2025/12/26 更新

写真a

ナカニシ　ナオ
中西　奈央
NAKANISHI NAO
所属
理学院 助教
職名
助教
外部リンク

経歴

論文

  • “CY1” Chondrites Produced by Impact Dehydration of the CI Chondrite Parent Body

    Ke Zhu, Nao Nakanishi, Jan Render, Quinn R. Shollenberger, Tetsuya Yokoyama, Akira Ishikawa, Lu Chen

    The Astrophysical Journal Letters   984 ( 2 )   L54 - L54   2025年5月

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    掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

    Abstract

    The recently proposed Yamato-type (CY) chondrites share significant similarities with CI chondrites and Ryugu. We present major and trace elemental, Re–Os, and mass-independent Ti, Cr, and Fe isotope data for seven CY chondrites. The elemental data along with isotopic compositions reveal two distinct lithologies, here designated as CY1 and CY2, potentially originating from two different parent bodies. Although sharing similarities with CM chondrites, CY2 chondrites have distinct Cr isotope compositions, arguing against a close genetic relationship. The CY1 lithology exhibits elemental abundances similar to CI chondrites/Ryugu as well as Fe, Ti, and Cr isotope compositions that closely overlap with those of CI chondrites/Ryugu. This suggests that CI chondrites, CY1 chondrites, and Ryugu accreted in the same region of the solar system and may even originate from the same parent body. In fact, we find that the reduced water content and certain volatile element abundances alongside increased sulfide content and mass-dependent O isotope enrichments observed in CY1 compared to CI chondrites could be attributed to an impact-induced heating event on the CI parent body. This impact likely disrupted the CI parent body, resulting in the ejection of both CI and CY1 lithologies. Furthermore, given that there are presently only five known CI meteorite specimens, the close chemical composition between CY1 and CI chondrites substantially expands the data set for comparisons and referrals to the bulk solar system composition for nonvolatile elements. Finally, we propose that the “CY1” chondrites could be called “CI1T,” while the designation “CY” chondrites could be restricted to “CY2” samples.

    DOI： 10.3847/2041-8213/adc89c

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    その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/adc89c/pdf

  • Zirconium isotope composition indicates s‐process depletion in samples returned from asteroid Ryugu

    Maria Schönbächler, Manuela A. Fehr, Tetsuya Yokoyama, Ikshu Gautam, Nao Nakanishi, Yoshinari Abe, Jérôme Aléon, Conel Alexander, Sachiko Amari, Yuri Amelin, Ken‐ichi Bajo, Martin Bizzarro, Audrey Bouvier, Richard W. Carlson, Marc Chaussidon, Byeon‐Gak Choi, Nicolas Dauphas, Andrew M. Davis, Tommaso Di Rocco, Wataru Fujiya, Ryota Fukai, Makiko K. Haba, Yuki Hibiya, Hiroshi Hidaka, Hisashi Homma, Peter Hoppe, Gary R. Huss, Kiyohiro Ichida, Tsuyoshi Iizuka, Trevor Ireland, Akira Ishikawa, Shoichi Itoh, Noriyuki Kawasaki, Noriko T. Kita, Koki Kitajima, Thorsten Kleine, Shintaro Komatani, Alexander N. Krot, Ming‐Chang Liu, Yuki Masuda, Mayu Morita, Kazuko Motomura, Frédéric Moynier, Izumi Nakai, Kazuhide Nagashima, Ann Nguyen, Larry Nittler, Morihiko Onose, Andreas Pack, Changkun Park, Laurette Piani, Liping Qin, Sara Russell, Naoya Sakamoto, Lauren Tafla, Haolan Tang, Kentaro Terada, Yasuko Terada, Tomohiro Usui, Sohei Wada, Meenakshi Wadhwa, Richard J. Walker, Katsuyuki Yamashita, Qing‐Zhu Yin, Shigekazu Yoneda, Edward D. Young, Hiroharu Yui, Ai‐Cheng Zhang, Tomoki Nakamura, Hiroshi Naraoka, Takaaki Noguchi, Ryuji Okazaki, Kanako Sakamoto, Hikaru Yabuta, Masanao Abe, Akiko Miyazaki, Aiko Nakato, Masahiro Nishimura, Tatsuaki Okada, Toru Yada, Kasumi Yogata, Satoru Nakazawa, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Yuichi Tsuda, Sei‐ichiro Watanabe, Makoto Yoshikawa, Shogo Tachibana, Hisayoshi Yurimoto

    Meteoritics & Planetary Science   2024年11月

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    掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

    Abstract

    Nucleosynthetic isotope variations are powerful tracers to determine genetic relationships between meteorites and planetary bodies. They can help to link material collected by space missions to known meteorite groups. The Hayabusa 2 mission returned samples from the Cb‐type asteroid (162173) Ryugu. The mineralogical, chemical, and isotopic characteristics of these samples show strong similarities to carbonaceous chondrites and in particular CI chondrites. The nucleosynthetic isotope compositions of Ryugu overlap with CI chondrites for several elements (e.g., Cr, Ti, Fe, and Zn). In contrast to these isotopes, which are of predominately supernovae origin, s‐process variations in Mo isotope data are similar to those of carbonaceous chondrites, but even more s‐process depleted. To further constrain the origin of this depletion and test whether this signature is also present for other s‐process elements, we report Zr isotope compositions for three bulk Ryugu samples (A0106, A0106‐A0107, C0108) collected from the Hayabusa 2 mission. The data are complemented with that of terrestrial rock reference materials, eucrites, and carbonaceous chondrites. The Ryugu samples are characterized by distinct 96Zr enrichment relative to Earth, indicative of a s‐process depletion. Such depletion is also observed for carbonaceous chondrites and eucrites, in line with previous Zr isotope work, but it is more extreme in Ryugu, as observed for Mo isotopes. Since s‐process Zr and Mo are coupled in mainstream SiC grains, these distinct s‐process variations might be due to SiC grain depletion in the analyzed materials, potentially caused by incomplete sample digestion, because the Ryugu samples were dissolved on a hotplate only to avoid high blank levels for other elements (e.g., Cr). However, local depletion of SiC grains cannot be excluded. An alternative, equally possible scenario is that aqueous alteration redistributed anomalous, s‐process‐depleted, Zr on a local scale, for example, into Ca‐phosphates or phyllosilicates.

    DOI： 10.1111/maps.14279

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  • Nucleosynthetic s-Process Depletion in Mo from Ryugu samples returned by Hayabusa2

    N. Nakanishi, T. Yokoyama, A. Ishikawa, R.J. Walker, Y. Abe, J. Aléon, C.M.O'D. Alexander, S. Amari, Y. Amelin, K.-I. Bajo, M. Bizzarro, A. Bouvier, R.W. Carlson, M. Chaussidon, B.-G. Choi, N. Dauphas, A.M. Davis, T. Di Rocco, W. Fujiya, R. Fukai, I. Gautam, M.K. Haba, Y. Hibiya, H. Hidaka, H. Homma, P. Hoppe, G.R. Huss, K. Ichida, T. Iizuka, T.R. Ireland, S. Itoh, N. Kawasaki, N.T. Kita, K. Kitajima, T. Kleine, S. Komatani, A.N. Krot, M.-C. Liu, Y. Masuda, M. Morita, K. Motomura, F. Moynier, I. Nakai, K. Nagashima, A. Nguyen, L. Nittler, M. Onose, A. Pack, C. Park, L. Piani, L. Qin

    Geochemical Perspectives Letters   2023年12月

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    掲載種別：研究論文（学術雑誌）  

    DOI： 10.7185/geochemlet.2341

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  • Dissipation of Tungsten-182 Anomalies in the Archean Upper Mantle: Evidence from the Black Hills, South Dakota, USA

    Nao Nakanishi, Igor S. Puchtel, Richard J. Walker, Peter I. Nabelek

    Chemical Geology   2023年2月

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    記述言語：英語   掲載種別：研究論文（学術雑誌）  

    DOI： 10.1016/j.chemgeo.2022.121255

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  • Geochemical constraints on the formation of chondrules: Implication from Os and Fe isotopes and HSE abundances in metals from CR chondrites

    Nao Nakanishi, Tetsuya Yokoyama, Satoki Okabayashi, Hikaru Iwamori, Takafumi Hirata

    Geochimica et Cosmochimica Acta   2022年2月

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    記述言語：英語   掲載種別：研究論文（学術雑誌）  

    DOI： 10.1016/j.gca.2021.11.009

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  • Tungsten-182 evidence for an ancient kimberlite source

    Nao Nakanishi, Andrea Giuliani, Richard Carlson, Mary Horan, Jon Woodhead, Graham Pearson, Richard Walker

    Proceedings of the National Academy of Sciences   2021年5月

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    記述言語：英語   掲載種別：研究論文（学術雑誌）  

    <jats:title>Significance</jats:title>
    <jats:p>
    Kimberlites are igneous rocks derived from deep mantle sources. Recent studies have suggested that certain kimberlites originated from a mantle source with relatively primitive chemical composition that was created by early Earth processes. We present W isotope data for a global suite of kimberlites with variable formation ages and find their mantle source(s) to be characterized by
    <jats:sup>182</jats:sup>
    W/
    <jats:sup>184</jats:sup>
    W averaging ∼6 ppm lower than the upper mantle ratio. This result is consistent with derivation of some kimberlites from one or more early formed mantle reservoirs. The low
    <jats:sup>182</jats:sup>
    W/
    <jats:sup>184</jats:sup>
    W of these kimberlites is indicative of an ancient mantle source modified by some form of core–mantle interaction, an early silicate fractionation event, an overabundance of late-accreted materials, or a combination of these.
    </jats:p>

    DOI： 10.1073/pnas.2020680118

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  • Refinement of the Micro‐Distillation Technique for Isotopic Analysis of Geological Samples with pg‐Level Osmium Contents

    Nao Nakanishi, Tetsuya Yokoyama, Akira Ishikawa

    Geostandards and Geoanalytical Research   2019年6月

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    記述言語：英語   掲載種別：研究論文（学術雑誌）  

    <jats:p>In recent years, the <jats:sup>187</jats:sup>Re–<jats:sup>187</jats:sup>Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐<jats:styled-content style="fixed-case">TIMS</jats:styled-content>). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(<jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os) expressed as <jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution <jats:styled-content style="fixed-case">ICP</jats:styled-content>‐<jats:styled-content style="fixed-case">MS</jats:styled-content> method after the addition of <jats:sup>190</jats:sup>Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (<jats:styled-content style="fixed-case">HB</jats:styled-content>r) by H<jats:sub>2</jats:sub>O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of <jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os by N‐<jats:styled-content style="fixed-case">TIMS</jats:styled-content> of varying test portions of the geological reference material <jats:styled-content style="fixed-case">BIR</jats:styled-content>‐1a. The resulting <jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os ratios of <jats:styled-content style="fixed-case">BIR</jats:styled-content>‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.</jats:p>

    DOI： 10.1111/ggr.12262

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  • Fractionation of highly siderophile elements in metal grains from unequilibrated ordinary chondrites: Implications for the origin of chondritic metals

    Satoki Okabayashi, Tetsuya Yokoyama, Nao Nakanishi, Hikaru Iwamori

    Geochimica et Cosmochimica Acta   244   197 - 215   2019年1月

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    掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV  

    DOI： 10.1016/j.gca.2018.10.003

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  • Re‐Os isotope systematics and fractionation of siderophile elements in metal phases from <scp>CB</scp>a chondrites

    Nao Nakanishi, Tetsuya Yokoyama, Satoki Okabayashi, Tomohiro Usui, Hikaru Iwamori

    Meteoritics &amp; Planetary Science   2018年5月

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    記述言語：英語   掲載種別：研究論文（学術雑誌）  

    <jats:title>Abstract</jats:title><jats:p>We report Os isotope compositions of metal grains in two <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma–mass spectrometry. The <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> metal grains presented <jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os ratios akin to carbonaceous chondrites with limited variations (0.1257–0.1270). Most of the <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> metal grains were scattered along a <jats:sup>187</jats:sup>Re‐<jats:sup>187</jats:sup>Os reference isochron of <jats:styled-content style="fixed-case">IIIAB</jats:styled-content> iron meteorites, indicating that the <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> metals experienced limited Re‐Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> metals, recast from the observed <jats:sup>187</jats:sup>Os/<jats:sup>188</jats:sup>Os ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the <jats:styled-content style="fixed-case">CB</jats:styled-content><jats:sub>a</jats:sub> metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at ~10<jats:sup>−4</jats:sup> bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.</jats:p>

    DOI： 10.1111/maps.13050

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共同研究・競争的資金等の研究課題

  • 複数元素同位体を用いたマントル深部リザバーの実態解明

    研究課題/領域番号：25K00026  2025年2月 - 2029年3月

    日本学術振興会  科学研究費助成事業  基盤研究(B)

    中西 奈央

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    配分額：18850000円 （ 直接経費：14500000円 、 間接経費：4350000円 ）

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  • 衝突イジェクタの微小領域分析による地球飛来物質の起源解明

    研究課題/領域番号：23KK0066  2023年9月 - 2027年3月

    日本学術振興会  科学研究費助成事業  国際共同研究加速基金(海外連携研究)

    佐藤 峰南, 石川 晃, 中西 奈央

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    配分額：20930000円 （ 直接経費：16100000円 、 間接経費：4830000円 ）

    本研究では，巨大天体衝突により供給された宇宙物質を高濃度で含む世界各地の堆積物を対象として，ミリグラムレベルでの微小試料から衝突物質の混入率とその起源を求める新たな地球化学的分析法を確立することを目的としている．研究初年度の2023年度は，国際共同研究者らと連携し，白亜紀-古第三紀境界に形成されたチチュルブクレーター内部の掘削試料を対象として研究を行った．特に，隕石成分が濃集していると考えられる掘削試料下部の衝突溶融岩および衝撃角礫岩に焦点を当て，試料ごとの親鉄元素濃度バリエーションを求める作業に着手した．具体的には，各試料に対して研磨片を作成し，蛍光X線分析装置を用いた元素マッピング分析を行い，岩石片中の親鉄元素高濃度層の検出を試みた．本研究では，元素マッピング分析により隕石由来元素濃集領域を見出し，湿式分析用試料のサンプリング範囲を数ミリまで絞り込むことが，研究結果を大きく左右すると考えられる．そのため，親鉄元素高濃度層がわずかにでも検出された試料に対しては，分析領域やスポットサイズを変えて繰り返しマッピング分析を行い，本研究遂行のための最適試料の取得を試みた．また，これまでの国際共同研究により隕石由来元素を高濃度で含むことが明らかとなったクレーター上部の古第三紀緑色泥灰岩を用いて，宇宙物質混入率に鋭敏に反応する強親鉄性元素組成およびオスミウム同位体比のテスト分析を行った．分析用試料作成の際は，同一分解試料を一部分取し，衝突天体の推定に有用なクロム安定同位体分析を行うための準備を進めている．

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