 1752-0894 2011/10 [Refereed][Not invited]
   

  The mantle transition zone, located at depths of 410-660 km between the lower and upper mantle, is an important water reservoir in the Earth's interior(1-4). However, there are regional-scale heterogeneities in the distribution of water(4,5). The zone beneath northeast China, in particular, is remarkably hydrous(4), but when and how it became hydrous remains uncertain. Here we combine analyses of the geochemistry of late Cenozoic basalts in northeast China with published geochemical analyses. We find a spatial correlation between basalt geochemistry and the distribution of a low-velocity zone in the underlying mantle that is interpreted as a plume upwelling from the mantle transition zone(6). We therefore use the basalt geochemistry to infer the composition of the mantle transition zone. The basalts have high Ba/Th and Pb-207/Pb-206 ratios, which we suggest record an ancient hydration event in the transition zone that occurred more than one billion years ago, probably as a result of dehydration of a subducted slab. We suggest that this ancient hydration event, combined with a more recent hydration event linked to dehydration of the subducted Pacific slab(7), can account for the hydrous nature of the mantle transition zone beneath China. Our results demonstrate that the mantle transition zone can remain as a stable water reservoir in Earth's interior for timescales of more than a billion years.
• Geochemical evolution of historical lavas from Askja Volcano, Iceland: Implications for mechanisms and timescales of magmatic differentiation

  Takeshi Kuritani, Tetsuya Yokoyama, Hiroshi Kitagawa, Katsura Kobayashi, Eizo Nakamura

  GEOCHIMICA ET COSMOCHIMICA ACTA 75 (2) 570 - 587 0016-7037 2011/01 [Refereed][Not invited]
   

  The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjoll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as U-238-Th-230-Ra-226 systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., Sr-87/Sr-86 increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at similar to 600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, Sr-87/Sr-86 tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at similar to 100 MPa beneath the Oskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (Ra-226/Th-230), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <similar to 3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used. (C) 2010 Elsevier Ltd. All rights reserved.
• Internal differentiation of Kutsugata lava flow from Rishiri Volcano, Japan: Processes and timescales of segregation structures' formation

  Takeshi Kuritani, Takeyoshi Yoshida, Yoshitaka Nagahashi

  JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 195 (1) 57 - 68 0377-0273 2010/08 [Refereed][Not invited]
   

  Internal differentiation processes in a solidifying lava flow were investigated for the Kutsugata lava flow from Rishiri Volcano in northern Japan. In a representative 6-m thick lava flow that was investigated in detail in this study, segregation products darker than the host lavas manifested mainly in the form of pipes (vesicle cylinders) and layers (vesicle sheets), occurring around 0.5-23 m and 2.0-4.0 m above the base, respectively. Both the cylinders and sheets are significantly richer in incompatible elements such as TiO(2) and K(2)O than the host lavas, which suggest that these products essentially represent residual melt produced during solidification of the lava flow. Field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders. On the other hand, the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression. The vesicle cylinders have a remarkably higher MgO content (up to 8 wt.%) than the host lava (<6 wt.%), whereas the vesicle sheets display MgO depletion (as low as 3.5 wt.%). The relatively high MgO content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt. It is suggested that the vesicle cylinders were produced by the extraction of olivine-bearing interstitial melt from an augite-plagioclase network in the host lava, whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase, augite, and olivine in the host lava into platy fractures. We infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward-migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava. The vesicle cylinders are considered to have formed in similar to 1 day after the lava flow came to rest, while relatively large vesicle sheets (>1 cm thick) appeared much later (after similar to 9 days). The formation of these segregation products was essentially complete within 20 days after the lava emplacement. (C) 2010 Elsevier B.V. All rights reserved.
• Intraplate magmatism related to deceleration of upwrelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China

  Takeshi Kuritani, Jun-Ichi Kimura, Tsuyoshi Miyamoto, Haiquan Wei, Taketo Shimano, Fukashi Maeno, Xu Jin, Hiromitsu Taniguchi

  LITHOS 112 (3-4) 247 - 258 0024-4937 2009/10 [Refereed][Not invited]
   

  The mechanisms of continental intraplate magmatism were investigated with respect to Cenozoic basaltic shield lavas from the Changbaishan area, northeast China, using a comprehensive data set of major and trace elements, Sr, Nd, and Pb isotopic compositions, and K-Ar ages. The shield lavas show a significant variation in TiO(2) content (1-4 wt%), which exhibits a positive covariance with K(2)O and a negative covariance with SiO(2) for most samples. The compositional variation of the lavas cannot be explained solely by magma chamber processes, such as fractional crystallization and crustal assimilation, and may in part be attributed to processes in the magmatic source region. The variation in TiO(2) is considered to mainly reflect variations in the degree of partial melting of the source mantle, and the negative covariance of SiO(2) and TiO(2) suggests that lower-degree melts were produced at greater depths. On the basis of the observed negative covariance of TiO(2) with K-Ar ages in the main lavas, it would appear that magmas segregated at progressively greater depths with time, since at least 4.2 Ma. This feature cannot be explained in terms of temporal changes in the mantle potential temperature and/or bulk source composition. Instead, it appears likely that the rate of asthenosphere upwelling beneath the Changbaishan area decreased with time, whereby the melting depth gradually increased, resulting from a reduction in the heat flux from the upwelling mantle to the overlying lithosphere. The inferred deceleration of the asthenospheric mantle is considered to have been related to a change from an extensional to compressive tectonic stress regime during the late Pliocene. Our study suggests that long-term volcanic activity in the Changbaishan area will continue to decrease unless there is an increase in the upwelling rate of the asthenospheric mantle. (C) 2009 Elsevier B.V. All rights reserved.
• The relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers

  Takeshi Kuritani

  LITHOS 110 (1-4) 247 - 261 0024-4937 2009/06 [Refereed][Not invited]
   

  In a cooling magma chamber, magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body (homogeneous fractionation) and by mixing of the main magma with fractionated melt derived from low-temperature mush zones (boundary layer fractionation). In this study, the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics-based mass balance model. Model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures (<similar to 50 MPa) or when magmas are less hydrous (<similar to 1 wt.%), such as mid-ocean ridge basalt and intraplate basalt, because of the low efficiency of melt transport from the mush zones to the main magma. Therefore, magmas evolve principally by homogeneous fractionation. If crystal-melt separation does not occur effectively in the main magma, the magma becomes crystal-rich in the early stages of magmatic evolution. On the other hand, boundary layer fractionation can occur effectively when magmas are hydrous (>similar to 2 wt.%), such as arc basalt, and the magma chambers are located at depth (>similar to 100 MPa). Because the melt derived from mush zones is enriched in alkalis and H(2)O, crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature. Therefore, homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active. If magmatic differentiation proceeds primarily by boundary layer fractionation, magmas can remain free of crystals for long periods during magmatic evolution. (C) 2009 Elsevier B.V. All rights reserved.
• Highly unradiogenic lead isotope ratios from the Horoman peridotite in Japan

  Sanjeewa P. K. Malaviarachchi, Akio Makishima, Masaaki Tanimoto, Takeshi Kuritani, Eizo Nakamura

  NATURE GEOSCIENCE 1 (12) 859 - 863 1752-0894 2008/12 [Refereed][Not invited]
   

  Basalts at mid-ocean ridges are generated by partial melting of the Earth's upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting(1-3). Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle(2-7), recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts(8). Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains-if abundant in the mantle-may constitute a hitherto unrecognized reservoir with highly unradiogenic lead.
• Geochemistry and geochronology of Gaima basaltic lava plateau, Northeast China

  T. Kuritani, J. I. Kimura, T. Miyamoto, T. Shimano, F. Maeno, H. Wei, X. Jin, H. Taniguchi

  GEOCHIMICA ET COSMOCHIMICA ACTA 72 (12) A505 - A505 0016-7037 2008/07 [Refereed][Not invited]
  
• Generation of rear-arc magmas induced by influx of slab-derived supercritical liquids: Implications from alkali basalt lavas from Rishiri Volcano, Kurile arc

  Takeshi Kuritani, Tetsuya Yokoyama, Eizo Nakamura

  JOURNAL OF PETROLOGY 49 (7) 1319 - 1342 0022-3530 2008/07 [Refereed][Not invited]
   

  Magma generation processes were investigated for alkali basalt lavas from Rishiri Volcano, located at the rear of the Kurile arc, using major and trace elements and Sr, Nd, Pb and Th isotopic data. The Numaura and the Araragiyama lava flows, investigated in this study, show a significant variation in TiO2 contents (1.0-1.4wt %) despite a limited variation in SiO2 content (48.5-50.0wt %); TiO2 contents correlate positively with Nd-143/Nd-144 and negatively with Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204. The compositional variations of the lavas cannot be explained by magma chamber processes, such as fractional crystallization, crustal assimilation and magma mixing, and they are suggested to have formed principally during magma generation. The variation of the TiO2 contents essentially reflects a variation of the degree of partial melting (from similar to 2 to similar to 3%) of the source mantle, and it is inferred that the melting degree correlated positively with amounts of slab-derived materials influxed into the melting region. The melting appears to have occurred progressively under isothermal and isobaric conditions, as slab-derived materials were continuously supplied. The geochemical variations in the lavas can be explained by mixing of depleted mid-ocean ridge basalt source mantle with slab-derived materials consisting of an altered oceanic crust component and a sediment component. The slab-derived materials are likely to have contained not only Sr, Ba, Pb and U, but also significant amounts of Nd and Th that are not highly soluble in aqueous fluids. The materials are thus suggested to have been supercritical liquids, and it is suggested that magma generation occurred at depths greater than that at which supercritical liquids were decomposed into aqueous fluid and silicate melt components. The lava samples show U-238-(230) Th disequilibrium with 10-20% of Th-230 excess; this Th-230 enrichment resulted primarily from the high-Th nature of the slab-derived materials.
• Rates of thermal and chemical evolution of magmas in a cooling magma chamber: a chronological and theoretical study on basaltic and andesitic lavas from Rishiri Volcano, Japan

  Takeshi Kuritani, Tetsuya Yokoyama, Eizo Nakamura

  JOURNAL OF PETROLOGY 48 (7) 1295 - 1319 0022-3530 2007/07 [Refereed][Not invited]
   

  Rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from Rishiri Volcano, northern Japan, by dating of these lavas using U-238-Th-230 radioactive disequilibrium and C-14 dating methods, in combination with theoretical analyses. We obtained the eruption age of the basaltic lavas to be 29.3 +/- 0.6 ka by C-14 dating of charcoals. The eruption age of the andesitic lavas was estimated to be 20.2 +/- 3.1 ka, utilizing a whole-rock isochron formed by U-Th fractionation as a result of degassing after lava emplacement. Because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber, the difference of the ages (i.e. similar to 9 kyr) is a timescale of magmatic evolution. The thermal and chemical evolution of the Rishiri magma chamber was modeled using mass and energy balance constraints, as well as quantitative information obtained from petrological and geochemical observations on the lavas. Using the timescale of similar to 9 kyr, the thickness of the magma chamber is estimated to have been about 1.7 km. The model calculations show that, in the early stage of the evolution, the magma cooled at a relatively high rate (>0.1 degrees C/year), and the cooling rate decreased with time. Convective heat flux from the main magma body exceeded 2W/m(2) when the magma was basaltic, and the intensity diminished exponentially with magmatic evolution. Volume flux of crustal materials to the magma chamber and rate of convective melt exchange (compositional convection) between the main magma and mush melt also decreased with time, from similar to 0.1m/year to similar to 10(-3) m/year, and from similar to 1m/year to similar to 10(-2) m/year, respectively, as the magmas evolved from basaltic to andesitic compositions. Although the mechanism of the cooling (i. e. thermal convection and/or compositional convection) of the main magma could not be constrained uniquely by the model, it is suggested that compositional convection was not effective in cooling the main magma, and the magma chamber is considered to have been cooled by thermal convection, in addition to heat conduction.
• Ancient recycled crust beneath the ontong Java plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands

  Akira Ishikawa, Takeshi Kuritani, Akio Makishima, Eizo Nakamura

  EARTH AND PLANETARY SCIENCE LETTERS 259 (1-2) 134 - 148 0012-821X 2007/07 [Refereed][Not invited]
   

  We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic gamet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-gaenet clinopyroxenite, whose major element compositions remain intact, has lower Pb-206/(204) Pb-Nd-143/(144) Nd and higher (87)sr/Sr-86-Pb-207/(204) Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf-Nd isotopic decoupling and the good coincidence of Ph isotopes to the Stacey-Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5-1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau. (c) 2007 Elsevier B.V. All rights reserved.
• Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from U-238-Th-230-Ra-226 systematics

  T Yokoyama, T Kuritani, K Kobayashi, E Nakamura

  GEOCHIMICA ET COSMOCHIMICA ACTA 70 (11) 2885 - 2901 0016-7037 2006/06 [Refereed][Not invited]
   

  In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show U-238-Th-230-Ra-226 disequilibria with excess 238 U and Ra-226, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (Th-230/Th-232) ratio compared to older Miyakejima eruptives, yielding the youngest U-238-Th-230 model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (Th-230/Th-232)-(U-238/Th-232) diagram and (2) a positive linear correlation in a (Ra-226/Th-230)(0)-1/Th-230 diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of Th-230 and Ra-226, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (Th-230/Th-232) and (Ra-226/Th-230)(0) ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (Ra-226/Th-230) ratio are excellent geochemical tracers of magmatic evolution. (c) 2006 Elsevier Inc. All rights reserved.
• Accurate isotopic and concentration analyses of small amounts of Pb using isotope dilution coupled with the double spike technique

  Takeshi Kuritani, Tomohiro Usui, Tetsuya Yokoyama, Eizo Nakamura

  GEOSTANDARDS AND GEOANALYTICAL RESEARCH 30 (3) 209 - 220 1639-4488 2006 [Refereed][Not invited]
   

  A new method has been developed for the simultaneous determination of Pb abundance and Pb isotopic composition with high precision and accuracy for small test portion masses by thermal ionisation mass spectrometry. In this method, a Pb-205-Pb-204 double spike is added to samples prior to the chemical separation of Pb, and the isotopic composition of the spike-sample mixture is determined rigorously by the double spike technique using a Pb-207-Pb-204 spike. The isotopic composition and concentration of Pb in the sample are then obtained by utilising the principle of isotope dilution. Using this technique, replicate determinations of Pb from NIST SRM 981 and GSJ JP-1 (peridotite; 0.07 mu g g(-1) Pb) were performed. The measured concentration and isotopic data were identical, within uncertainty, to published data or to data that were determined independently in this study. The application of this method to U-Pb dating and the determination of the "initial" Pb isotopic composition was also tested. Lead isotopic compositions and the concentrations of Pb, Th and U were determined for a single batch of samples, through the addition of Pb-205-Pb-204, Th-230 and U-235 spikes to samples prior to chemical separation. Also in these experiments, we confirmed that this routine gives accurate data for Pb, Th and U concentrations and Pb isotopic compositions.
• Elemental fractionation in lavas during post-eruptive degassing: Evidence from trachytic lavas, Rishiri Volcano, Japan

  T Kuritani, E Nakamura

  JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 149 (1-2) 124 - 138 0377-0273 2006/01 [Refereed][Not invited]
   

  Trachytic lavas of Rishiri Volcano, northern Japan, show a peculiar geochemical variation across lava flow units. Samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 in are nearly homogeneous in major element compositions. However, some trace elements, including Li, B and Cs, are considerably depleted in samples collected from the main part of the flow unit, compared to those obtained from the surface of the lava flow (clinker layer). In particular, Cs content of the main flow unit is as low as similar to 30% of the clinker layer. Br-11.B-10 ratios of samples from the main flow unit are also slightly lower than those of the clinker samples, and the isotope compositions positively correlate with boron concentrations. These geochemical variations cannot be explained by magmatic processes in magma chambers, post-eruptive weathering, or alteration process. Rather, we infer these systematics resulted from escape of these elements from the lava flow unit during post-eruptive degassing. Vapor phases in which Li, B and Cs dissolved ire suggested to have been transported through veins formed in the main flow unit as fractures due to slight,,hearing along the flow planes after lava emplacement. In the Tanetomi lava, only rocks of the clinker layer preserve original composition of magmas, although they are porous and brownish due to extensive oxidization. On the other hand, rocks of the main flow unit do not retain original magma compositions, although they are dense and grayish, and seem to tic much fresher compared to the clinkers, A similar geochemical modification of lavas can occur in other volcanic systems, especially for lavas consisting of relatively thick flow units. (c) 2005 Elsevier B.V. All rights reserved.
• Assimilation and fractional crystallization controlled by transport process of crustal melt: Implications from an alkali basalt-dacite suite from Rishiri Volcano, Japan

  T Kuritani, H Kitagawa, E Nakamura

  JOURNAL OF PETROLOGY 46 (7) 1421 - 1442 0022-3530 2005/07 [Refereed][Not invited]
   

  Mechanisms of fractional crystallization with simultaneous crustal assimilation (AFC) are examined for the Kutsagata and Tanetomi lavas, an alkali basalt-dacite suite erupted sequentially from Rishiri Volcano, northern Japan. The major element variations within the suite can be explained by boundary layer fractionation; that is, mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor. Systematic variations in SiO2 correlate with variations in the Pb, Sr and Nd isotopic compositions of the lavas. The geochemical variations of the lavas are explained by a constant and relatively low ratio of assimilated mass to crystallized mass ('r value). In the magma chamber in which the Kutsogata and Tanetomi magmas evolved, a strong thermal gradient was present and it is suggested that the marginal part of the reservoir was completely solidified. The assimilant was transported by crack flow from the partially fused floor crust to the partially crystallized floor mush zone through fractures in the solidified margin, formed a ny by thermal stresses resultingfrom cooling of the solidified margin and heating of the crust. The crustal melt was then mixed with the fractionated interstitial melt in the mushy zone, and the mixed melt was further transported by compositional convection to the main magma, causing its geochemical evolution to be characteristic of AFC. ne volume flux of the assimilant from the crust to the magma chamber is suggested to have decreased progressively with time (proportional to t(-1/2)), and was about 3 x 10(-2) m/year at t = 10years and I x 10(-2) m/year at t = 100years. It has been commonly considered that the heat balance between magmas and the surrounding crust controls the coupling of assimilation and fractional crystallization processes (i.e. absolute value of r). However, it is inferred from this study that the ratio of assimilated mass to crystallized mass can be controlled by the transport Process of the assimilant from the crust to magma chambers.
• Magmatic differentiation examined with a numerical model considering multicomponent thermodynamics and momentum, energy and species transport

  T Kuritani

  LITHOS 74 (3-4) 117 - 130 0024-4937 2004/06 [Refereed][Not invited]
   

  Magmatic differentiation processes in a cooling magma body were examined using a numerical model considering multicomponent thermodynamics and momentum, energy and species transport. The model accounts for melt transport induced by its density variation, resulting primarily from solid-liquid phase change. The equilibrium mineral assemblages, their mass fractions and their chemical compositions are determined by multicomponent thermodynamic models, and these parameters are linked with the calculations of velocity, pressure, temperature and species fields at each iteration and time step. For simplicity, solid phases are assumed to be stationary, and only olivine and plagioclase that are the earliest crystallization phases in common basaltic magmas are considered as fractionating phases. Application of the model to natural magmatic system shows that crystallization occurs selectively along the chilled boundaries soon after the cooling, and the magma body is separated into high-crystallinity mush zones and mostly crystal-free main magma. Because of the melt exchange between the mush zones and the main magma by compositional convection, the main magma evolves progressively in composition and the spatial chemical heterogeneity is grown with time. The melt segregated from the sidewall mush zone is accumulated below the roof mush zone, forming compositional stratification in the upper part of the main magma. On the other hand, the fractionated melt from the floor mush zone principally mixes with the overlying ambient magma in the middle and lower parts of the main magma body. This study shows that the direct treatment of nonlinear coupling among momentum, energy and species transport provides useful information of the thermal and chemical evolution of magma chambers as a function of time and space. (C) 2003 Elsevier B.V. All rights reserved.
• Highly precise and accurate isotopic analysis of small amounts of Pb using Pb-205-Pb-204 and Pb-207-Pb-204, two double spikes

  T Kuritani, E Nakamura

  JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY 18 (12) 1464 - 1470 0267-9477 2003/12 [Refereed][Not invited]
   

  A new, simple, and practical method has been developed for the accurate and precise isotopic analysis of extremely small amounts of Pb (<&SIM;3 ng) by thermal ionization mass spectrometry. Two different types of double spikes, one consisting of Pb-205 and Pb-204 and another enriched in Pb-207 and Pb-204, are used to reduce "Pb-204 error" and to correct mass fractionation during mass spectrometry. Using this technique, replicate analyses of 1.5 ng of Pb from NBS981 were performed, and an external, precision of 0.02% (2σ) was attained for the Pb-208/Pb-204 ratio. Compared with results for 1.5 ng of Pb by the normal double spike method using a single Pb-207-Pb-204 double spike, the external precision for the isotopic ratios involving Pb-204 is reduced by about 60-70%, simply by addition of the Pb-205-Pb-204 double spike. The two double spikes method was also applied to isotopic analyses of natural rock samples, and we obtained an external precision of 0.06% (2σ) for Pb-208/Ph-204 in &SIM;1.5 ng of Pb separated from fresh peridotite; in which the Pb concentration is as low as. 3.2 ppb. This new technique is superior to any previous method for precise and accurate isotopic analyses of extremely small amounts of Pb, and will be a highly powerful tool in the future of earth science and environmental science.
• Shift and rotation of composition trends by magma mixing: 1983 eruption at Miyake-jima volcano, Japan

  T Kuritani, T Yokoyama, K Kobayashi, E Nakamura

  JOURNAL OF PETROLOGY 44 (10) 1895 - 1916 0022-3530 2003/10 [Refereed][Not invited]
   

  Pre-emption processes are investigated for magmas erupted in 1983 from Miyake-jima volcano, which is one of the most active volcanoes in Japan. The whole-rock compositional trends of the eruptive products are principally smooth and linear. Magmas erupted from some fissures have compositions that deviate from the main linear trend. Phenocryst contents of samples displaced from the linear compositional trends are significantly lower than those of samples on the main trends. Anorthite-rich plagioclase phenocrysts, present throughout the 1983 products, are too calcic to have crystallized from the erupted magma composition, and were derived from a basaltic magma through magma mixing. Although the linear whole-rock composition trends favor simple two-component magma mixing.. this cannot explain the presence of samples that deviate from the main trend. Instead, the observed composition trends were formed by mixing of a homogeneous basaltic magma with andesitic magmas exhibiting compositional diversity. The original linear composition trends of the andesitic end-member magma were rotated and shifted to the direction of the basaltic end-member magma by magma mixing. The samples out of the main trends represent magmas with less basaltic component than those on the trend. The density and viscosity of the basaltic end-member magma were comparable with those of the andesitic end-member magmas. The basaltic magma, discharged from one magma chamber at similar to2 kbar pressure, was injected into a magma chamber at lower pressure occupied by the chemically zoned andesite magma (similar to1 kbar), and possibly as a fountain. To establish the characteristic mixing trend of the 1983 magma, the basaltic component must have been distributed systematically in the zoned andesite magma. A requirement is that the basaltic magma spread laterally and mixed with the andesite magma at various levels of ascent of the fountain in the host andesite magma. Analysis of compositional zoning in titanomagnetite crystals revealed that the eruption of the 1983 magmas was initiated soon after the replenishment of the basaltic magma in the 1 kbar magma chamber.
• Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U-230Th-226Ra disequilibria of Miyakejima volcano, Izu arc, Japan

  Tetsuya Yokoyama, Katsura Kobayashi, Takeshi Kuritani, Eizo Nakamura

  Journal of Geophysical Research B: Solid Earth 108 (7) 2329 - 2329 0148-0227 2003/07/10 [Refereed][Not invited]
   

  238U-230Th-226Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1-4) starting >7000 BP. 238U-230Th-226Ra disequilibria observed in lavas with large 238U and 226Ra excesses imply metasomatism of depleted mantle by fluid-related processes. This metasomatism is also suggested by trace element and Sr-Nd-Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial (230Th/232Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab-derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid-release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3 . Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several-kiloyear timescale.
• Timing and trigger of arc volcanism controlled by fluid flushing from subducting slab

  T Yokoyama, E Nakamura, K Kobayashi, T Kuritani

  PROCEEDINGS OF THE JAPAN ACADEMY SERIES B-PHYSICAL AND BIOLOGICAL SCIENCES 78 (7) 190 - 195 0386-2208 2002/09 [Refereed][Not invited]
   

  We present U-238-Th-230-Ra-226 systematics in lavas from Miyakejima volcano, Izu arc, Japan, together with major element, trace element and Sr-Nd-Pb isotopic compositions, to estimate the timescale of magmatic processes beneath an island arc. U-238-Th-230-Ra-226 disequilibria observed in Miyakejima lavas with large excesses of U-238 and Ra-226 to Th-230 imply metasomatism of depleted mantle by fluid related processes. Our data reveal that fluid components released from the slab ascend rapidly through the mantle wedge (> 50 m/yr), and that individual stages of subaerial volcanism are induced by flushing of different fluid batches to the mantle wedge with a periodicity of 1-13 kyr. This indicates that arc magmatism is controlled by pulsed release of fluid from the slab occurring on such a small timescale, which might be related to the occurrence of earthquakes in the subducting slab.
• Precise isotope analysis of nanogram-level Pb for natural rock samples without use of double spikes

  T Kuritani, E Nakamura

  CHEMICAL GEOLOGY 186 (1-2) 31 - 43 0009-2541 2002/06 [Refereed][Not invited]
   

  A simple technique has been developed for the precise analysis of lead isotope in natural rock samples by thermal ionization mass spectrometry (TIMS). Two-stage column chromatography, using 100 and 10 mul columns, was used to minimize the amounts of impurities in separated lead samples. This dramatically improved the reproducibility of mass fractionation during mass spectrometry using an ion emitter made of a mixture of silicic acid and phosphoric acid. This improvement made it possible to precisely determine the Pb isotopic compositions of very small sample sizes, employing "zero-time correction" for mass discrimination, without requiring a double-spike technique. Using the present method, analytical reproducibility of Pb-208/(204)pb of 0.02% and 0.06% (2sigma) was attained for 100 and 1 ng of Pb, respectively, separated from natural rock samples. Furthermore, we obtained a reproducibility of 0.06% (2sigma) for Pb-208/Pb-204 for 10 ng of Pb separated from GSJ JP-1 (peridotite), in which the Pb concentration was 0.09 ppm. The measured isotope compositions of USGS standard rocks AGV-1 and BCR-1 were comparable with the published values using the double-spike technique. These observations suggest that our simple technique is reliable in terms of both accuracy and precision for the determination of the Pb isotopic compositions of natural rock samples irrespective of rock chemistry and sample sizes from 1 to 100 ng of Pb. (C) 2002 Elsevier Science B.V. All rights reserved.
• Replenishment of a mafic magma in a zoned felsic magma chamber beneath Rishiri Volcano, Japan

  Takeshi Kuritani

  Bulletin of Volcanology 62 (8) 533 - 548 0258-8900 2001 [Refereed][Not invited]
   

  The trachytic Tanetomi lava from Rishiri Volcano, northern Japan, provides useful information concerning how a replenished mafic magma mixes with a compositionally zoned felsic magma in a magma chamber. The Tanetomi lava was erupted in the order of Lower lava 1 (LL1, 59.2-59.8 wt.% in SiO2), Lower lava 2 (LL2, 58.4-59.1 wt.%), and Upper lava (UL, 59.9-65.1 wt.%). Evidence for mixing with a mafic magma is observed only in the LL2, in which a greater amount of crystals derived from the mafic magma occurs in rocks with higher SiO2 content. The whole-rock compositional trend of the Tanetomi lavas is fairly smooth except for the LL2 lava composition, which scatter along the main composition trend. There is no reasonable composition of basaltic magma on the extrapolation of the LL2 composition trend, and the trend cannot be explained by a simple two-component magma mixing. Before the replenishment, the felsic magma was zoned in composition (58-65 wt.% in SiO2) and temperature (1030-920°C) in the magma chamber located at the pressure of ∼2 kbar. The compositional variation of the main felsic magma was produced by extraction of a fractionated interstitial melt from mush zones along the chamber walls and its subsequent mixing with the main magma (boundary layer fractionation). The LL1 magma tapped the magma chamber soon after the replenishment, before the mafic magma mixed with the overall felsic magma. Then the basalt magma mixed heterogeneously with the upper part of the felsic magma by forced convection as a fountain during injection. The mixing of the basalt magma with compositionally zoned felsic magma resulted in the characteristic composition trend of the LL2. The fraction of basaltic magma in the LL2 magma is estimated to be at most 10%. Despite such a small proportion, the basalt magma was mixed completely with the felsic magma, probably because the crystallinity of undercooled basalt magma was low enough to behave as a liquid. © Springer-Verlag 2001.
• Boundary layer fractionation constrained by differential information from the Kutsugata lava flow, Rishiri Volcano, Japan

  Takeshi Kuritani

  Journal of Geophysical Research B: Solid Earth 104 29401 - 29417 0148-0227 1999/12/01 [Refereed][Not invited]
   

  Detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber. Kutsugata lava, a suitable target of such study, is a Quaternary alkali basalt (51.3-53.2 wt% in SiC>2) from Rishiri Volcano, northern Japan. Despite the narrow range in the whole rock compositional variation, chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition. In the North lava (51.3-51.9 wt% in SiOa) the less differentiated portion of the Kutsugata lava, most crystals which include low-Ni olivine and plagioclase were derived from the mushy boundary layer. The main part of the magma body was principally aphyric (<0.5 vol% crystals). Estimated chemical compositions of fractionated mineral phases during differentiation of the magma coincide with the observed compositions of low-Ni olivine and plagioclase crystals. This indicates that the main magma was differentiated by separation of crystals grown in the mush zone. The low-density interstitial melt is suggested to have been extracted from the floor mush zone with average crystallinity of >30 vol% by such mechanisms as compaction and compositional convection. This fractionated melt was mixed with the overlying main magma, causing differentiation of the Kutsugata magma. The average temperature of the extracted melt is 1010°C, significantly lower than 1100°C estimated for the main magma. A quantitative model of magmatic differentiation, which includes thermal and compositional evolution of a mushy boundary layer, can successfully reproduce the observed compositional trends of the North lava. Copyright 1999 by the American Geophysical Union.
• Thermal and compositional evolution of a cooling magma chamber by boundary layer fractionation: Model and its application for primary magma estimation

  T Kuritani

  GEOPHYSICAL RESEARCH LETTERS 26 (13) 2029 - 2032 0094-8276 1999/07 [Refereed][Not invited]
   

  A boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented. The model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body. The magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer. The consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space. The model is used to evaluate a primary magma estimation from volcanic rock series. It is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation. This can cause significant errors in estimating the conditions at which the primary magma was generated.
• Phenocryst crystallization during ascent of alkali basalt magma at Rishiri Volcano, northern Japan

  T Kuritani

  JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 88 (1-2) 77 - 97 0377-0273 1999/01 [Refereed][Not invited]
   

  Phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers. A fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions, on the basis of their large sizes. However, this assumption is not always true as demonstrated here for a porphyritic alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan. All phenocryst phases in the Kutsugata lava, plagioclase, olivine, and augite, have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals. Rarely, a core region with distinct composition is present in all phenocryst phases. Phenocrysts, excluding this core, are occasionally in direct contact with each other, forming crystal aggregates. The equilibrium liquidus temperature of plagioclase, the dominant phase (similar to 35 vol%) in the Kutsugata lava, can never exceed the estimated magmatic temperature, unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent. This suggests that most phenocrysts in the Kutsugrata lava were formed by decompression of the magma during ascent in a conduit, rather than by cooling during residence in a magma reservoir. In the magma chamber before eruption, probably located at depth of more than 7 km, only cores of the phenocrysts were present and the magma was nearly aphyric (< 5 vol% crystals), though the observed rock is highly porphyritic with up to 40 vol% crystals. The Kutsugata magma is inferred to have been rich in dissolved H2O (> 4 wt.%) in the magma chamber, and liquidus temperatures of phenocryst phases were significantly suppressed. Large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution. Low ascent rate of the Kutsugata magma, which is suggested by pahoehoe lava morphology and no association of pyroclastics, gave sufficient time for crystallization. Furthermore, the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size, which characterizes the porphyritic nature of the Kutsugata lava. Alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems. (C) 1999 Elsevier Science B.V. All rights reserved.
• Boundary layer crystallization in a basaltic magma chamber: Evidence from Rishiri Volcano, Northern Japan

  T Kuritani

  JOURNAL OF PETROLOGY 39 (9) 1619 - 1640 0022-3530 1998/09 [Refereed][Not invited]
   

  The evolution of basaltic magma by mushy boundary layer processes in a magma chamber is documented for an alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan, on the basis of zoning patterns of plagioclase phenocrysts and their spatial distribution in the lava. Plagioclase phenocrysts with complex zoning patterns can be divided into four types. Type 1 is characterized by an extremely An-rich core (An(71-90)), which is commonly corroded and filled with sodic plagioclase. Type 2 has an Ab-rich core surrounded by a calcic mantle characteristic of partial dissolution. Type 3 is characterized by an Ab-rich core that is commonly surrounded by a reversely zoned slightly calcic mantle. Type 4 has an Ab-rich core without a calcic mantle. The An-rich cores of the Type 1 plagioclase are too calcic to have crystallized from a liquid represented by any whole-rock composition at the estimated temperatures of the Kutsugata magma system. The An-rich cores are inferred to have been formed in a mushy boundary layer along the wall of a magma chamber, where the magma was relatively cool and rich in water because of significant olivine fractionation and possibly because of addition of water expelled from the chilled margin and/or groundwater from the surrounding crust. The Ab-rich cores of Types 2-4 plagioclase phenocrysts are interpreted to have crystallized in the main magma body of the reservoir. The Types 2 and 3 Ab-rich cores were brought to the mushy layer by convection. The temporal and spatial variations of zoning patterns of plagioclase phenocrysts in basaltic lava flows give useful information on the evolution of mafic magma chambers.

Association Memberships

• Geochemical Society   アメリカ地球物理学連合   日本火山学会   日本岩石鉱物鉱床学会   

Research Projects

• Evolution of lithosphere-asthenosphere boundary during the 600 Myr Phanerozoic time in northeast Africa

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2023/04 -2026/03 

  Author : 小澤 一仁, 栗谷 豪, 秋澤 紀克
• U－Th放射非平衡解析から探るカルデラ噴火後の珪長質マグマ系の再活性化過程

  日本学術振興会：科学研究費助成事業

  Date (from‐to) : 2023/04 -2026/03 

  Author : 栗谷 豪
• Origin of water in the Earth inferred from hydrogen isotope ratios of volcanic glass and melt inclusions

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2021/04 -2024/03 

  Author : 清水 健二, 羽生 毅, 栗谷 豪
• The lithosphere-asthenosphere boundary zone at steady state

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research

  Date (from‐to) : 2020/04 -2023/03 

  Author : 小澤 一仁, 秋澤 紀克, 栗谷 豪, 柵山 徹也
• 巨大カルデラ形成噴火におけるマグマ蓄積プロセスの特異性の解明

  日本学術振興会：科学研究費助成事業 基盤研究(B)

  Date (from‐to) : 2020/04 -2023/03 

  Author : 栗谷 豪

   

  本研究は、国内の代表的なカルデラ火山を対象に U－Th 放射非平衡を軸とした物質科学的解析を行い、マグマが多量に蓄積されるに至った特異性の要因を実証的に明らかにすることを目的とする。初年度は、１）洞爺・屈斜路・肘折・阿蘇の４か所を対象とした試料採取および基礎的な化学分析、２）U-Th放射非平衡測定法の確立、を行う予定であった。１）については、コロナ禍のために洞爺・阿蘇についての試料採取が行えなかったことから、研究室内に保管されていた屈斜路・支笏の代表的試料を対象に全岩化学組成（主に微量元素濃度・Sr-Nd-Pb同位体比）の測定を行い、珪長質マグマの生成プロセスについての検討を行った。２）については、マルチコレクタ型質量分析計へのRPQフィルターの装着（230Thへの232Thのテーリング効果の低減）、脱溶媒システムの使用、複数の標準試料（アメリカ地質調査所のBHVO-2とW-2）を使用したデータ補正の試み、などを行った。その結果、230Th/232Th比について0.5%程度の繰り返し再現性での測定が可能になり、またモニター試料として測定したBCR-2の同位体比についても推奨値と一致していることが確認された。確立したU－Th放射非平衡分析法の有効性を確認するため、白頭山10世紀噴火の噴出物について、分析・解析を行った。その結果、主要な珪長質マグマは2万年以上前から準備されていたことなどを明らかにするに至り、確立した分析法とともに、Lithos誌に掲載されるに至った。
• Long-term eruption potential assessment for active volcanoes in eastern Japan

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

  Date (from‐to) : 2016/04 -2019/03 

  Author : Kuritani Takeshi

   

  In this study, petrological analyses on volcanic rocks from representative active volcanoes and seismological analyses on the crust and mantle were conducted for the Northeast Japan arc and Kurile arc to estimate magma production rates at the representative volcanoes. We have accumulated information about the temperatures and the fluid fractions of the mantle at the depths of the magma generation for many volcanoes, and also obtained high-resolution tomographic images in the NE Japan and Kurile arcs. By combining the results of the petrological and geophysical analyses, we estimated the paths for slab-derived fluids for some volcanoes. However, we cannot complete the estimations of the magma production rates for the representative volcanoes in the three years of the research period, because of the delay of the research.
• Research on diversity and preparing processes of caldera-forming silicic eruptions

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

  Date (from‐to) : 2015/04 -2019/03 

  Author : Nakagawa Mitsuhiro

   

  The formation and eruption processes of the silicic magma plumbing system of a caldera volcano was investigated, with a special attention to the time scale of magma processes. We focused on caldera-forming eruptions of Kutcharo, Shikotsu and Kikai volcanoes. Petrological analysis revealed that the voluminous silicic magma in each eruption was the mixing product of two silicic magma, and that the silicic products had been developed at least since several hundred years before each eruption. Then, we also clarified that each caldera-forming eruption was triggered by the injection of mafic magmas into the mixed silicic magma occurring just before the eruption. In addition, we point out that each caldera-forming eruption was complex, and can be divided into several eruption phases by the presence of dormant periods.
• The rate of dehydration of the stagnant slab in the mantle transition zone

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research

  Date (from‐to) : 2016/04 -2018/03 

  Author : Kuritani Takeshi

   

  Geophysical observation has suggested that the mantle transition zone beneath NE China is hydrous, and the subducted Pacific slab is stagnated in the transition zone. To estimate the rate of the dehydration of the stagnant slab, petrological and geochemical analyses were conducted on volcanic rocks of variable ages (60-0 Ma) above the stagnant slab. Unfortunately, the rate of the dehydration could not have been estimated because of no significant temporal variation in the water contents of the source mantle. However, this study showed that the mantle transition zone have been hydrous between 60 and 0 Ma. In addition, this study proposed a general model about the origin of the intraplate magmatism over the stagnant slab.
• Initiation and eruption style of silicic volcanism: Roles of outgassing and rheology of crystal- and bubble-bearing magmas

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

  Date (from‐to) : 2015/04 -2018/03 

  Author : Okumura Satoshi

   

  Crystal-rich magmas with high viscosity often cause explosive volcanism. To ascend rapidly to the surface, the magma must be lubricated during the eruption. We performed decompression experiments for bubble- and crystal-bearing magmas to simulate magma ascent process. We also measured the viscosity of magmas with gas bubbles and crystals. Based on these experiments, we found that crystal-rich magma can lubricate due to the breakage of crystal network with gas bubble formation under high decompression rates. In contrast, gas bubbles escaped under low decompression rates. Thus, we infer that explosive volcanism by crystal-rich magmas can cause when high decompression rate is achieved by some mechanisms such as large overpressure in a magma reservoir.
• Spars-modeling view of natural history of the solid bodies in the solar system: Exploration strategy of small bodies after the Hayabusa mission

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)

  Date (from‐to) : 2013/06 -2018/03 

  Author : Miyamoto Hideaki

   

  Asteroids are classified based on their reflectance spectra, which are compared with those of meteorites, which are known to be mostly originated from asteroids. However, some types of asteroids do not really match with meteorites, and their direct comparisons are generally difficult without professional skills. We applied the concept of the sparse modelling to connect asteroids with meteorites to search for the optimal integration scheme for two different databases without relying on preliminary knowledge. We developed large databases of asteroids and meteorites for easy application of sparse modelling. Through our analyses including principal component analysis, Bayesian spectral deconvolution and dimensionality reduction, we found that our data-driven approach can extract potential information consistent with previous studies. Our methods show a new type of data handling scheme for asteroid and meteorite data, potentially having a significant contribution for future missions.
• Water content of the mantle transition zone constrained by volcanic rocks

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

  Date (from‐to) : 2013/04 -2016/03 

  Author : Kuritani Takeshi, YOKOYAMA TETSUYA, NAKAMURA MICHIHIKO

   

  Geophysical observation has suggested that the mantle transition zone beneath NE China is hydrous, and intraplate magmatism of this region has been caused by a transition zone-derived mantle plume. In this study, petrological and geochemical analyses were carried out on basaltic rocks in NE China to estimate the water content of the mantle transition zone. It is suggested that the water content of the source mantle is up to 350 ppm, which is significantly higher than that of the normal upper mantle (~120 ppm). As a preliminary result, the water content of the mantle transition zone of ~0.7 wt.% is obtained.
• Chemical differentiation of a stagnant slab

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)

  Date (from‐to) : 2011 -2012 

  Author : KURITANI Takeshi

   

  To understand geochemical processes associated with slab stagnation in the mantle transition zone, petrological and geochemical analyses were carried out on Quaternary volcanic rocks in NE China. It is suggested that the mantle transition zone has been me
• A study on the mechanism of shallow-level degassing of conduit magmas as a controlling factor of eruption styles

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)

  Date (from‐to) : 2009 -2012 

  Author : NAKAMURA Michihiko, KURITANI Takesi, YASUI Maya, OKUMURA Satoshi, YOSHIA Takeyoshi

   

  We examined the crystal size distribution (CSD) of ground crystals in the erupted materials of the 2011 Shinmoe-dake volcano. We found that magma residence time in the shallow conduit in the explosive subplinian phase is shorter than in the less-explosive vulcanian and lava dome phases. We also conducted textural relaxation experiments of andesitic pumices in shallow conduit conditions and clarified the vapor pressure dependence of the relaxation and bubble coagulation. We conclude that the textural relaxation enhances formation of “gas pockets" via compaction of shallow melt foam.
• Quantitative understanding of thermal evolution of magma chambers

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)

  Date (from‐to) : 2008 -2009 

  Author : KURITANI Takeshi

   

  This study has been carried out to understand the mechanism of the thermal and chemical evolution of magma chambers. Thus, the following conclusions were reached : 1)Chemical differentiation trend of magmas can be affected by the amounts of bubbles. 2)In basaltic magma chamber, boundary layer fractionation can be important relative to homogeneous fractionation when magmas are hydrous and the magma chambers are located at depth. 3)It took ~3000 yr for basaltic magma to evolve to andesite at Askja volcano, Iceland.
• Joint research among Japan, Korea and China on the active history and magma of Baitoushan Volcano

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)

  Date (from‐to) : 2005 -2008 

  Author : TANIGUCHI Hiromitsu, KURITANI Takeshi, MIYAMOTO Tsuyoshi, NAGASE Toshirou, KANNO Hitoshi, GOTO Akio, NAKAGAWA Mitsuhiro, OKUNO Mituru, BAN Masao, MAENO Fukashi, SHIMANO Taketo, ITAYA Tetsumaru, YASUDA Yoshinori, UEKI Sadato, FURUHATA Toru, KOJIMA Yoshitaka

   

  頭山およびそれを包括する蓋馬溶岩台地に関して、現地調査、衛星データー解析、採集した資料の化学分析・年代分析、国内の関連地層の調査・年代分析などの手法を用いて、白頭山10世紀巨大噴火の概要、白頭山及び蓋馬溶岩台地の火山学的な実態を明らかにしようとした。開始してから1年後に北朝鮮のミサイル問題・核開発問題などの諸問題が発生し、現地での調査や研究者との交流などの実施が徐々に困難になっていった。そのため、すでに収集していた試料の分析、衛星データーの解析及び国内での調査に研究の主力を移し、可能な限りの成果を得ようとした。その結果、近年発生している白頭山における地震多発とマグマ活動との関係、存在は知られているが分布や内容が全く未知である蓋馬溶岩台地の概要が明らかになり、更に、地下におけるマグマの成因についても一定の結論を得た。混乱状態にある白頭山10世紀噴火の年代問題をふくめ、また、北朝鮮からの論文を含め、研究成果は12編の論文として論文集にまとめられつつある。
• マグマ分化プロセスの時間スケールの定量的解明

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)

  Date (from‐to) : 2005 -2007 

  Author : 栗谷 豪

   

  本研究の主目的は、地殻下のマグマ溜り内において、どのような時間スケールでマグマの熱・物質進化が進行するのかについて、定量的な理解を得ることである。その目的達成のため、昨年度まで利尻火山下マグマ溜まりにおける、玄武岩質マグマから安山岩質マグマへの進化の時間スケールを推定し、さらにモデル計算に基づいてマグマの熱進化を時間の関数として表現した。そこで今年度はまず、熱進化の理解の一般性を図るべく、モデル計算の結果の解析を行った。マグマ溜まり内において、熱対流の駆動力は、天井部の固液境界層と主要部マグマ間の熱境界層の温度差であり、その温度差は結晶の過冷却度に対応していると考えられているが、モデル計算で得られた過冷却度と固液境界層の成長速度の関係が、実験で得られているカンラン石の過冷却度と結晶成長速度の関係に比較的近いことが明らかになった。このことから、将来的には結晶の過冷却度と成長速度の関係から、マグマの熱進化をある程度予測できる可能性が示された。本年度はさらに、マグマ溜まり内における、主要部での結晶分化作用(均質分化)と固液境界層由来の分化液が主要部マグマに混入することによって引き起こされる分化作用(境界層分化)との結合作用について考察を行った。その結果、含水量が高いマグマが地下の比較的深部でマグマ溜まりを形成した場合、固液境界層から主要部マグマへのメルトの輸送(境界層分化)が活発になり、さらに主要部マグマのリキダス温度降下を引き起こして結晶化を抑えることによって均質分化を妨げ、その結果、境界層分化がさらに支配的になることが予測された。その一方で、地下の比較的浅部のマグマ溜まり内では、固液境界層から主要部マグマへのメルトの輸送が起こりにくいため、マグマは主に均質分化作用で進化することが明らかになった。
• マグマ分化の時間スケールの定量的解明

  科学研究費補助金

  Date (from‐to) : 2005 -2005
• 地殻におけるマグマの熱物質進化モデルの構築

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for JSPS Fellows

  Date (from‐to) : 2000 -2002 

  Author : 栗谷 豪

   

  本年度の当初の研究計画は、マグマ溜まり内における温度・組成構造の進化モデルの仕上げであった。マグマ溜まりの熱物質進化について、結晶の運動を無視するという条件付ながら、多成分系熱力学及び、物質・運動量・エネルギー保存を考慮した2次元モデルの構築を、今年度の前半に終えることができた。その後結晶の運動について、モデルへの組み込みに取り組んできたが、結晶の核密度についての予測が必要であったり、固相が支配的である領域での固相-液相の相対運動を支配するパラメータが未知であったりするといった壁に阻まれ、今年度中の完成には至らなかった。上記の研究と同時進行で、表面電離型質量分析計における鉛同位体比の測定法の開発及び改良を行った。測定時のマス分別効果の補正についてDouble spike法を導入した結果、従来の世界のあらゆる測定法よりも高確度・高精度で鉛同位体比が測定できるようになった。その成果については現在国際誌に投稿中である。また本年度は、昨年度に採取したアイスランドの岩石の解析も行った。解析の結果、アスキヤ火山の玄武岩は大規模なマグマ混合及び地殻物質の混染の影響を受けており、マグマの分化プロセスの情報を引き出すという当初の目的にそぐわないことが判明した。しかし鉛同位体比等のデータから、マグマ溜まり周囲の地殻物質が約2Maに形成されたものであること、またアスキヤ火山の玄武岩マグマはMORBのマグマと地下深部由来のマントルプリューム起源のマグマが噴火直前に混合していた可能性が示された。
• 地殻におけるマグマの進化過程の定量的解明

  Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for JSPS Fellows

  Date (from‐to) : 1998 -1999 

  Author : 栗谷 豪