研究者総覧

西 真之 (ニシ マサユキ)

  • 地球深部ダイナミクス研究センター 助教
Last Updated :2020/09/16

研究者情報

学位

  • 博士(2010年03月 九州大学)

論文上での記載著者名

  • MASAYUKI NISHI

ホームページURL

J-Global ID

研究キーワード

  • 超高圧実験   放射光X線   含水鉱物   中心核   地球物理学   水循環   マントル   地球惑星進化   

研究分野

  • 自然科学一般 / 固体地球科学

経歴

  • 2020年04月 - 現在  愛媛大学地球深部ダイナミクス研究センター准教授
  • 2013年10月 - 現在  東京工業大学地球生命研究所協力研究者
  • 2017年09月 - 2020年03月  愛媛大学地球深部ダイナミクス研究センター講師
  • 2014年09月 - 2017年08月  愛媛大学地球深部ダイナミクス研究センター助教
  • 2013年10月 - 2014年08月  愛媛大学地球深部ダイナミクス研究センターWPI研究員
  • 2010年03月 - 2013年09月  愛媛大学地球深部ダイナミクス研究センター日本学術振興会特別研究員

学歴

  • 2007年04月 - 2010年03月   九州大学   大学院理学府   地球惑星科学専攻 博士課程
  • 2005年04月 - 2007年03月   九州大学   大学院理学府   地球惑星科学専攻 修士課程
  • 2001年04月 - 2005年03月   九州大学   理学部   地球惑星科学科

所属学協会

  • 応用物理学会   American Geophysical Union   日本高圧力学会   日本鉱物科学会   日本地球惑星科学連合   

研究活動情報

論文

  • Masayuki Nishi, Yasuhiro Kuwayama, Jun Tsuchiya
    Icarus 338 113539  Elsevier {BV} 2020年03月 [査読有り]
  • Chaowen Xu, Masayuki Nishi, Toru Inoue
    American Mineralogist 104 1416 - 1420 2019年10月 
    © 2019 Walter de Gruyter GmbH, Berlin/Boston. Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth's surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ϵ-FeOOH with CaCl2-type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH-FeOOH binary system at 15-25 GPa and 700-1200 °C. X-ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ϵ-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl2-type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.
  • M. Nishi, J. Tsuchiya, Y. Kuwayama, T. Arimoto, Y. Tange, Y. Higo, T. Hatakeyama, T. Irifune
    Journal of Geophysical Research: Solid Earth 124 10 10231 - 10239 2019年10月 [査読有り]
     研究論文(学術雑誌)
  • Takeshi Arimoto, Tetsuo Irifune, Masayuki Nishi, Yoshinori Tange, Takehiro Kunimoto, Zhaodong Liu
    Physics of the Earth and Planetary Interiors 295 2019年10月 研究論文(学術雑誌) 
    © 2019 Elsevier B.V. We have developed techniques to perform precise determinations of phase transition boundaries under the pressure and temperatures corresponding to those of the deep lower mantle, using Kawai-type multianvil apparatus (KMA) with sintered diamond anvils. By optimizing the materials and dimensions of the high-pressure furnace assembly, we achieved stable generation of high temperature to 2300 K, at pressures up to ~64 GPa, which is the highest temperature reported in KMA under such high pressure conditions. Phase relations in the MgSiO3-FeSiO3 system have been studied at pressures from 27 to 64 GPa and temperatures of 1800–2300 K by in situ X-ray diffraction measurements, combined with quench experiments, using synthetic pyroxene samples with chemical compositions of FeSiO3 and (Mg0.4Fe0.6)SiO3 as the starting materials. Our results show that the iron content in MgSiO3 bridgmanite increases almost linearly up to 42 mol% of the FeSiO3 component with increasing pressure in this pressure and temperature range. For higher iron proportions, an assemblage of bridgmanite + magnesiowüstite + stishovite is formed, while the oxide mixture of magnesiowüstite and stishovite is stable for the compositions close to FeSiO3. Our results suggest that bridgimanite with the pure FeSiO3 composition should only be formed at pressures significantly higher than 64 GPa, contradictory to some recent studies using laser-heated diamond anvil cell. It is also suggested that magnesiowüstite inclusions with various iron contents found in some natural diamonds may reflect the formation of magnesiowüstite in former oceanic crust material subducted into the uppermost lower mantle.
  • Nishi Masayuki, Greaux Steeve, Tateno Shigehiko, Kuwayama Yasuhiro, Kawai Kenji, Trifune Tetsuo, Maruyama Shigenori
    GEOSCIENCE FRONTIERS 9 6 1859 - 1870 2018年11月 [査読有り]
  • Nishi, M., Tsuchiya, J., Arimoto, T., Kakizawa, S., Kunimoto, T., Tange, Y., Higo, Y., Irifune, T.
    Physics and Chemistry of Minerals 1 - 7 2018年 研究論文(学術雑誌) 
    © 2018 Springer-Verlag GmbH Germany, part of Springer Nature Phase H (MgSiO4H2) is the high-pressure form of dense hydrous silicate that could deliver surface water into the lower mantle. In this study, we determined the thermal equations of the state of phase H using in situ X-ray diffraction measurements, under conditions ranging from 34 to 62 GPa and 300 and 1300 K, using a multianvil apparatus. Analysis of the data, based on the Mie–Grüneisen–Debye model using third-order Burch–Murnaghan equations at a reference pressure of 35 GPa, yielded the following results Vref = 49.61 ± 0.01 Å3, Kref = 344.6±4.1 GPa, (Formula presented.) = 3.05 ± 0.32, θref = 974 ± 146 K, γref = 1.8 ± 0.1, and q = 1.79 ± 0.55. The compressibility of phase H observed in this study agrees well with that derived from theoretical calculations in pressure regions where hydrogen bond symmetrization is predicted. It was also found that the volume and compressibility of phase H and δ-AlOOH were similar.
  • Steeve Gréaux, Masayuki Nishi, Shigehiko Tateno, Yasuhiro Kuwayama, Naohisa Hirao, Kenji Kawai, Shigenori Maruyama, Tetsuo Irifune
    Physics of the Earth and Planetary Interiors 274 184 - 194 2018年01月 [査読有り]
     研究論文(学術雑誌) 
    The phase relations, mineral chemistry and density of KREEP basalt were investigated at pressures of 12–125 GPa and temperatures up to 2810 K by a combination of large volume multi-anvil press experiments and in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. Our results showed that grossular-rich majorite garnet, liebermannite and Al-bearing stishovite are dominant in the upper-to-middle part of the upper mantle while in the lowermost transition zone a dense Ti-rich CaSiO3 perovskite exsoluted from the garnet, which becomes more pyropic with increasing pressure. At lower mantle conditions, these minerals transform into an assemblage of bridgmanite, Ca-perovskite, Al-stishovite, the new aluminium-rich (NAL) phase and the calcium-ferrite type (CF) phase. At pressures higher than 50 GPa, NAL phase completely dissolved into the CF phase, which becomes the main deposit of alkali metals in the lower mantle. The density of KREEP estimated from phase compositions obtained by energy dispersive X-ray spectroscopy (EDS) in scanning (SEM) and transmission (TEM) electron microscopes, was found substantially denser than pyrolite suggesting that the Earth primordial crust likely subducted deep into the Earth's mantle after or slightly before the final solidification of magma ocean at 4.53 Ga. Radiogenic elements U, Th and 40K which were abundant in the final residue of magma ocean were brought down along the subduction of the primordial crust and generate heat by decay after the settlement of the primordial crust on top of the CMB, suggesting the non-homogeneous distribution of radiogenic elements in the Hadean mantle with implications for the thermal history of the Earth.
  • Zhaodong Liu, Masayuki Nishi, Takayuki Ishii, Hongzhan Fei, Nobuyoshi Miyajima, Tiziana Boffa Ballaran, Hiroaki Ohfuji, Takeshi Sakai, Lin Wang, Svyatoslav Shcheka, Takeshi Arimoto, Yoshinori Tange, Yuji Higo, Tetsuo Irifune, Tomoo Katsura
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 122 10 7775 - 7788 2017年10月 研究論文(学術雑誌) 
    Phase relations in the system MgSiO3-Al2O3 were investigated at pressures of 27-45GPa and temperatures of 1700, 2000, and 2300K using sintered diamond and tungsten carbide anvils in a multianvil apparatus. The bulk compositions in the MgSiO3-Al2O3 binary system crystallize a phase assemblage of pyrope and corundum at pressures below 27GPa and an assemblage of bridgmanite and corundum at pressures above 27GPa regardless of temperatures. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increases significantly with increasing temperature. The solubility of Al2O3 in bridgmanite increases from 6.7mol% at 1700K to 21.8mol% at 2500K under a constant pressure of 27GPa. Bridgmanite becomes more aluminous with increasing pressure from 27 to 45GPa at a given temperature. The MgSiO3 content in corundum increases with increasing pressure at pressure lower than 27GPa, while it decreases at pressure higher than 27GPa. Our results suggest that bridgmanite can incorporate a considerably higher Al2O3 content than that of the pyrope composition (25mol% Al2O3). The present study further suggests that the entire Al2O3 component is accommodated into bridgmanite in the pyrolite lower mantle. However, Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs in the shallow part of the lower mantle, and therefore, additional phases such as MgAl2O4 with calcium ferrite-type structure are necessary to host the excess Al2O3. Plain Language Summary Here we determined the phase relations in the system MgSiO3-Al2O3 up to 2300K under lower mantle pressures and found that the solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum increase with increasing temperature. All Al2O3 can be completely accommodated into bridgmanite in the pyrolite composition, while the Al2O3 cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs, and therefore, additional phases are required to host the excess Al2O3.
  • T. Kimura, H. Ohfuji, M. Nishi, T. Irifune
    NATURE COMMUNICATIONS 8 2017年06月 [査読有り]
     研究論文(学術雑誌) 
    Periclase (MgO) is the second most abundant mineral after bridgmanite in the Earth's lower mantle, and its melting behaviour under pressure is important to constrain rheological properties and melting behaviours of the lower mantle materials. Significant discrepancies exist between the melting temperatures of MgO determined by laser-heated diamond anvil cell (LHDAC) and those based on dynamic compressions and theoretical predictions. Here we show the melting temperatures in earlier LHDAC experiments are underestimated due to misjudgment of melting, based on micro-texture observations of the quenched samples. The high melting temperatures of MgO suggest that the subducted cold slabs should have higher viscosities than previously thought, suggesting that the inter-connecting textural feature of MgO would not play important roles for the slab stagnation in the lower mantle. The present results also predict that the ultra-deep magmas produced in the lower mantle are peridotitic, which are stabilized near the core-mantle boundary.
  • 有本 岳史, 入舩 徹男, 西 真之, 大藤 弘明, 國本 健広, 丹下 慶範
    日本鉱物科学会年会講演要旨集 2017 0 112 - 112 一般社団法人日本鉱物科学会 2017年 [査読有り]
     
    MgSiO3-FeSiO3系の相関係を(Mg0.4Fe0.6)SiO3組成の出発物質を用いて、64 GPa、2300 Kまで実験を行い決定した。その
    温度・圧力条件下においてブリッジマナイト、マグネシオウスタイト、スティショバイトの生成を観察した。ブリッジマナイト中のFeSiO3成分は温度・圧力の増加とともに増加し、60 GPa、2000 Kにおいて最大38 mol%も固溶することが判明した。
  • Nishi, M., Kuwayama, Y., Tsuchiya, J., Tsuchiya, T.
    Nature 547 7662 205 - 208 2017年 [査読有り]
     研究論文(学術雑誌)
  • Zhaodong Liu, Tetsuo Irifune, Masayuki Nishi, Yoshinori Tange, Takeshi Arimoto, Toru Shinmei
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 257 18 - 27 2016年08月 [査読有り]
     研究論文(学術雑誌) 
    Phase relations in the system MgSiO3-Al2O3 have been determined for pressures from 15 to 52 GPa and at a temperature of 2000 K from in situ synchrotron X-ray diffraction measurements obtained using sintered diamond anvils in a multi-anvil apparatus. A wide two-phase region of garnet and corundum is found to exist up to 27 GPa; at higher pressures, a phase assemblage of aluminous bridgmanite and corundum is stabilized. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum are highly dependent on pressure; the Al2O3 content in bridgmanite increases from 12 mol% at 27 GPa to 29 mol% at 52 GPa, forming bridgmanite with the pyrope composition (25 mol% Al2O3) at about 45 GPa. In contrast, the MgSiO3 content in corundum decreases from 24 mol% at 27 GPa to 9 mol% at 52 GPa. The minimum pressure where the pyropic bridgmanite is formed is significantly higher than the result of the earlier study (similar to 37 GPa) on the pyrope composition. These results suggest the Al2O3 content in bridgmanite and the MgSiO3 content in corundum are good pressure indicators in the mixed-phase region at pressures above 30 GPa, where virtually no pressure references are available for quench experiments. The new phase diagram in the system MgSiO3-Al2O3 confirms that bridgmanite is the dominant host mineral for Al2O3 in the pyrolitic or peridotitic compositions under the pressure and temperature conditions of the Earth's lower mantle. (C) 2016 Elsevier B.V. All rights reserved.
  • Yu Nishihara, Genta Maruyama, Masayuki Nishi
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 257 220 - 229 2016年08月 [査読有り]
     研究論文(学術雑誌) 
    Growth kinetics of forsterite (Fo) reaction rims between periclase (Per) and enstatite (En) were studied experimentally at pressure (P) and temperature (T) conditions of 3.0-11.1 GPa and 1473-1873 K, respectively. Pt markers originally placed at the Per-En interface were always observed at the Per-Fo interface, which indicates that Mg and O are the diffusing species in Fo rim growth (Mg-O coupled diffusion). The presence of some En inclusions in Fo grains and the growth rate of the Fo rim suggests that grain boundary diffusion is dominant rather than lattice diffusion. Considering the very fast grain boundary diffusion of O in olivine, the Mg-O coupled grain boundary diffusion in Fo is deduced to be rate-limited by the diffusivity of Mg. Based on an analysis of data collected under dry conditions, the product of the Mg grain boundary diffusion coefficient (D-gb) and the effective grain boundary width (5) was determined to be delta D-gb = delta D(gb,0)exp[-(E* + PV*)/RT] with delta D-gb,D-0 = 10(-9.68 +/- 1.51) m(3)/s, E* = 379 +/- 44 kJ/mol and V* = -1.9 +/- 1.4 cm(3)/mol. Our results, combined with previously reported data on Mg lattice diffusion in Fo, suggest that for Mg, the significance of grain boundary diffusion increases with depth in the Earth's upper mantle, although lattice diffusion is still dominant for typical mantle grain sizes of 1-10 mm. (C) 2016 Elsevier B.V. All rights reserved.
  • M. Nishi, T. Irifune, S. Greaux, Y. Tange, Y. Higo
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 245 52 - 58 2015年08月 [査読有り]
     研究論文(学術雑誌) 
    Phase transformations in natural serpentine and chlorite have been studied at 25-50 GPa and 900-1280 degrees C by in-situ X-ray diffraction measurements using the multianvil apparatus with sintered diamond anvils. Antigorite was found to transform to a phase assemblage including phase H, at pressures above about 35-40 GPa corresponding to those in the upper part of the lower mantle. The zero pressure bulk modulus K-0 of phase H was determined to be 160 GPa, assuming its pressure derivative K' = 4. The thermal stability of phase H is significantly enhanced by a solid solution with a delta-AlOOH component, suggesting that aluminous phase H in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle. (C) 2015 Elsevier B.V. All rights reserved.
  • Luca Bindi, Masayuki Nishi, Tetsuo Irifune
    AMERICAN MINERALOGIST 100 7 1637 - 1640 2015年07月 [査読有り]
     研究論文(学術雑誌) 
    The crystal structure of the two dense hydrous magnesium silicates Phase D, MgSi2H2O6, and Phase H, MgSiH2O4, synthesized at 45 GPa and 1000 degrees C and coexisting in the same micrometer-sized grain, was investigated by single-crystal X-ray diffraction to study the preferential partition of A I between the two structures. In agreement with the literature, Phase D was found to be trigonal, space group P (3) over bar 1m, with lattice parameters a = 4.752(2), c = 4.314(2) angstrom, V = 84.37(6) angstrom(3) (R-1 = 0.020), and Phase H was found to be orthorhombic, space group Pnnm, with lattice parameters a = 4.730(2), b = 4.324(2), c = 2.843(2) angstrom, V = 58.15(5) angstrom(3) (R-1 = 0.024). The estimated proportion (vol%) of the two phases from the refinement is 27(2)(PHD) - 73(PhH). The analysis of the geometric details of the two structures shows that Phase D hosts almost all the Al available, whereas Phase H is nearly identical to pure MgSiH2O4. Overexposed electron-microprobe X-ray maps of the same grain used for the X-ray diffraction study together with WDS spots on the two phases confirmed the structural results. Thus, our results suggest that when Phase D and Phase H coexist, Al is strongly partitioned into Phase D at the expense of coexisting Phase H. At pressure above similar to 50 GPa, where Phase D is no longer stable, Phase H is able to incorporate the high aluminum contents present in hydrous peridotitic compositions in the deep lower mantle and be stabilized at the expense of Phase D and magnesium silicate perovskite.
  • Tomohiro Ohuchi, Yu Nishihara, Yusuke Seto, Takaaki Kawazoe, Masayuki Nishi, Genta Maruyama, Mika Hashimoto, Yuji Higo, Ken-ichi Funakoshi, Akio Suzuki, Takumi Kikegawa, Tetsuo Irifune
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 243 1 - 21 2015年06月 [査読有り]
     研究論文(学術雑誌) 
    Simple-shear deformation experiments on polycrystalline olivine and olivine single-crystal were conducted at pressures of 1.3-3.8 GPa and temperatures of 1223-1573 K to understand the achievement of steady-state fabric strength and the process of dynamic recrystallization. Development of crystallographic preferred orientation (CPO) of olivine was evaluated from two-dimensional X-ray diffraction patterns, and shear strain was measured from X-ray radiographs. The steady-state fabric strength of the A-type fabric was achieved within total shear strain of gamma = 2. At strains higher than gamma = 1, an increase in concentration of the [010] axes mainly contributes to an increase in fabric strength. At strains higher than gamma = 2, the magnitude of V-SH/V-SV (i.e., ratio of horizontally and vertically polarized shear wave velocities) scarcely increased in most of the runs. The V-SH/V-SV of peridotite (70 vol.% olivine + 30 vol.% minor phases) having the steady-state A-type olivine fabric coincides with that of recent global one-dimensional models under the assumption of horizontal flow, suggesting that the seismic anisotropy observed in the shallow upper mantle is mostly explained by the development of A-type olivine fabric. Experimental results on the deformation of single-crystal olivine showed that the CPO of olivine is influenced by the initial orientation of the starting single crystal because strain is concentrated in the recrystallized areas and the relic of the starting single crystal remains. In the upper mantle, the old CPO of olivine developed in the past may affect the olivine CPO developed in the present. (C) 2015 Elsevier B.V. All rights reserved.
  • 西 真之
    高圧力の科学と技術 25 2 172 - 180 2015年 [査読有り][招待有り]
     研究論文(学術雑誌) 
    © 2015 Japan Society of High Pressure Science and Technology. All rights reserved. Atomic diffusion rates and transformation mechanims of mantle minelars provide important constraints for understanding many physical and chemical processes in the Earth's interior, including mantle rheology and chemical transportation. Here I introduce two recent experimental studies on the atomic diŠusivity and transformation mechanisms of mantle minerals under lower mantle conditions up to 50 GPa using a Kawai-type high-pressure apparatus combined with sintered diamond anvils. DiŠusion-controlled growth kinetics of the polycrystalline MgSiO3 bridgmanite enabled us to estimate the grain boundary diŠusivity of bridgmanite in the lower mantle. Also, metastability of the deep subducted plate was disscussed based on the mechanism changes through the post-garnet transformation under large over pressure conditions.
  • 西 真之
    高圧力の科学と技術 24 2 136 - 144 2014年 [査読有り][招待有り]
     研究論文(学術雑誌) 
    Untransformed metastable minerals exist under the low-temperature conditions of the subducting plates. Here I report the experimental studies on the transformation kinetics of pyroxene and garnet at high-pressure and hightemperature conditions. Slow kinetics of the pyroxene-garnet transformation due to the low atomic diffusion rate indicates that large low-density metastable regions would exist in the slab, greatly contributing to the slab stagnation around the mantle transition region.
  • Nishi, M., Irifune, T., Tsuchiya, J., Tange, Y., Nishihara, Y., Fujino, K., Higo, Y.
    Nature Geoscience 7 3 224 - 227 2014年 [査読有り]
     研究論文(学術雑誌)
  • Bindi, L., Nishi, M., Tsuchiya, J., Irifune, T.
    American Mineralogist 99 8-9 1802 - 1805 2014年 [査読有り]
     研究論文(学術雑誌)
  • Akira Shimojuku, Tomoaki Kubo, Takumi Kato, Takashi Yoshino, Masayuki Nishi, Tomoki Nakamura, Ryuji Okazaki, Yuki Kakazu
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 226 28 - 38 2014年01月 [査読有り]
     研究論文(学術雑誌) 
    We have determined the pressure and temperature dependencies of Si volume diffusion rates in a single crystal of Pyp(75)Alm(15)Gr(10) garnet at 6-25 GPa and 1400-1800 degrees C by the Si-29 tracer diffusion method. High-pressure experiments were conducted using the Kawai-type multi-anvil high-pressure apparatus. Diffusion profiles were obtained using secondary ion mass spectrometry in the depth-profiling mode. The Si diffusion coefficient in garnet (D-gt) is expressed by the Arrhenius equation: D-gt = D-0 exp(-(E* +- PV*)/RT), with log(10)D(0) = -7.1 +/- 1.7 m(2)/s, E* = 345 +/- 56 kJ/mol, and V* = 4.7 +/- 0.5 cm(3)/mol. Si diffusion seems to be the slowest in the major constituent elements in a pyrope-rich garnet and control rates of plastic deformation. The comparison between Si diffusion rates in garnet and wadsleyite/ringwoodite suggests that garnet has similar or slightly lower strength (at most similar to 2 times) compared with wadsleyite, and has similar or slightly higher strength (at most similar to 3 times) compared with ringwoodite at temperatures ranging from 900 to 1300 degrees C. Thus, strength contrast between subducted oceanic crust and underlying peridotite layer may be small at the mantle transition zone conditions. This result suggests that the separation of subducted oceanic crust from an underlying peridotite layer may not occur. (C) 2013 Elsevier B.V. All rights reserved.
  • Masayuki Nishi, Yu Nishihara, Tetsuo Irifune
    EARTH AND PLANETARY SCIENCE LETTERS 377 191 - 198 2013年09月 [査読有り]
     研究論文(学術雑誌) 
    The growth rate of MgSiO3 perovskite reaction rims between periclase and stishovite was investigated at 24-50 GPa and 1650-2150 K using a Kawai-type high-pressure apparatus. The textural observations of the recovered samples and rim growth kinetic data revealed that the reaction is controlled by coupled grain boundary diffusion of MgO and grain coarsening in the perovskite reaction layer. Assuming a high diffusivity of O compared with Mg, the grain boundary diffusivity of Mg in the perovskite was determined to be delta D-gb(Mg) [m(3)/s] = 10(-15.1) exp{-[176,000 + (P - 24) x 3.8 x 10(3)]/RT), which is similar to 3-5 gb orders of magnitude faster than that of Si. We found that the bulk diffusivity of Mg in polycrystalline perovskite is affected by the grain boundary when we consider the possible grain sizes and temperatures in the lower mantle. Accordingly, grain boundary diffusion in perovskite may be an effective mechanism for chemical transportation of divalent cations in the lower mantle. (C) 2013 Elsevier B.V. All rights reserved.
  • Masayuki Nishi, Tomoaki Kubo, Hiroaki Ohfuji, Takumi Kato, Yu Nishihara, Tetsuo Irifune
    EARTH AND PLANETARY SCIENCE LETTERS 361 44 - 49 2013年01月 [査読有り]
     研究論文(学術雑誌) 
    Si4+ + M2+ <-> 2Al(3+) (M=Mg+Fe+Ca) interdiffusion rates in majoritic garnet have been determined at pressure of 17 GPa and temperatures of 1550-1700 degrees C using a Kawai-type high-pressure apparatus. The diffusion profiles were measured across the interface between pyropic garnet and majoritic garnet samples by an analytical transmission electron microscope, and the rate of the Si4+ + M2+ <-> 2Al(3+) interdiffusion was determined to be a Dsi-Al (m(2) s(-1))=2 x 10(-1) exp [-364 (kJ mol(-1))/RT]. We found that the diffusion rate is low compared to the Si diffusion rate in other silicate minerals relevant to the deep mantle. Slow kinetics of the pyroxene-garnet transformation due to the low diffusion rate indicates that large low-density metastable regions may exist in the slab, greatly contributing to the slab stagnation around the transition region. Stagnant slabs possibly stay in this region over 10(8) yr because of the sluggish pyroxene-garnet transformation, but may eventually fall into the lower mantle after the completion of the transformation. (C) 2012 Elsevier B.V. All rights reserved.
  • Masayuki Nishi, Tetsuo Irifune, Hiroaki Ohfuji, Yoshinori Tange
    GEOPHYSICAL RESEARCH LETTERS 39 2012年12月 [査読有り]
     研究論文(学術雑誌) 
    The mechanism of the post-garnet transformation in natural single-crystalline pyropic garnet has been examined under large overpressure conditions of similar to 31-43 GPa using the multianvil apparatus with sintered diamond anvils. Intracrystalline nucleation was found to be dominant above 3538 GPa, while only grain-boundary nucleation is responsible at lower pressures. In addition, natural pyropic garnet transformed to a single phase of perovskite without decomposing to multiple phases above 38 GPa. Both intracrystalline nucleation and polymorphic growth of the post-garnet transformation under large overpressure conditions would strongly enhance the transformation kinetics. Consequently, the post-garnet transformation may pwroceed rapidly in the subducting slab at depths near similar to 950 km, which would cause substantial rheological weakening of the subducting slab. The metastable post-garnet transformation may also cause the seismic discontinuities at a depth of 900-1080 km depth, which is indeed observed beneath some subduction zones. Citation: Nishi, M., T. Irifune, H. Ohfuji, and Y. Tange (2012), Intracrystalline nucleation during the post-garnet transformation under large overpressure conditions in deep subducting slabs, Geophys. Res. Lett., 39, L23302, doi:10.1029/2012GL053915.
  • Masayuki Nishi, Tomoaki Kubo, Takumi Kato, Aiko Tominaga, Ken-ichi Funakoshi, Yuji Higo
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 189 1-2 47 - 55 2011年11月 [査読有り]
     研究論文(学術雑誌) 
    Mechanisms and kinetics of exsolution of majoritic garnet from Ca-Tschermak-rich clinopyroxene were examined at 14.0-15.6 GPa, 1250-1590 degrees C, and H(2)O-undersaturated conditions (800-1170 wt. ppm H(2)O) by time-resolved in situ synchrotron X-ray diffraction measurements. Textural observations of recovered samples revealed that majoritic garnet had exsolved along the grain boundaries of parent clinopyroxene. Analyses of the kinetic data obtained indicate that exsolved majoritic garnet grew by diffusion-controlled growth, and the growth kinetics can be described by k(2) (m(2)/s) = 7.4 x 10(-5) exp[-389(kJ mol(-1))/RT]. The exsolution of majoritic garnet from clinopyroxene would be kinetically inhibited at temperatures below similar to 800 degrees C in geological time scales (10(5)-10(6) years) when the parent grain size is larger than 1 mm. We constructed possible phase relations of subducting mid-ocean ridge basalt (MORB) taking the exsolution kinetics into consideration. When the effect of reaction kinetics is considered, the density contrast between the MORB component of a subducting slab and surrounding mantle becomes significantly reduced relative to that calculated for the equilibrium MORB assemblage. (C) 2011 Elsevier B.V. All rights reserved.
  • Takashi Yoshino, Eiji Ito, Tomoo Katsura, Daisuke Yamazaki, Shuanming Shan, Xinzhuan Guo, Masayuki Nishi, Yuji Higo, Ken-ichi Funakoshi
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 116 2011年04月 [査読有り]
     研究論文(学術雑誌) 
    The pressure-induced electronic spin transition of iron in ferropericlase was investigated as a function of iron content in ferropericlase by in situ electrical conductivity measurement. The electrical conductivity of ferropericlase, (Mg1-x,Fe-x)O (x = 0.07, 0.10, 0.13, 0.17, 0.24), was measured up to 53 GPa and 600 K using the Kawai-type multianvil apparatus equipped with sintered diamond anvils. At pressures up to 25 GPa, the electrical conductivity of ferropericlase generally increases with increasing pressure, and both the activation energy and activation volume of ferropericlase decrease with increasing iron content. For the samples with x = 0.07 and 0.10, the electrical conductivity shows a slight initial decrease and becomes constant between 25 and 40 GPa, upon which it increases slightly as the pressure increases. For the samples with higher iron content, the electrical conductivity constantly increases with pressure over the investigated pressure range. If these changes in the electrical conductivity are due to the isosymmetric high-to low-spin transition of iron in ferropericlase, this conductivity change suggests that the spin transition pressure significantly decreases with decreasing iron content in ferropericlase. Because the amount of iron in ferropericlase that coexists with the Al-bearing perovskite seems to be less than that in the Al-free perovskite, the influence of the iron partitioning between perovskite and ferropericlase by the spin transition appears in a pressure range of about 30-40 GPa in the lower mantle of the Earth.
  • Masayuki Nishi, Tomoaki Kubo, Takumi Kato, Aiko Tominaga, Akira Shimojuku, Naoko Doi, Ken-ichi Funakoshi, Yuji Higo
    GEOPHYSICAL RESEARCH LETTERS 37 2010年05月 [査読有り]
     研究論文(学術雑誌) 
    Majoritic garnets discovered as diamond inclusion in several localities originated from the deep upper mantle or the mantle transition zone. Some of them show evidence of partial transformation to low-pressure phases of pyropic garnet and clinopyroxene during transportation to the Earth's surface. The degree of partial transformation can be used potentially as a unique speedometer on the average ascent rate of diamond through the entire upper mantle. Here we present experimental results on the back transformation kinetics of majoritic garnet into pyropic garnet and clinopyroxene by time-resolved in-situ synchrotron X-ray diffraction measurements. We observed that back transformation occurs by grain-boundary nucleation and diffusion-controlled growth at 6.5-7.5 GPa and 1020-1300 degrees C. The transformation mechanisms are similar to those observed in natural diamond. The temperature dependence of diffusion-controlled growth rates were determined from the kinetic data obtained, which was used to constrain the survival time of majoritic garnet in diamond. We found that the ascending velocity in the normal mantle convection is far too slow to satisfy the time limitation for the survival of majoritic garnet, indicating that such diamonds have been transported directly from the deep mantle by the rapid kimberlite magma within a few days. Citation: Nishi, M., T. Kubo, T. Kato, A. Tominaga, A. Shimojuku, N. Doi, K. Funakoshi, and Y. Higo (2010), Survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle, Geophys. Res. Lett., 37, L09305, doi:10.1029/2010GL042706.
  • D. Yamazaki, E. Ito, T. Yoshino, X-G Guo, S. Shun, M. Nishi, Y. Higo, K. Funakoshi
    INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, JOINT AIRAPT-22 AND HPCJ-50 215 012096  2010年 [査読有り]
     研究論文(国際会議プロシーディングス) 
    Phase transformation from perovskite to "post-perovskite" and reserve transition in CaRuO3 was observed by in situ X-ray diffraction at pressure of 20-22 GPa and temperature of 1173-1523 K. These pressure and temperature conditions are 1-2 GPa lower than previous results by quenched method. The axial compressibility is estimated to be a/a(0)>b/b(0)>c/c(0). This result is inconsistent with previous result on CaIrO3.
  • Tomoaki Kubo, Makoto Kimura, Takumi Kato, Masayuki Nishi, Aiko Tominaga, Takumi Kikegawa, Ken-ichi Funakoshi
    NATURE GEOSCIENCE 3 1 41 - 45 2010年01月 研究論文(学術雑誌) 
    Shocked meteorites that were formed when their parent body underwent shock metamorphism often contain the mineral plagioclase either in an amorphous form or in its high-pressure phase. The peak pressures in shock metamorphism can be constrained by shock-recovery experiments that determine the amorphization pressures of plagioclase(1-3). However, in these experiments temperatures have been unrealistically low and timescales much shorter than those in natural events. Here we present in situ X-ray diffraction measurements of two kinds of plagioclase feldspar in conditions of increasing pressures and temperatures. We find that the amorphization pressure of plagioclase decreases with increasing temperature, suggesting that previous studies overestimated this parameter(1-3). We also found that the mineral jadeite forms first from amorphous plagioclase, whereas the nucleation of other minerals such as stishovite or garnet is significantly delayed. The occurrence of jadeite in shocked meteorites that do not contain stishovite(4-8) can therefore be explained as a result of the crystallization kinetics. We conclude that the study of plagioclase breakdown can constrain the pressure-temperature-time history of shock events, and thus help to reconstruct the collisional history of asteroids in the early Solar System.
  • Masayuki Nishi, Tomoaki Kubo, Takumi Kato
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 104 3 192 - 198 2009年06月 [査読有り]
     研究論文(学術雑誌) 
    The eclogite-garnetite transformation experiments were conducted at 11 GPa and 1000-155 degrees C using synthesized polycrystalline eclogite with MORB composition as a starting material. We found that the eclogite-garnetite transformation proceeds by two stages, (1) precipitation of majoritic garnet from clinopyroxene and (2) formation of majoritic garnet from original pyropic garnet by absorption of clinopyroxene. The 1st stage in the transformation proceeds at 1000 C for 180 minutes, however the 2nd stage was not observed even at the high temperature of 1550 C on the same time scale. The differences in kinetics are due to contrasting diffusivities between clinopyroxene and garnet. The kinetic effect would modify the mineralogy and rock microstructures in MORB, which provides important implications for dynamics of subducting oceanic crust by changing the density and viscosity relation.
  • Takashi Yoshino, Masayuki Nishi, Takuya Matsuzaki, Daisuke Yamazaki, Tomoo Katsura
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 170 3-4 193 - 200 2008年11月 [査読有り]
     研究論文(学術雑誌) 
    Electrical conductivities of majorite garnet with compositions of pyrolite minus olivine (pyrolite majorite) and mid-ocean ridge basalt (MORB majorite) were measured under physical conditions of the mantle transition zone (18 and 23 GPa and temperatures up to 2000 K) in a Kawai-type multi-anvil apparatus. The samples with MORB composition are mainly composed of majorite, which has higher Fe and Al contents, and contain a small amount of stishovite. The conductivity of the MORB majorite is more than twice higher than those of the pyrolite majorite at the same temperature. The activation energies of these majorites are both 1.4 eV at temperature of 1000-1600 K suggesting that the dominant mechanism of charge transportation is Fe2(+)-Fe3(+) hopping (small polaron) conduction. At higher temperatures (>1600 K), corresponding to temperature conditions of the transition zone. conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction. The pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite, respectively, and will not largely change the conductivity-depth profile predicted for the dry mantle transition zone. The laboratory-based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi-global conductivity-depth profile obtained from electromagnetic study beneath Pacific. On the other hand, the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of MORB majorite is significantly higher than those of wadsleyite and ringwoodite. Conductivity values of MORB majorite agree with those of the stagnant slab beneath the northeastern China. (C) 2008 Elsevier B.V. All rights reserved.
  • Masayuki Nishi, Takumi Kato, Tomoaki Kubo, Takumi Kikegawa
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 170 3-4 274 - 280 2008年11月 [査読有り]
     研究論文(学術雑誌) 
    An experimental study has been conducted to clarify the conditions and kinetic aspects of the pyroxene-garnet transformation by using synthetic polycrystals (two pyroxenes + garnet) in the CaO-MgO-Al(2)O(3)-SiO(2) system. The run products recovered from the in situ X-ray diffraction experiments in a double-stage multi-anvil system at 15.5-20.4 GPa and 800-1600 degrees C were carefully analyzed by scanning electron microscopy. Our results show that the pyroxene-garnet transformation proceeds by overgrowth of garnet absorbing the surrounding pyroxene components above 1550 degrees C. At 18.4 GPa and 1320 degrees C, pyroxenes (enstatite + diopside) directly decomposed to the assemblages (wadsleyite + stishovite + Ca-perovskite) without dissolving in garnet. The pyroxene-garnet transformation requires much higher temperatures than those needed for transformation of olivine to wadsleyite (similar to 1000 degrees C) and of pyroxenes to their high-pressure phases (similar to 1300 degrees C) in laboratory time (similar to 10(3) s) and grain (similar to 10(-5) m) scales. Therefore, the pyroxene-garnet transformation is likely to be kinetically inhibited at low temperatures in the subducting plate, and the pyropic garnet could survive in the transition zone instead of equilibrium majoritic garnet. (C) 2008 Elsevier B.V. All rights reserved.

MISC

  • The pyrite-type high-pressure form of FeOOH
    西 真之 SPring8 Research Frontiers 2017 80 -81 2018年 [招待有り]
  • 古田大祐, 入舩徹男, 入舩徹男, 西真之, 西真之, 國本健広, 有本岳史, 新名亨 高圧討論会講演要旨集 58th 159 2017年10月
  • 柿澤翔, 井上徹, 井上徹, 西真之, 西真之, 有本岳史, 門林宏和, 丹下慶範, 肥後祐司, 中野智志, 平井寿子 高圧討論会講演要旨集 58th 102 2017年10月
  • 伊賀文俊, 菊地翔弥, 植松直之, 羽賀浩人, 横道啓省, 平野航, 西山宣正, 新名亨, 西真之, 國本健広, 入舩徹男, 丹下義範, 近藤晃弘, 金道浩一, 小山内湧人, 山中昭司 高圧討論会講演要旨集 57th 165 2016年10月
  • Transportation of water into the deep lower mantle by new type of dense hydrous magnesium silicate
    NISHI Masayuki SPring-8 Research Frontiers 2014 記事・総説・解説・論説等(大学・研究所紀要) 80 -81 2015年 [招待有り]
  • Masayuki Nishi NATURE GEOSCIENCE 記事・総説・解説・論説等(商業誌、新聞、ウェブメディア) 8 (1) 9 -10 2015年01月 [招待有り]
  • Back transformation kinetics of majoritic garnet and implication for ascending rate of diamond
    NISHI Masayuki SPring-8 Research Frontiers 2010 記事・総説・解説・論説等(大学・研究所紀要) 114 -115 2011年 [招待有り]

受賞

  • 2014年11月 日本高圧力学会 奨励賞
     
    受賞者: 西 真之
  • 2011年09月 Scientist Presentation Award (The GL-GRC Inter-Institutional Science Symposium)
  • 2011年09月 日本鉱物学科学会 論文賞
     
    受賞者: 西 真之

共同研究・競争的資金等の研究課題

  • 高温高圧変形実験で探る内核のレオロジー
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2019年04月 -2024年03月 
    代表者 : 西原 遊, 西 真之, 大内 智博
  • 地球マントルー中心核境界および氷惑星深部における水の挙動
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2019年04月 -2022年03月 
    代表者 : 西 真之, 土屋 旬, 桑山 靖弘
  • 核-マントル物質の精密高圧実験技術の開発
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2015年06月 -2020年03月 
    代表者 : 入舩 徹男, 山崎 大輔, 小松 一生, 境 毅, 西 真之, 石松 直樹
     
    下部マントル及び核領域の精密実験技術と多様な実験技術の開発を、超硬合金(WC)、焼結ダイヤモンド(SD)、ナノ多結晶ダイヤモンド(NPD=ヒメダイヤ)、及び単結晶ダイヤモンドを使い分けて進めるとともに、これらの技術に基づき、他計画研究と連携し、領域の先端的研究を推進した。本年度得られた主な成果は以下のとおりである。 1)NPD合成技術の改良による、安定した大型・高品質NPD合成法の最適化と、実験に必要なNPD合成・加工を行い、本計画研究内及び他計画研究へ提供。また様々な粒径のNPD試料に対して放射光X線その場観察実験を行い、圧縮挙動の粒径依存性を明らかにした。 2)NPDアンビルを用いたダイヤモンドアンビル装置(NPD-DAC)による500GPa領域の超高圧発生に成功するとともに、1000K程度までの外熱加熱システムの開発を行った。また2段式DACを用いた高温高圧下での300GPa領域での変形実験技術を開発した。大型NPD-DACを用いた中性子実験用高圧装置の開発を行い、40GPa領域での圧力発生に成功した。 3)SDアンビルを用いたマルチアンビル装置(SD-MA)による、65GPa領域の圧力下における2300K領域の高温発生に成功するとともに、この技術を用いた下部マントル下部領域までのマントル主要鉱物の相関係の解明を行った。またWCアンビルを用いて25GPa程度の圧力下で3000Kまでの安定的高温発生を可能にした。 4)上記の技術及び他の技術を組み合わせ、また他の計画研究メンバーと協力することにより、沈み込んだ海洋地殻物質の下部マントルにおける挙動、Al2SiO5の新高圧相の発見とその構造解明、MgSiO3-FeSiO3系の下部マントル中部までの相関係、CaSiO3-MgSiO3系の融解関係、ハロゲン元素等の分配挙動の解明と地球始源物質への制約、等に関する研究成果を発表した。
  • 新しい含水高圧相Phase Hの安定性と下部マントル領域での水の挙動
    日本学術振興会:科学研究費助成事業 若手研究(A)
    研究期間 : 2015年04月 -2019年03月 
    代表者 : 西 真之
     
    前年度までの本研究課題において、100万気圧を超える高圧条件下において熱力学的に安定な新含水相FeOOHとAlOOHを発見し、これらの含水鉱物がマントル最深部へ水を供給する可能性を示した。この結果を受け、当該年度では、より大型惑星深部を想定し、より高い圧力における含水鉱物の安定領域の決定と新たな相転移を探索することを目標として研究を進めた。新含水相FeOOHの構造はパイライト型であることを論文として公表したが、新含水鉱物AlOOHについてはその構造を正確に決定することができておらず、さらなる検討を行った。270万気圧におよぶ高圧実験と数値計算による分析により、AlOOHは近年理論計算により予測されたAlOOHの超高圧型単斜晶系相(Verma et al., 2018)であると結論付けた。本鉱物は地球より大きな惑星(例えばスーパーアースや氷惑星)の内部に存在する可能性があるとして、国際雑誌に投稿である。また、下部マントル条件下の含水鉱物はAlOOH、FeOOH、MgSiO4H2の端成分において広い組成範囲で固溶体を形成することを実験により確認し、こちらも国際雑誌に投稿中である。このように、本研究課題で目標とした新規含水相の発見と、多成分系における含水鉱物の組成の理解についての成果が得られてた。本研究の結果を受けた新たな研究が国内外の研究者により行われているが、沈み込んだ水が地球のマントルと中心核の境界でどのような挙動をとるかはいまだ謎が多く、さらなる研究が必要である。
  • コンドライトの融解で探る初期惑星の形成と分化
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2014年04月 -2018年03月 
    代表者 : GREAUX Steeve, 西 真之, 河合 研志, 市川 浩樹, 丹下 慶範, スタグノ ヴィンセンツォ, 舘野 繁彦
     
    本研究では,コンドライト隕石の融解温度と相関係を高圧実験によって決定した.目的は,核形成,マントル全岩組成,始原マントルの酸化還元状態など,岩石型惑星形成初期の物質分化過程の理解である.融解実験は,天然の炭素質コンドライト隕石(CI2,タギシュ・レイク)とエンスタタイトコンドライト隕石(EH,サハラ)に対してマルチアンビル型高圧合成装置を用いて行い,両隕石の融点への揮発物質の影響を見積もった.これにより,鉄-ニッケル硫化物と炭素・水和物の共存安定性を考慮した,岩石型始原惑星の集積や核分離のモデル化に有用な相平衡関係を確立した.これは岩石型惑星の始原マントルの進化過程へも重要な示唆を与える.
  • 下部マントルの化学組成と初期地球の分化過程
    日本学術振興会:科学研究費助成事業 基盤研究(S)
    研究期間 : 2013年05月 -2018年03月 
    代表者 : 入舩 徹男, 大藤 弘明, 境 毅, 西 真之, 肥後 祐司, 丹下 慶範
     
    川井型多アンビル装置を中心とした実験技術開発に基づき、下部マントル上部に対応する高温高圧下での弾性波速度測定技術を確立するとともに、下部マントル深部に対応する温度圧力の安定発生を可能にした。これらの実験技術に基づき、下部マントル条件下での(1)主要高圧相の弾性波速度の精密測定、(2)マントル関連物質の融点・融解関係と元素分配、(3)沈み込むスラブ関連物質や含水高圧相の安定性、等に関して実験解明をおこなった。これらの実験結果と地震学・地球化学などから得られる情報を対比することにより、下部マントルを中心とした地球深部領域の化学組成と進化過程、また物質循環とダイナミクスに対して重要な制約を与えた。
  • 相転移カイネティクスとダイヤモンド包有物に基づく地球深部の鉱物構成の解明
    日本学術振興会:科学研究費助成事業 特別研究員奨励費
    研究期間 : 2011年 -2013年 
    代表者 : 西 真之
     
    (研究1)鉱物の原子拡散速度は地球内部の流動特性や元素移動特性に影響する。昨年度から今年度にかけて、マントル構成鉱物の非平衡性や流動特性を理解することを目的として、MgOとSio_2結晶間の拡散律速反応であるMgSi03 perovskiteの反応縁成長速度を実験によりを明らかにした。24-50GPa, 1650-2100℃, 15-1500分の条件下で、計10回の実験を行った。回収試料の組織観察から、すべての実験条件でMgOとSiO_2の反応に伴うperovskite反応縁が確認された。また、得られた反応縁成長カイネティクスから、MgO成分の粒界拡散係数を、粒成長の効果を考慮して計算した。この結果から、下部マントル領域での鉱物の粒界拡散特性の議論が可能となった。 (研究2)地球内部に存在する水は、相転移カイネティクスや岩石の流動特性に強く影響する。地球表層環境を構成する水は、地表の岩石との反応により含水鉱物を作り、プレートの沈み込みとともにマントル内に持ち込まれる。そのため含水鉱物の安定領域を知ることは、地球内部の水の循環機構とその規模を知るための重要な手掛かりとなる。今回、マルチアンビル型高圧発生装置と、焼結ダイヤモンドアンビル、貴金属製の試料容器を用いた超高圧下での含水実験技術を開発し、下部マントルにおける含水鉱物の相関係を研究した。その結果1200kmの深さを超えて、下部マントル深くまで存在できうる含水ケイ酸鉱物(Phase H)を新しく発見し、phase Hによる、地球表層から地球中心核(コア)までの大規模な水の循環の可能性を示した。
  • パイロキシン-ガーネット相転移カイネティクスと沈み込むスラブの非平衡ミネラロジー
    日本学術振興会:科学研究費助成事業 特別研究員奨励費
    研究期間 : 2009年 -2010年 
    代表者 : 西 真之
     
    地球深部に沈み込むプレートの内部は周囲のマントルよりも低温の環境であり、鉱物の反応速度が非常に遅い。そのため、プレート内部のマントル鉱物は完全に平衡相関係に従うわけでなく、高圧相安定領域でも低圧相のまま、準安定相として存在する可能性が指摘されている。地震波トモグラフィーに映るプレートの停滞領域は、予想される非平衡鉱物の存在領域でもある。従って、相転移速度の解明は、粘性や密度等、物性値に密接に結びつくため、プレート滞留のメカニズムを考える上で非常に重要な課題である。今年度は沈み込むプレート内で起こると考えられているpyroxene-garnet相転移速度を測定するために、この相転移速度を律速するgarnet粒内の原子の拡散係数を比較的低含水量条件下で測定した。出発物質として、マントル遷移層領域に存在すると考えられる化学組成を持つメージャライトガーネットと、天然のパイロープ的組成のガーネットを用いた。これらのサンプルを研磨し、お互いを接触させ、高温高圧条件下で反応させた。高温高圧実験にはマルチアンビル型高圧発生装置を用い、回収試料の化学組成の分析には透過型電子顕微鏡を用いた。得られた拡散係数から計算したpyroxene-garnet相転移速度は非常に遅く、沈み込むプレート内でこの相転移が進行しないことが示唆された。また、この相転移が進行しないことを考慮して沈み込むプレートの鉱物構成を構築し、その密度を計算した。その結果、沈み込むプレートはマントル遷移層領域で周囲のマントルより賢くなり、この領域でプレートが浮力を持つことが示唆された。

愛媛大学教員活動実績

教育活動(B)

担当授業科目(B01)

  • 2019, 前期, 学部, 地球科学実験Ⅰ
  • 2019, 前期, 修士, 地球科学高等実験Ⅰ
  • 2019, 前期, 修士, 地球科学高等実験Ⅱ
  • 2019, 前期, 修士, 地球科学高等実験Ⅴ
  • 2019, 前期, 修士, 地球科学高等実験Ⅵ
  • 2019, 前期, 修士, 地球科学プレゼンテーション特別実習Ⅰ
  • 2019, 前期, 修士, 地球科学プレゼンテーション特別実習Ⅲ
  • 2019, 前期, 修士, 地球科学ゼミナールⅠ
  • 2019, 前期, 修士, 地球科学プレゼンテーション特別実習Ⅱ
  • 2019, 前期, 修士, 地球科学高等実験Ⅲ
  • 2019, 前期, 学部, 地球科学実験I


Copyright © MEDIA FUSION Co.,Ltd. All rights reserved.