研究者総覧

境 毅 (サカイ タケシ)

  • 地球深部ダイナミクス研究センター 助教
Last Updated :2020/11/10

研究者情報

学位

  • 博士(理学)(東北大学)

論文上での記載著者名

  • Takeshi, Sakai
  • 境, 毅

科研費研究者番号

  • 90451616

J-Global ID

研究キーワード

  • マルチメガバール   2段式ダイヤモンドアンビルセル   状態方程式   ダイヤモンドアンビルセル   超高圧地球科学   

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
  • 2014年04月 - 2019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
  • 2012年08月 - 2014年03月  愛媛大学地球深部ダイナミクス研究センター助教
  • 2011年05月 - 2012年07月  東北大学大学院理学研究科地学専攻助教
  • 2007年04月 - 2011年04月  東北大学国際高等研究教育機構国際高等融合領域研究所助教

学歴

  • 2004年04月 - 2007年03月   東北大学   理学研究科   地学専攻(博士課程)
  • 2002年04月 - 2004年03月   東北大学   理学研究科   地学専攻(修士課程)
  • 1998年04月 - 2002年03月   東北大学   理学部   地球惑星物質科学科

所属学協会

  • アメリカ地球物理学連合   日本惑星科学会   日本鉱物科学会   日本高圧力学会   日本地球惑星科学連合   

研究活動情報

論文

  • Takehiko Yagi, Takeshi Sakai, Hirokazu Kadobayashi, Tetsuo Irifune
    HIGH PRESSURE RESEARCH 40 1 148 - 161 2020年01月 [査読有り]
     
    Current anvil designs and problems associated with various efforts to generate static high pressures beyond the limit of conventional diamond anvil cells (DACs) (similar to 400 GPa) are reviewed. Pressures of up to 1 TPa have been reported by one research group using the double-stage DAC (ds-DAC) technique, but no other research group has successfully reproduced this high pressure result. Some research groups have used toroidal anvils, achieving pressures of >400 GPa. We have conducted numerous ds-DAC experiments and investigated the problems associated with such experiments. They include problems associated with various pressure scales in the multi-megabar region, difficulties in obtaining reliable X-ray diffraction patterns from micron-sized samples, and physical property measurements of tiny materials that may be harder than diamond. Each of these problems is discussed, following the summary of various experiments.
  • Kanta Fukimoto, Masaaki Miyahara, Takeshi Sakai, Hiroaki Ohfuji, Naotaka Tomioka, Yu Kodama, Eiji Ohtani, Akira Yamaguchi
    Meteoritics and Planetary Science 2020年 [査読有り]
     研究論文(学術雑誌) 
    © The Meteoritical Society, 2020. We investigated the back-transformation mechanisms of ringwoodite and majorite occurring in a shock-melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back-transformed into olivine and enstatite, respectively. Ringwoodite (Fa~18) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, fine-grained polycrystalline olivine becomes dominant instead of ringwoodite. The back-transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface-controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs16–17En82–83Wo1) occurs in the SMV as a fine-grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back-transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.
  • Phase and melting relations of Fe3C to 300 GPa and carbon in the core.
    Suguru Takahashi, Eiji Ohtani, Takeshi Sakai, Seiji Kamada, Shin Ozawa, Tatsuya Sakamaki, Masaaki Miyahara, Yoshinori Ito, Naohisa Hirao, Yasuo Ohishi
    Carbon in Earth’s interior, AGU Monograph volume 25 - 36 2019年11月 [査読有り][招待有り]
     研究論文(国際会議プロシーディングス)
  • Norimasa Nishiyama, Julia Langer, Takeshi Sakai, Yohei Kojima, Astrid Holzheid, Nico A. Gaida, Eleonora Kulik, Naohisa Hirao, Saori I. Kawaguchi, Tetsuo Irifune, Yasuo Ohishi
    Journal of the American Ceramic Society 102 4 2195 - 2202 2019年04月 研究論文(学術雑誌) 
    © 2018 The American Ceramic Society Phase relations in silicon and germanium nitrides (Si3N4 and Ge3N4) were investigated using a Kawai-type multianvil apparatus and a laser-heated diamond anvil cell combined with a synchrotron radiation. The pressure-induced phase transition from the β to γ (cubic spinel-type structure) phase was observed in both compositions. We observed the coexistence of the β and γ phases in Si3N4 at 12.4 GPa and 1800°C, while the appearance of single phase γ-Ge3N4 was observed at pressures above 10 GPa. Our observations under higher pressures revealed that γ-Si3N4 and γ-Ge3N4 have wide stability fields and no postspinel transition was observed up to 98 GPa and 2400°C in both compositions. Using the room-temperature compression curves of these materials, the bulk moduli (K0) and their pressure derivatives (K′0) were determined: K0 = 317 (16) GPa and K′0 = 6.0 (8) for γ-Si3N4; K0 = 254 (13) GPa and K′0 = 6.0 (7) for γ-Ge3N4.
  • K. Kuramochi, N. Ishimatsu, T. Sakai, N. Kawamura, T. Irifune
    High Pressure Research 2019年 [査読有り]
     研究論文(学術雑誌) 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. We present the first application of double-stage diamond anvil cells (ds-DACs) to X-ray absorption spectroscopy (XAS) of Rhenium (Re) metal. By using highly stable and sub-micron sized X-ray beam, XAS spectra were successfully observed at the (Formula presented.) -edges from the sample encapsulated in the small space between the second-stage anvils. The pressure generation exceeding 300 GPa was evaluated by the pressure-induced shift of extended X-ray absorption fine structure (EXAFS) oscillation at the (Formula presented.) -edge. The profile of white-line at the (Formula presented.) -edges broadened with increasing pressure, which indicates the modification of the Re (Formula presented.) band structure. The present results demonstrate the high ability of XAS technique combined with ds-DACs as a probe for crystal structure and electronic structure under ultrahigh pressure.
  • Takeshi Sakai, Takehiko Yagi, Ryosuke Takeda, Toshiki Hamatani, Yuki Nakamoto, Hirokazu Kadobayashi, Hideto Mimori, Saori I. Kawaguchi, Naohisa Hirao, Keitaro Kuramochi, Naoki Ishimatsu, Takehiro Kunimoto, Hiroaki Ohfuji, Yasuo Ohishi, Tetsuo Irifune, Katsuya Shimizu
    High Pressure Research 2019年01月 
    © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Both micro-paired and conical support type double-stage diamond anvil cells (ds-DAC) were tested using a newly synthesized ultra-fine nano-polycrystalline diamond (NPD). Well-focused X-ray sub-micron beam and the conically supported 2nd stage anvils (micro-anvils) with 10 μm culet enable us to obtain good quality X-ray diffraction peaks from the sample at around 400 GPa. The relationship between confining pressure and sample pressure depends heavily on the initial height (thickness) of micro-anvils, the difference of a few micrometers leads to a quite different compression path. The conical support type is a solution to retain both enough thickness and strength of micro-anvils at higher confining pressure conditions. All conical support ds-DAC experiments terminated by the failure of the 1st stage anvil instead of 2nd one. The combination of ultra-fine NPD 2nd stage anvil and NPD 1st stage anvil opens a new frontier for measurement of the X-ray absorption spectrum above 300 GPa.
  • Tomohiro Ohuchi, Xinglin Lei, Yuji Higo, Yoshinori Tange, Takeshi Sakai, Kiyoshi Fujino
    Contributions to Mineralogy and Petrology 173 2018年10月 
    © 2018, Springer-Verlag GmbH Germany, part of Springer Nature. The role of aqueous fluid in fracturing in subducting slabs was investigated through a series of deformation experiments on dunite that was undersaturated (i.e., fluid-free) or saturated with water (i.e., aqueous-fluid bearing) at pressures of 1.0–1.8 GPa and temperatures of 670–1250 K, corresponding to the conditions of the shallower regions of the double seismic zone in slabs. In situ X-ray diffraction, radiography, and acoustic emissions (AEs) monitoring demonstrated that semi-brittle flow associated with AEs was dominant and the creep/failure strength of dunite was insensitive to the dissolved water content in olivine. In contrast, aqueous fluid drastically decreased the creep/failure strength of dunite (up to ~ 1 GPa of weakening) over a wide range of temperatures in the semi-brittle regime. Weakening of the dunite by the aqueous fluid resulted in the reduction of the number of AE events (i.e., suppression of microcracking) and shortening of time to failure. The AE hypocenters were located at the margin of the deforming sample while the interior of the faulted sample was aseismic (i.e., aseismic semi-brittle flow) under water-saturated conditions. A faulting (slip rate of ~ 10−3 to 10−4 s−1) associated with a large drop of stress (Δσ ~ 0.5 to 1 GPa) and/or pressure (ΔP ~ 0.5 GPa) was dominant in fluid-free dunite, while a slow faulting (slip rate < 8 × 10−5 s−1) without any stress/pressure drop was common in water-saturated dunite. Aseismic semi-brittle flow may mimic silent ductile flow under water-saturated conditions in subducting slabs.
  • Takeshi Sakai, Takehiko Yagi, Tetsuo Irifune, Hirokazu Kadobayashi, Naohisa Hirao, Takehiro Kunimoto, Hiroaki Ohfuji, Saori Kawaguchi-Imada, Yasuo Ohishi, Shigehiko Tateno, Kei Hirose
    High Pressure Research 38 2 107 - 119 2018年04月 [査読有り]
     研究論文(学術雑誌) 
    © 2018 Informa UK Limited, trading as Taylor & Francis Group. We have developed a double stage diamond anvil cell (ds-DAC) technique for reproducible pressure by precisely fabricating 2nd stage anvils using a focused ion beam system. We used 2nd stage micro-anvils made of ultra-fine (< 10 nm) nano-polycrystalline diamond with various shapes and dimensions synthesized from glassy carbon at high pressure and temperature. The X-ray diffraction patterns from the rhenium sample always showed very broad peaks due to large pressure gradients in the culet of the micro-anvils. Deconvolution of the broad 101 diffraction peak results in compression of rhenium to V/V0= 0.633 for the smallest d-spacing. The calculated pressure for this minimum volume varies from 430 to 630 GPa, depending on the choice of the equation of state of rhenium. We conclude that the most likely pressure achieved for the minimum volume of rhenium is in a range of 430–460 GPa based on a calibration using the platinum pressure scale to 280 GPa and the latter value of 630 GPa is unreasonably high, suggesting that the pressures in an earlier study for the equation of state of rhenium would have been significantly overestimated.
  • Takeshi Sakai
    Japanese Magazine of Mineralogical and Petrological Sciences 47 1 27 - 33 2018年 研究論文(学術雑誌) 
    The equation of state (EoS) at multi-megabar condition should include a parameter at in- finite pressure such as K'∞ in Keane EoS. The Keane EoS model was adopted for the first time to extract meaningful physical properties for MgSiO3 post-perovskite (PPv) phase. The thermal EoSs of PPv were determined by using both laser-heated diamond anvil cell and density- functional theoretical techniques, within a multi-megabar pressure range, corresponding to the conditions of a super-Earth's mantle. The experimentally determined Gruä neisen parameter, which is one of the thermal EoS parameters, and its volume dependence were consistent with their theoretically obtained values. Both the experimental and theoretical EoS were also found to be in very good agreement for volumes up to 300 GPa and 5000 K, respectively. Our newly developed EoS is applicable to a super-Earth's mantle, as well as the Earth's core-mantle boundary region. On the other hand, the double stage diamond anvil cell (ds-DAC) technique was developed using a focused ion beam (FIB) system in order to generate the Tera pascal (TPa) regime corresponding to conditions of exoplanet's interior. Micro-manufacturing using a FIB system enables us to control shapes of 2nd stage micro-anvils, process any materials, including nano-polycrystalline diamond (NPD) and single crystal diamond, and assemble the sample exactly in a very small space between the 2nd stage anvils. This method is highly reproducible and would allow us to open a new frontier.
  • 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 [査読有り]
     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.
  • Tomohiro Ohuchi, Xinglin Lei, Hiroaki Ohfuji, Yuji Higo, Yoshinori Tange, Takeshi Sakai, Kiyoshi Fujino, Tetsuo Irifune
    NATURE GEOSCIENCE 10 10 771 - + 2017年10月 [査読有り]
     研究論文(学術雑誌) 
    The occurrence of intermediate-depth and deep earthquakes at depths greater than 60 km in subducting slabs has long puzzled geoscientists. These earthquakes require some mechanism to accelerate the fault movement at high pressures above 1.8 GPa. Localized heating would contribute to faulting, but experimental evidence for this mechanism has been limited to pressures of up to 0.5 GPa. Here we conduct deformation experiments on dry dunite samples at pressures of 1.0 to 2.6 GPa and temperatures of 860 to 1,350 K-conditions close to those for relatively shallow intermediate-depth earthquakes. We observe plastic deformation of the dunite, followed by faulting and acoustic emissions at an accelerated strain rate of about 5 x 10(-5) s(-1) or higher. We find that ultrafine-grained gouge layers containing iron-rich melt films, which is indicative of a very high peak temperature of about 2,110 K along the fault planes. We also observe faulting in wet harzburgite-a dehydration product of antigorite-at natural stress levels of 0.3 to 0.4 gigapascals. We therefore suggest that intermediate-depth earthquakes can be induced by localized heating both in dry and wet subducting slabs, if the background strain rate exceeds a threshold value in the range from 10(-16) to 10(-13) s(-1).
  • Takanori Sakairi, Eiji Ohtani, Seiji Kamada, Takeshi Sakai, Tatsuya Sakamaki, Naohisa Hirao
    Progress in Earth and Planetary Science 4 2017年03月 研究論文(学術雑誌) 
    © The Author(s). 2017. The phase and melting relations in the Fe–S–Si system were determined up to 60 GPa by using a double-sided laser-heated diamond anvil cell combined with X-ray diffraction. On the basis of the X-ray diffraction patterns, we confirmed that hcp/fcc Fe–Si alloys and Fe 3 S are stable phases under subsolidus conditions in the Fe–S–Si system. Both solidus and liquidus temperatures are significantly lower than the melting temperature of pure Fe and both increase with pressure. The slopes of the Fe–S–Si liquidus and solidus curves determined here are smaller than the adiabatic temperature gradients of the liquid cores of Mercury and Mars. Thus, crystallization of their cores started at the core–mantle boundary region.
  • Takeshi Sakai, Haruhiko Dekura, Naohisa Hirao
    SCIENTIFIC REPORTS 6 2016年03月 [査読有り]
     研究論文(学術雑誌) 
    The MgSiO3 post-perovskite phase is the most abundant silicate phase in a super-Earth's mantle, although it only exists within the Earth's lowermost mantle. In this study, we established the thermal equation of state (EoS) of the MgSiO3 post-perovskite phase, which were determined by using both laser-heated diamond anvil cell and density-functional theoretical techniques, within a multi-megabar pressure range, corresponding to the conditions of a super-Earth's mantle. The Keane and AP2 EoS models were adopted for the first time to extract meaningful physical properties. The experimentally determined Gruneisen parameter, which is one of the thermal EoS parameters, and its volume dependence were found to be consistent with their theoretically obtained values. This reduced the previously reported discrepancy observed between experiment and theory. Both the experimental and theoretical EoS were also found to be in very good agreement for volumes at pressures and temperatures of up to 300 GPa and 5000 K, respectively. Our newly developed EoS should be applicable to a super-Earth's mantle, as well as the Earth's core-mantle boundary region.
  • Tatsuya Sakamaki, Eiji Ohtani, Hiroshi Fukui, Seiji Kamada, Suguru Takahashi, Takanori Sakairi, Akihiro Takahata, Takeshi Sakai, Satoshi Tsutsui, Daisuke Ishikawa, Rei Shiraishi, Yusuke Seto, Taku Tsuchiya, Alfred Q. R. Baron
    SCIENCE ADVANCES 2 2 2016年02月 [査読有り]
     研究論文(学術雑誌) 
    Hexagonal close-packed iron (hcp-Fe) is a main component of Earth's inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density-sound velocity data of Earth's core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (V-P) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch's law for hcp-Fe, which gives us the V-P of pure hcp-Fe up to core conditions. We find that Earth's inner core has a 4 to 5% smaller density and a 4 to 10% smaller V-P than hcp-Fe. Our results demonstrate that components other than Fe in Earth's core are required to explain Earth's core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth's core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.
  • Takeshi Sakai, Takehiko Yagi, Hiroaki Ohfuji, Tetsuo Irifune, Yasuo Ohishi, Naohisa Hirao, Yuya Suzuki, Yasushi Kuroda, Takayuki Asakawa, Takashi Kanemura
    REVIEW OF SCIENTIFIC INSTRUMENTS 86 3 2015年03月 [査読有り]
     研究論文(学術雑誌) 
    Micron-sized diamond anvils with a 3 mu m culet were successfully processed using a focused ion beam (FIB) system and the generation of high pressures was confirmed using the double stage diamond anvil cell technique. The difficulty of aligning two second-stage micro-anvils was solved via the paired micro-anvil method. Micro-manufacturing using a FIB system enables us to control anvil shape, process any materials, including nano-polycrystalline diamond and single crystal diamond, and assemble the sample exactly in a very small space between the second-stage anvils. This method is highly reproducible. High pressures over 300 GPa were achieved, and the pressure distribution around the micro-anvil culet was evaluated by using a well-focused synchrotron micro-X-ray beam. (C) 2015 AIP Publishing LLC.
  • Izumi Mashino, Eiji Ohtani, Naohisa Hirao, Takaya Mitsui, Ryo Masuda, Makoto Seto, Takeshi Sakai, Suguru Takahashi, Satoshi Nakano
    AMERICAN MINERALOGIST 99 8-9 1555 - 1561 2014年08月 [査読有り]
     研究論文(学術雑誌) 
    Valence, spin states, and crystallographic sites of Fe in (Mg,Fe)SiO3 perovskite were investigated using energy-domain Fe-57-synchrotron Mossbauer spectroscopy and powder X-ray diffraction up to 86 GPa. The volumes of Fe3+-bearing perovskite in this study are slightly smaller than those of Mg end-member perovskite. Our Mossbauer data suggest that Fe3+ prefers A sites coupled with Mg vacancies, which is consistent with previous data at ambient conditions. Fe3+ in the A site remains in a high-spin state up to 86 GPa, and some fraction of the A site is occupied by Fe2+ at pressures above 30 GPa. Fe2+ in the A sites is also in a high-spin state up to 86 GPa. The coupled substitution from Mg2+ to a high-spin state of Fe3+ and Mg2+ vacancy would make the volume of perovskite smaller than that of Mg end-member perovskite. If the lower mantle is saturated in silica, perovskite containing high-spin Fe3+ in A site has a higher density. Such silica oversaturated regions could sink by the density difference.
  • Masaaki Miyahara, Eiji Ohtani, Akira Yamaguchi, Shin Ozawa, Takeshi Sakai, Naohisa Hirao
    PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 111 30 10939 - 10942 2014年07月 [査読有り]
     研究論文(学術雑誌) 
    Howardite-eucrite-diogenite meteorites (HEDs) probably originated from the asteroid 4 Vesta. We investigated one eucrite, Bereba, to clarify a dynamic event that occurred on 4 Vesta using a shock-induced high-pressure polymorph. We discovered high-pressure polymorphs of silica, coesite, and stishovite originating from quartz and/or cristobalite in and around the shock-melt veins of Bereba. Lamellar stishovite formed in silica grains through a solid-state phase transition. A network-like rupture was formed and melting took place along the rupture in the silica grains. Nanosized granular coesite grains crystallized from the silica melt. Based on shock-induced high-pressure polymorphs, the estimated shock-pressure condition ranged from similar to 8 to similar to 13 GPa. Considering radiometric ages and shock features, the dynamic event that led to the formation of coesite and stishovite occurred ca. 4.1 Ga ago, which corresponds to the late heavy bombardment period (ca. 3.8-4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the origin of HEDs is thought to be related to dynamic events that formed the basins ca. 1.0 Ga ago, our findings are at variance with that idea.
  • Takeshi Sakai, Suguru Takahashi, Naoya Nishitani, Izumi Mashino, Eiji Ohtani, Naohisa Hirao
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 228 114 - 126 2014年03月 [査読有り]
     研究論文(学術雑誌) 
    Pure iron and Fe0.9Ni0.1 alloy were compressed to a pressure of 279 GPa and 272 GPa, respectively. The compression data sets were fitted using the third order Birch-Murnaghan equation of state based on six different pressure scales. Our result shows the nickel increase the density and the c/a ratio, decrease the bulk modulus. The high temperature data obtained the thermal equation of state parameters. At high temperature, although the nickel effect on density slightly decrease due to the a little larger thermal expansion, the bulk modulus difference increase to 7.3-7.8% at 329 GPa and 5000 K. Nickel effects on elasticity might be important at multimegabar pressure and especially high temperature such as the inner core condition. The core density deficit was estimated to be 3.4(1)-5.1(1)% for pure iron and 4.7(1)-6.5(2)% for Fe0.9Ni0.1 alloy if the temperature at the inner core boundary is 5000 K. (C) 2014 Elsevier B.V. All rights reserved.
  • Seiji Kamada, Eiji Ohtani, Hidenori Terasaki, Takeshi Sakai, Suguru Takahashi, Naohisa Hirao, Yasuo Ohishi
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 228 106 - 113 2014年03月 [査読有り]
     研究論文(学術雑誌) 
    The equation of state of Fe3S was investigated up to 200 GPa at room temperature using a diamond anvil cell. Fe3S adopts a tetragonal structure up to 200 GPa and no phase transition was observed. The fourth-order Birch-Murnaghan equation of state (EOS) was fitted to present compression data at room temperature. The elastic parameters, such as bulk modulus (K-0), its pressure derivative (K-0'), and K-0 ''(dK'/dP) were determined to be 122.4(50) GPa, 5.36(48), and -0.066(30) GPa(-1), respectively by fixing the zero pressure volume, V-0, to be 377 angstrom(3). Based on fourth-order Birch-Murnaghan EOS of Fe3S, the maximum amount of S in the inner core was estimated to be 11.4(14) at.% based on the density deficit of the inner core. (C) 2013 Elsevier B.V. All rights reserved.
  • Seiji Kamada, Eiji Ohtani, Hiroshi Fukui, Takeshi Sakai, Hidenori Terasaki, Suguru Takahashi, Yuki Shibazaki, Satoshi Tsutsui, Alfred Q. R. Baron, Naohisa Hirao, Yasuo Ohishi
    AMERICAN MINERALOGIST 99 1 98 - 101 2014年01月 [査読有り]
     研究論文(学術雑誌) 
    We measured the sound velocity of Fe3S at room temperature up to 85 GPa employing inelastic X-ray scattering to better constrain the constitution of the inner core. The density of Fe3S was also determined by X-ray diffraction under the same conditions. The relation of the P-wave velocity (v(P)) and density (rho) of Fe3S follows Birch's law, v(P)(m/s) = 1.14(5) x rho(kg/m(3)) - 2580(410). Based on Birch's law determined here for Fe3S and that for epsilon-Fe reported previously, we found that sulfur decreases both density and compressional velocity of hcp-Fe at the core pressure and 300 K.
  • Ohira, I., Ohtani, E., Sakai, T., Miyahara, M., Hirao, N., Ohishi, Y., Nishijima, M.
    Earth and Planetary Science Letters 401 12 - 17 2014年 [査読有り]
     研究論文(学術雑誌)
  • Hiroshi Fukui, Takeshi Sakai, Tatsuya Sakamaki, Seiji Kamada, Suguru Takahashi, Eiji Ohtani, Alfred Q. R. Baron
    REVIEW OF SCIENTIFIC INSTRUMENTS 84 11 2013年11月 [査読有り]
     研究論文(学術雑誌) 
    A compact system for generating extreme pressures and temperatures was developed for versatile experiments based on laser-heated diamond anvil cell technique. This system has been used for inelastic X-ray scattering measurements for iron. (C) 2013 AIP Publishing LLC.
  • Eiji Ohtani, Yuki Shibazaki, Takeshi Sakai, Kenji Mibe, Hiroshi Fukui, Seiji Kamada, Tatsuya Sakamaki, Yusuke Seto, Satoshi Tsutsui, Alfred Q. R. Baron
    Geophysical Research Letters 40 19 5089 - 5094 2013年10月 [査読有り]
     研究論文(学術雑誌) 
    We determined the compressional velocity of hcp-Fe using high-resolution inelastic X-ray scattering combined with in situ X-ray powder diffraction: Our measurements extend up to 174 GPa at room temperature, to 88 GPa at 700 K, and to 61.5 GPa at 1000 K. Our data, including those obtained at high temperature, are well described by a linear relation to density, extending the range of verification of Birch's law and suggesting only small temperature dependence up to 1000 K. This result, once compared to the preliminary reference Earth model seismologically based model, indicates that there is either a strong temperature effect on Birch's law above 1000 K or the composition of the core is rather different than expected, containing, e.g., heavy impurities. Noting that both recent theoretical calculations and shock wave velocity measurements are consistent with modification of Birch's law at high temperature, we favor the former explanation. Key Points We measured the velocity of hcp-Fe to 174 GPa and 300 K by IXS We confirmed no temperature dependence on Birch's law to 60 GPa and 1000 K We argued the light elements in the inner core based on the present study ©2013. American Geophysical Union. All Rights Reserved.
  • Masaaki Miyahara, Shin Ozawa, Eiji Ohtani, Makoto Kimura, Tomoaki Kubo, Takeshi Sakai, Toshiro Nagase, Masahiko Nishijima, Naohisa Hirao
    EARTH AND PLANETARY SCIENCE LETTERS 373 102 - 108 2013年07月 [査読有り]
     研究論文(学術雑誌) 
    Albitic feldspar in shocked ordinary chondrites (Yamato 791384 L6 and Yamato 75100 H6) and albite recovered from static high-pressure and high-temperature synthetic experiments (Kubo et al., 2010) were investigated with a transmission electron microscope (TEM) subsequent to a conventional micro-Raman spectroscopy analysis to clarify albite dissociation reaction under high-pressure and high-temperature condition. When jadeite forms from albite, SiO2 phase as a residual phase of albite dissociation reaction should accompany jadeite from the stoichiometry. However, albitic feldspar in and adjacent to shock-melt veins of the shocked chondrites dissociates into jadeite+residual amorphous (or poorly-crystallized) material having varied chemical compositions between jadeite and SiO2 phase. TEM observations of albitic feldspar in the shocked chondrites and albite recovered from the static high-pressure and high-temperature synthetic experiments show that jadeite crystallization is initiated by grain refinement of albite (or albitic feldspar). Nucleation occurs along grain-boundaries or at triple-junctions of the fine-grained albite crystal assemblage. Jadeite crystal starts to grow from the nucleus through grain-boundary diffusion. Considering pressure condition recorded in the shock-melt veins of the shocked chondrites, stishovite is the most likely as a residual SiO2 phase accompanying jadeite. High-pressure and high-temperature condition induced by a dynamic event is very short. Stishovite would be hardy formed through a dynamic event due to sluggish nucleation rate of stishovite compared with that of jadeite, thus leading to induce heterogeneous and incomplete albite dissociation reaction; albite dissociates into jadeite+residual amorphous material. (C) 2013 Elsevier B.V. All rights reserved.
  • Masaaki Miyahara, Shohei Kaneko, Eiji Ohtani, Takeshi Sakai, Toshiro Nagase, Masahiro Kayama, Hirotsugu Nishido, Naohisa Hirao
    NATURE COMMUNICATIONS 4 2013年04月 [査読有り]
     研究論文(学術雑誌) 
    Many craters and thick regoliths of the moon imply that it has experienced heavy meteorite bombardments. Although the existence of a high-pressure polymorph is a stark evidence for a dynamic event, few high-pressure polymorphs are found in a lunar sample. alpha-PbO2-type silica (seifertite) is an ultrahigh-pressure polymorph of silica, and is found only in a heavily shocked Martian meteorite. Here we show evidence for seifertite in a shocked lunar meteorite, Northwest Africa 4734. Cristobalite transforms to seifertite by high-pressure and -temperature condition induced by a dynamic event. Considering radio-isotopic ages determined previously, the dynamic event formed seifertite on the moon, accompanying the complete resetting of radio-isotopic ages, is similar to 2.7 Ga ago. Our finding allows us to infer that such intense planetary collisions occurred on the moon until at least similar to 2.7 Ga ago.
  • Seiji Kamada, Eiji Ohtani, Hidenori Terasaki, Takeshi Sakai, Masaaki Miyahara, Yasuo Ohishi, Naohisa Hirao
    EARTH AND PLANETARY SCIENCE LETTERS 359 26 - 33 2012年12月 [査読有り]
     研究論文(学術雑誌) 
    In situ X-ray diffraction experiments in the Fe-Fe3S system were performed up to 175 GPa and 3500 K using a laser-heated diamond anvil cell to investigate melting relationships in the system. Partial melting in the Fe-Fe3S system was observed based on the disappearance of X-ray diffraction peaks of solid Fe3S and texture observation of the recovered samples. The melting relationship of the Fe-Fe3S system as a function of pressure is evaluated based on Kraut-Kennedy law. Our results of melting relationships suggest that the temperature at the inner core boundary is between 4700(160) and 4930(330) K if sulfur is the only light element in the Earth's core. Assuming the adiabatic temperature gradient in the outer core, the temperature at the core-mantle boundary is estimated to be in the range of 3600-3770 K. The present temperature profile of the core is consistent with the core-mantle boundary temperature that can explain the core heat flux to maintain the core dynamo and the seismic structure at the base of the lower mantle. (C) 2012 Elsevier B.V. All rights reserved.
  • Hidenori Terasaki, Eiji Ohtani, Takeshi Sakai, Seiji Kamada, Hidetoshi Asanuma, Yuki Shibazaki, Naohisa Hirao, Nagayoshi Sata, Yasuo Ohishi, Tatsuya Sakamaki, Akio Suzuki, Ken-ichi Funakoshi
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 194 18 - 24 2012年03月 [査読有り]
     研究論文(学術雑誌) 
    The hydrous mineral, delta-AlOOH, is stable up to at least the core-mantle boundary, and therefore has been proposed as a water carrier to the Earth's deep mantle. If delta-AlOOH is transported down to the core-mantle boundary by a subducting slab or the mantle convection, then the reaction between the iron alloy core and delta-AlOOH is important in the deep water/hydrogen cycle in the Earth. Here we conducted an in situ X-ray diffraction study to determine the behavior of hydrogen between Fe-Ni alloys and delta-AlOOH up to near the core-mantle boundary conditions. The obtained diffraction spectra show that fcc/dhcp Fe-Ni hydride is stable over a wide pressure range of 19-121 GPa at high temperatures. Although the temperature of formation of Fe-Ni hydride tends to increase up to 1950 K with increasing pressure to 121 GPa, this reaction temperature is well below the mantle geotherm. delta-AlOOH was confirmed to coexist stably with perovskite, suggesting that delta-AlOOH can be a major hydrous phase in the lower mantle. Therefore, when delta-AlOOH contacts with the core at the core-mantle boundary, the hydrogen is likely to dissolve into the Earth's core. Based on the present results, the amount of hydrogen to explain the core density deficit is estimated to be 1.0-2.0 wt.%. (C) 2012 Elsevier B.V. All rights reserved.
  • Takeshi Sakai, Eiji Ohtani, Seiji Kamada, Hidenori Terasaki, Naohisa Hirao
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 117 2012年02月 [査読有り]
     研究論文(学術雑誌) 
    Fe88.1Ni9.1S2.8 alloy was compressed up to 335 GPa, corresponding to the pressure at the Earth's inner core, and the hexagonal close-packed structure was found to be stable. The axial (c/a) ratio gradually decreased with increasing pressure. A linear fit as a function of pressure gave c/a = 1.605(2)-6.1(9) x 10(-5)P for P in GPa. The compression curve of Fe88.1Ni9.1S2.8 alloy was expressed by the third-order Birch-Murnaghan equation of state, giving K-0 = 167.0 +/- 15.0 GPa, K-0' = 4.46 +/- 0.14, and V-0 = 22.93 +/- 0.29 angstrom(3). Our results indicate that the hcp Fe-5 wt % Ni-5.7 wt % S alloy can account for the density of the inner core at 328.9 GPa, assuming a linear relationship exists between the density and the nickel and sulfur content.
  • Yuki Shibazaki, Eiji Ohtani, Hiroshi Fukui, Takeshi Sakai, Seiji Kamada, Daisuke Ishikawa, Satoshi Tsutsui, Alfred Q. R. Baron, Naoya Nishitani, Naohisa Hirao, Kenichi Takemura
    EARTH AND PLANETARY SCIENCE LETTERS 313 79 - 85 2012年01月 [査読有り]
     研究論文(学術雑誌) 
    We have determined the density evolution of the sound velocity of dhcp-FeHx (x approximate to 1) up to 70 GPa at room temperature, by inelastic X-ray scattering and by X-ray diffraction. We find that the variation of V-p with density is different for the ferromagnetic and nonmagnetic dhcp-FeHx, and that only nonmagnetic dhcp-FeHx follows Birch's law. Combining our results with Birch's law for iron and assuming an ideal two-component mixing model, we obtain an upper bound of the hydrogen content in the Earth's inner core, 0.23(6) wt.% H, corresponding to FeH0.13(3). The iron alloy with 0.23(6) wt.% H can satisfy the density, and compressional and shear sound velocities of the PREM inner core, assuming that there are no other light elements in the inner core. (C) 2011 Elsevier B.V. All rights reserved.
  • Hidetoshi Asanuma, Eiji Ohtani, Takeshi Sakai, Hidenori Terasaki, Seiji Kamada, Naohisa Hirao, Yasuo Ohishi
    EARTH AND PLANETARY SCIENCE LETTERS 310 1-2 113 - 118 2011年10月 [査読有り]
     研究論文(学術雑誌) 
    The pressure-volume equations of state of iron-nickel-silicon alloy Fe0.83Ni0.09Si0.08 (Fe-9.8 wt.% Ni-4.0 wt.% Si) and iron-silicon alloy Fe0.93Si0.07 (Fe-3.4 wt.% Si) have been investigated up to 374 GPa and 252 GPa, respectively. The present compression data covered pressures of the Earth's core. We confirmed that both Fe0.83Ni0.09Si0.08 and Fe0.93Si0.07 alloys remain in the hexagonal close packed structure at all pressures studied. We obtained the density of these alloys at the pressure of the inner core boundary (ICB), 330 GPa at 300 K by fitting the compression data to the third order Birch-Murnaghan equation of state. Using these density values combined with the previous data for hcp-Fe, hcp-Fe0.8Ni0.2, and hcp-Fe0.84Si0.16 alloys and comparing with the density of the PREM inner core, we estimated the Ni and Si contents of the inner core. The Si content of the inner core estimated here is slightly greater than that estimated previously based on the sound velocity measurement of the hcp-Fe-Ni-Si alloy at high pressure. (C) 2011 Elsevier B.V. All rights reserved.
  • Takeshi Sakai, Eiji Ohtani, Hidenori Terasaki, Seiji Kamada, Naohisa Hirao, Masaaki Miyahara, Masahiko Nishijima
    AMERICAN MINERALOGIST 96 10 1490 - 1494 2011年10月 [査読有り]
     研究論文(学術雑誌) 
    An in situ synchrotron powder X-ray diffraction study on (Fe-0.89,Ni-0.11)(3)S was conducted up to 141 GPa and 1590 K. (Fe-0.89,Ni-0.11)(3)S has a tetragonal structure, which is the same structure as Ni-free Fe3S. Fitting a third-order Birch-Murnaghan equation of state to data at ambient temperature yielded a bulk modulus of K-0 = 138.1(7.2) GPa and its pressure derivative K'(0) = 4.5(3) with a zero pressure volume V-0 = 375.67(4) angstrom(3). The density of (Fe-0.89,Ni-0.11)(3)S under the core-mantle boundary condition is 1.7% greater than that of Fe3S. The axial ratio (c/a) of (Fe-0.89,Ni-0.11)(3)S decreases with increasing pressure. The addition of nickel to Fe3S leads to a softening of the c-axis. Assuming that the nickel content of the outer core is about 5 at%, we estimated 12.3-20.8 at% sulfur in the outer core for the given 6-10% density deficit between the outer core and pure iron at 136 GPa.
  • First evidence for natural dissociation of olivine to silicate-perovskite and magnesiowustite in a shocked Martian meteorite DAG 735.
    Miyahara M, Ohtani E, Ozawa S, Kimura M, El Goresy A, Sakai T, Nagase T, Hiraga K, Hirao N, Ohishi Y
    Meteoritics & Planetary Science 46 SI, 1 A164 - A164 2011年07月 [査読有り]
     研究論文(学術雑誌)
  • Takeshi Sakai, Eiji Ohtani, Naohisa Hirao, Yasuo Ohishi
    GEOPHYSICAL RESEARCH LETTERS 38 2011年05月 [査読有り]
     研究論文(学術雑誌) 
    The phase relations of Fe, Fe90Ni10 and Fe87.9Ni4.4Si7.7 were investigated up to 273(6) GPa and 4490(560) K, 250(19) GPa and 2730(110) K and 304(3) GPa and 2780(210) K, respectively, and the hexagonal close-packed structure was found to be stable in all of these compounds under the pressure and temperature conditions studied. We did not observe other phases such as a body-centered cubic structure. The axial ratio (c/a) of Fe87.9Ni4.4Si7.7 at 300 GPa showed an almost constant value of 1.593-1.596 against temperature. The weak temperature dependency of the axial ratio of hcp-Fe-Ni-Si alloy indicates that the c-axis is still harder than the a-axis at high temperature condition. Therefore the alignment of hcp-Fe-Ni-Si alloy with the c-axis parallel to the Earth's rotation axis could account for the observation of the seismic wave anisotropy of the inner core. Citation: Sakai, T., E. Ohtani, N. Hirao, and Y. Ohishi (2011), Stability field of the hcp-structure for Fe, Fe-Ni, and Fe-Ni-Si alloys up to 3 Mbar, Geophys. Res. Lett., 38, L09302, doi: 10.1029/2011GL047178.
  • H. Terasaki, S. Kamada, T. Sakai, E. Ohtani, N. Hirao, Y. Ohishi
    EARTH AND PLANETARY SCIENCE LETTERS 304 3-4 559 - 564 2011年04月 研究論文(学術雑誌) 
    The solidus and liquidus temperatures of the Fe(75)O(5)S(20) alloy are determined up to 157 GPa using a laser-heated diamond anvil cell combined with in situ X-ray diffraction technique. Fe (fcc/hcp), Fe(3)S(2)/Fe(3)S. and FeO B1/B8/rhombohedral phases are stable under subsolidus conditions. First, Fe(3)S(2) or Fe(3)S phase melts at a temperature close to the eutectic point of the Fe-Fe(3)S system, suggesting that the alloying effect of 5 at.% oxygen on the eutectic temperature in the Fe-Fe(3)S system is minor. Then FeO melts at several hundreds of degrees Kelvin higher than the solidus, and Fe is a liquidus phase in this system. The liquidus temperature is 260-670 K lower than the melting temperature of pure Fe because of the alloying effect of S and O on the melting temperature of Fe. Based on our results, the temperatures at the core/mantle boundary (T(CMB)) and at the boundary of the inner/outer core (T(ICB)) are estimated to be 3600 +/- 200 < T(CMB)< 4310 +/- 350 K and T(ICB)similar to 5630 +/- 350 K, respectively. These results provide important constraints on the thermal structure of the Earth's core. (C) 2011 Elsevier B.V. All rights reserved.
  • Masaaki Miyahara, Eiji Ohtani, Shin Ozawa, Makoto Kimura, Ahmed El Goresy, Takeshi Sakai, Toshiro Nagase, Kenji Hiraga, Naohisa Hirao, Yasuo Ohishi
    PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 108 15 5999 - 6003 2011年04月 [査読有り]
     研究論文(学術雑誌) 
    We report evidence for the natural dissociation of olivine in a shergottite at high-pressure and high-temperature conditions induced by a dynamic event on Mars. Olivine (Fa(34-41)) adjacent to or entrained in the shock melt vein and melt pockets of Martian meteorite olivine-phyric shergottite Dar al Gani 735 dissociated into (Mg,Fe)SiO3 perovskite (Pv)+magnesiowustite (Mw), whereby perovskite partially vitrified during decompression. Transmission electron microscopy observations reveal that microtexture of olivine dissociation products evolves from lamellar to equigranular with increasing temperature at the same pressure condition. This is in accord with the observations of synthetic samples recovered from high-pressure and high-temperature experiments. Equigranular (Mg,Fe)SiO3 Pv and Mw have 50-100 nm in diameter, and lamellar (Mg,Fe)SiO3 Pv and Mw have approximately 20 and approximately 10 nm in thickness, respectively. Partitioning coefficient, KPv/Mw [FeO/MgO]/[FeO/MgO](Mw), between (Mg,Fe)SiO3 Pv and Mw in equigranular and lamellar textures are approximately 0.15 and approximately 0.78, respectively. The dissociation of olivine implies that the pressure and temperature conditions recorded in the shock melt vein and melt pockets during the dynamic event were approximately 25 GPa but 700 degrees C at least.
  • Takeshi Sakai, Eiji Ohtani, Naohisa Hirao, Yasuo Ohishi
    JOURNAL OF APPLIED PHYSICS 109 8 2011年04月 [査読有り]
     研究論文(学術雑誌) 
    We compressed NaCl-B2 to 304 GPa and presented a pressure scale based on the Birch-Murnaghan and Vinet equations of state for this phase. The pressure difference between the two formulations of the equation of state was less than 2.0% within the pressure range of 50-364 GPa. The present data indicates that the uniaxial stress of the platinum used in the sample is 2.4 GPa and comparable with that of runs using a helium pressure medium. The compression curve of NaCl-B2 was expressed by the Birch-Murnaghan equation of state with fixed volume at ambient pressure, where V(0) = 37.73(4.05) angstrom(3), K = 47.00(46) GPa and K' = 4.10(2) based upon Matsui et al.'s platinum scale. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3573393]
  • 鎌田誠司, 寺崎英紀, 大谷栄治, 境毅
    高圧力の科学と技術 21 2 77 - 83 2011年 
    The Earth's core has been considered to contain light elements, and sulfur, in particular, is one of the most plausible light elements. Knowledge of the melting relationships of the iron-sulfide system is thus essential in understanding of the physical and chemical properties of the core. In situ X-ray diffraction experiments in the Fe-Fe3S system were performed up to 220 GPa and 3300 K using a laser-heated diamond anvil cell. Hcp Fe and Fe3S coexisted stably up to 220 GPa and 3300 K. Both phases are therefore candidates of the constitution of the inner core. The solid iron (hcp Fe) contained 7.5 at% of sulfur at 126 GPa and 2370 K. This suggests that the inner core might be able to contain significant amount of sulfur. Our results revealed that the eutectic composition becomes nonsensitive to pressure. This is likely that the eutectic composition becomes to be constant around 20 at% of sulfur at pressures above 40 GPa.
  • Alfred Q.R. Baron, Masanori Inui, Ejii Ohtani, Hiroshi Fukui, Shinya Hosokawa, Tadahiko Masaki, Daisuke Ishikawa, Takeshi Sakai, Junpei T. Okada, Satoshi Tsutsui, Hiroshi Uchiyama
    Synchrotron Radiation News 23 6 17 - 25 2010年12月 研究論文(学術雑誌)
  • Takeshi Sakai, Eiji Ohtani, Hidenori Terasaki, Masaaki Miyahara, Masahiko Nishijima, Naohisa Hirao, Yasuo Ohishi, Nagayoshi Sata
    PHYSICS AND CHEMISTRY OF MINERALS 37 7 487 - 496 2010年07月 [査読有り]
     研究論文(学術雑誌) 
    Fe-Mg partitioning between post-perovskite and ferropericlase has been studied using a laser-heated diamond anvil cell at pressures up to 154 GPa and 2,010 K which corresponds to the conditions in the lowermost mantle. The composition of the phases in the recovered samples was determined using analytical transmission electron microscopy. Our results reveal that the Fe-Mg partition coefficient between post-perovskite and ferropericlase (K (D) (PPv/Fp) ) increases with decreasing bulk iron content. The compositional dependence of K (D) (PPv/Fp) on the bulk iron content explains the inconsistency in previous studies, and the effect of the bulk iron content is the most dominant factor compared to other factors, such as temperature and aluminum content. Iron prefers ferropericlase compared to post-perovskite over a wide compositional range, whereas the iron content of post-perovskite (X (Fe) (PPv) , the mole fraction) does not exceed a value of 0.10. The iron-rich ferropericlase phase may have significant influence on the physical properties, such as the seismic velocity and electrical conductivity at the core-mantle boundary region.
  • Hidetoshi Asanuma, Eiji Ohtani, Takeshi Sakai, Hidenori Terasaki, Seiji Kamada, Tadashi Kondo, Takumi Kikegawa
    PHYSICS AND CHEMISTRY OF MINERALS 37 6 353 - 359 2010年06月 [査読有り]
     研究論文(学術雑誌) 
    The melting temperature of Fe-18 wt% Si alloy was determined up to 119 GPa based on a change of laser heating efficiency and the texture of the recovered samples in the laser-heated diamond anvil cell experiments. We have also investigated the subsolidus phase relations of Fe-18 wt% Si alloy by the in-situ X-ray diffraction method and confirmed that the bcc phase is stable at least up to 57 GPa and high temperature. The melting curve of the alloy was fitted by the Simon's equation, P(GPa)/a = (T (m)(K)/T (0)) (c) , with parameters, T (0) = 1,473 K, a = 3.5 +/- A 1.1 GPa, and c = 4.5 +/- A 0.4. The melting temperature of bcc Fe-18 wt% Si alloy is comparable with that of pure iron in the pressure range of this work. The melting temperature of Fe-18 wt% Si alloy is estimated to be 3,300-3,500 K at 135 GPa, and 4,000-4,200 K at around 330 GPa, which may provide the lower bound of the temperatures at the core-mantle boundary and the inner core-outer core boundary if the light element in the core is silicon.
  • Seiji Kamada, Hidenori Terasaki, Eiji Ohtani, Takeshi Sakai, Takumi Kikegawa, Yasuo Ohishi, Naohisa Hirao, Nagayoshi Sata, Tadashi Kondo
    EARTH AND PLANETARY SCIENCE LETTERS 294 1-2 94 - 100 2010年05月 [査読有り]
     研究論文(学術雑誌) 
    In situ X-ray diffraction experiments in the Fe-FeS system were performed up to 220 GPa and 3300 K using a laser-heated diamond anvil cell. Fe3S and epsilon-Fe coexisted stably up to 220 GPa and 3300 K, and thus, Fe3S is likely to be the stable S-bearing iron alloy under the Earth's core conditions. The solid iron (E-Fe) also contained 7.6(0.8) at.% of sulfur at 86 GPa and 2200 K. The amount of sulfur in the solid iron increased with increasing pressure at the eutectic temperatures. If the sulfur content obtained in this study is extrapolated to the conditions at the inner core, all the sulfur in the solid inner core can be stored in epsilon-Fe. The eutectic composition becomes nonsensitive to pressure and seems to be constant around 20 at.% of sulfur at pressures above 40 GPa. The pressure gradient of the melting curve of the Fe-FeS system is 13.4 (0.7) K/GPa. Based on our results of melting relationship, the temperature at the core-mantle boundary should be greater than 2850(100) K, assuming that sulfur is the only light element in the Earth's liquid outer core. (C) 2010 Elsevier B.V. All rights reserved.
  • Daniel J. Frost, Yuki Asahara, David C. Rubie, Nobuyoshi Miyajima, Leonid S. Dubrovinsky, Christian Holzapfel, Eiji Ohtani, Masaaki Miyahara, Takeshi Sakai
    JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 115 2010年02月 [査読有り]
     研究論文(学術雑誌) 
    Experiments to investigate the partitioning of oxygen between liquid iron and (Mg,Fe)O magnesiowustite were conducted at 30-70 GPa and 2800-3500 K using a laser-heated diamond anvil cell. A thin foil was prepared from the reacted regions in the recovered samples using a focused ion beam. The compositions of coexisting quenched iron and magnesiowustite were measured using a transmission electron microscope equipped with energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy. In order to understand and model the results, additional experiments were performed to determine the activity of oxygen, or rather FeO, in liquid Fe metal. Multianvil experiments to measure the oxygen contents of coexisting immiscible metallic and ionic liquids in the Fe-FeO system were performed up to 25 GPa. The results were used to extract excess mixing properties for Fe-FeO liquids at high pressure and temperature. These properties were used to derive a model that describes oxygen partitioning in the Fe-Mg-O system that is independent of the actual experimental partitioning data. The model indicates that the oxygen content of liquid Fe becomes a strong nonlinear function of the FeO content of magnesiowustite at pressures greater than 25 GPa. This prediction is in excellent agreement with the experimental partitioning data, which is faithfully reproduced in most instances. The new results confirm that the Earth's core is undersaturated in oxygen with respect to the FeO content of the bulk mantle, which will result either in FeO being depleted from the very base of the mantle or lead to the development of an FeO-enriched outer layer of the core. These possibilities are not mutually exclusive.
  • Natural dissociation of olivine to perovskite and magnesiowustite in a Martian meteorite
    Miyahara M, Ohtani E, Ozawa S, Kimura M, El Goresy A, Sakai T, Nagase T, Hiraga K, Hirao N, Ohishi Y
    Annual report of Bayerisches Forschungsinstitut fur Experimentelle Geochemie und Geophysik, Universitat Bayreuth 173 - 176 2010年 [査読有り]
     研究論文(大学,研究機関等紀要)
  • Takeshi Sakai, Eiji Ohtani, Hidenori Terasaki, Naoki Sawada, Yusuke Kobayashi, Masaaki Miyahara, Masahiko Nishijima, Naohisa Hirao, Yasuo Ohishi, Takumi Kikegawa
    AMERICAN MINERALOGIST 94 7 921 - 925 2009年07月 [査読有り]
     研究論文(学術雑誌) 
    Fe-Mg partitioning between perovskite and ferropericlase in the MgO-FeO-SiO2 system has been studied up to about 100 GPa at around 2000 K using a laser-heated diamond anvil cell (LHDAC). The compositions of both phases were determined by using analytical transmission electron microscopy (ATEM) on the recovered samples. Present results reveal that the Fe-Mg apparent partition coefficient between perovskite and ferropericlase [K-D(Pv/Fp) = ((XFeXMgFp)-X-Pv)/((XMgXFeFp)-X-Pv)] decreases with increasing pressure for a constant FeO of the system, and it decreases with increasing FeO content of ferropericlase. The gradual decrease of K-D(Pv/FP) with increasing pressure is consistent with the spin transition in ferropericlase Occurring in the broad pressure range from 50 to 100 GPa at around 2000 K.
  • Eiji Ohtani, Takeshi Sakai
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 170 3-4 240 - 247 2008年11月 [査読有り]
     研究論文(学術雑誌) 
    We review recent advances in the study of phase transitions of minerals in the Earth's mantle, including unsolved problems regarding these processes. The phase boundaries of (Mg,Fe)(2)SiO4 in the mantle transition zone are modified under the wet conditions by changes in thermodynamic properties of its polymorphs because of differences in the water solubilities of these minerals. The shift of phase boundaries has significant implications for the topography of the 410 km and 660 km seismic discontinuities. The post-perovskite phase with CaIrO3 Structure exists at pressures above 110 GPa and at high temperature. The effects of Al and Fe on phase transition are under debate. Several post-stishovite phases have been reported but equilibrium boundaries and existence in the real lower mantle are also still debatable. Spin transition occurs in magnesiowustite, perovskite, and post-perovskite phases at high pressures of the lower mantle and core. The pressure interval in the spin transition can be large at high temperature and so the transition might not cause a discrete change in physical properties under real Earth conditions. (C) 2008 Elsevier B.V. All rights reserved.
  • Hidetoshi Asanuma, Eiji Ohtani, Takeshi Sakai, Hidenori Terasaki, Seiji Kamada, Naohisa Hirao, Nagayoshi Sata, Yasuo Ohishi
    GEOPHYSICAL RESEARCH LETTERS 35 12 2008年06月 [査読有り]
     研究論文(学術雑誌) 
    X-ray diffraction experiments were conducted to 257 GPa and high temperature in situ on an iron-silicon alloy containing 3.4 wt% silicon, a candidate for the Earth's inner core forming material. The results revealed that fcc and hcp phases coexist up to 104 GPa. A single hcp phase is stable at higher pressures at least up to 3600 K at 242 GPa and to 2400 K at 257 GPa. Dissolution of silicon in the liquid outer core following reaction with the silicate mantle during core formation strongly suggests the existence of silicon in the solid inner core. Our results revealed that the iron-3.4 wt% silicon alloy in the inner core is likely to possess an hcp structure, which can explain the inner core anisotropy observed in seismology.
  • Masaaki Miyahara, Takeshi Sakai, Eiji Ohtani, Yusuke Kobayashi, Seiji Kamada, Tadashi Kondo, Toshiro Nagase, Jung Ho Yoo, Masahiko Nishijima, Zahra Vashaei
    JOURNAL OF MINERALOGICAL AND PETROLOGICAL SCIENCES 103 2 88 - 93 2008年04月 [査読有り]
     研究論文(学術雑誌) 
    The conventional focused ion beam (FIB) sample preparation technique "lift-out method" was modified for the reliable analysis of a laser-heated diamond anvil cell (LHDAC) sample. Box-shaped grooves were prepared in the LHDAC specimen in order to recover a block using a gallium ion beam in the vicinity of the area to be characterized by transmission electron microscope (TEM). The recovered block was fixed on the stage of a copper (or molybdenum) grid and thinned by the gallium ion beam in order to obtain a TEM foil. Our modified lift-out method allows us to thin the entire vertical section (from the upper anvil surface to the lower anvil surface) of the LHDAC sample.
  • Asami Sano, Eiji Ohtani, Tadashi Kondo, Naohisa Hirao, Takeshi Sakai, Nagayoshi Sata, Yasuo Ohishi, Takumi Kikegawa
    GEOPHYSICAL RESEARCH LETTERS 35 3 2008年02月 [査読有り]
     研究論文(学術雑誌) 
    In-situ X-ray diffraction experiments were conducted to determine the stability field of the aluminous hydrous mineral delta-AlOOH over the entire conditions of the lower mantle. We observed delta-AlOOH at pressures from 33 to 134 GPa at 1350-2300 K, indicating that this hydrous mineral in the sediment layer of a subducting slab could transport hydrogen into the core-mantle boundary (CMB).
  • 境毅
    高圧力の科学と技術 18 1 38 - 43 2008年 
    Analytical transmission electron microscope (ATEM) is a powerful tool for analyses of the samples recovered from ultrahigh pressure experiments. Recently, Focused Ion Beam (FIB) system has been applied to prepare a TEM foil of sample recovered from laser heated diamond anvil cell (LHDAC). It has some advantages compared to conventional argon ion milling method. In this article, recent advances in the DAC sample preparation for TEM observation using FIB system are reviewed.
  • Naohisa Hirao, Eiji Ohtani, Tadashi Kondo, Takeshi Sakai, Takumi Kikegawa
    PHYSICS OF THE EARTH AND PLANETARY INTERIORS 166 1-2 97 - 104 2008年01月 [査読有り]
     研究論文(学術雑誌) 
    High-pressure and high-temperature experiments on the KAlSi(3)O(8) composition were conducted in a laser-heated diamond-anvil cell at pressures up to 128 GPa, which correspond to the lowermost mantle conditions. In situ synchrotron X-ray diffraction measurements revealed that the hollandite II phase in KAlSi(3)O(8) with a monoclinic symmetry of I2/m, was stable over the entire range of mantle conditions, and the tunnel structure formed by the double chains of edge-sharing (Si,AI)O(6) octahedra, which could accommodate a larger cation such as potassium, was sustained. The (Si,AI)O(6) octahedra in the KAlSi(3)O(8) hollandite II phase showed a similar compression behavior to those in high-pressure silicate structures, such as rutile-type and perovskite-type phases, and were found to be less compressible than the KO(8) polyhedra. The KAlSi(3)O(8) hollandite II phase is a potential host mineral for potassium under lower mantle conditions and, therefore, may have a significant influence on geochemistry if potassium feldspar KAlSi(3)O(8) in the Earth's crust is transported into the Earth's mantle through subduction. (C) 2007 Elsevier B.V. All rights reserved.
  • Takeshi Sakai, Tadashi Kondo, Eiji Ohtani, Hidenori Terasaki, Noriaki Endo, Toshiko Kuba, Toshiaki Suzuki, Takumi Kikegawa
    GEOPHYSICAL RESEARCH LETTERS 33 15 2006年08月 [査読有り]
     研究論文(学術雑誌) 
    Interaction between the lower mantle and core is essential for understanding the nature of D '' layer at the core-mantle boundary (CMB). Here, we report the reaction between post-perovskite (PPv) and metallic iron under the condition of the CMB, for example, 139 GPa and 3000 Kelvin. Analytical transmission electron microscope ( ATEM) analysis revealed that significant amount of oxygen up to 6.3 weight percent (wt.%) and silicon up to 4.0 wt.% can be dissolved into molten iron. The dihedral angle between PPv and molten iron is 67 degrees. Thus, a small amount of core metal of about 2 volume percent (vol.%) can be trapped without separation in the PPv region at the CMB. The amount of core metal trapped by this mechanism can produce the isotopic signature of the outer core in the plume source at the base of the lower mantle.

書籍

  • 地球科学の事典
    鳥海光弘 (担当:分担執筆, 範囲:3章3.9 核の物質科学)
    朝倉書店 2018年04月

MISC

受賞

  • 2019年04月 愛媛大学理学部 理学部ベストレクチャー賞
     
    受賞者: 境 毅
  • 2017年09月 日本鉱物科学会 日本鉱物科学会研究奨励賞
     
    受賞者: 境 毅

共同研究・競争的資金等の研究課題

  • 極高圧条件における内部整合圧力スケール群の構築
    日本学術振興会:基盤研究(B)
    研究期間 : 2017年04月 -2021年03月 
    代表者 : 境 毅
  • 静的圧縮による1テラパスカル発生への挑戦
    日本学術振興会:挑戦的萌芽研究
    研究期間 : 2015年04月 -2018年03月 
    代表者 : 境 毅
  • マルチメガバール領域における基礎物性データの取得~地球からスーパーアースへ~
    日本学術振興会:若手研究(A)
    研究期間 : 2011年04月 -2015年03月 
    代表者 : 境 毅
  • マルチメガバール領域での圧力スケールの確立と地球中心核像の再構築
    日本学術振興会:若手研究(B)
    研究期間 : 2009年04月 -2011年03月 
    代表者 : 境 毅
  • 地球の核圧力条件下での鉄-軽元素系の融点の決定と温度構造モデルの構築
    日本学術振興会:若手研究(スタートアップ)
    研究期間 : 2007年04月 -2009年03月 
    代表者 : 境 毅

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


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