Updated on 2025/06/02

写真a

 
NAKANISHI, Nao
 
Affiliation
Faculty of Education and Integrated Arts and Sciences, School of Education
Job title
Assistant Professor(without tenure)

Research Experience

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    日本地球化学会

 

Papers

  • “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  2025.05

    　View Summary

    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.

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  • 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    2024.11

    　View Summary

    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.

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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    2023.12

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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    2023.02

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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    2022.02

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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    2021.05

    　View Summary

    <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>

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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    2019.06

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    <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>

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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  2019.01

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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    2018.05

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    <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>

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Research Projects

  • 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

    日本学術振興会  科学研究費助成事業

    Project Year :

    2025.02
    -
    2029.03
     

    中西 奈央

  • Investigating the origin of large projectiles by micro-sampling in the impact ejecta

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

    Project Year :

    2023.09
    -
    2027.03
     

 

Syllabus

 

Internal Special Research Projects

  • メソシデライト・HED隕石を用いた天体ベスタ進化の解明

    2024  

    　View Summary

    182Wは短寿命核種182Hfから半減期890万年で生成する。親石性のHfと親鉄性のWという化学的特徴により、コアやコンドライトはマントル・地殻と大きく異なるW同位体組成を持つ。このことをHED隕石に利用してHED隕石の母天体と考えられている小天体Vestaの進化過程を明らかにすることを本研究は目的としている。そのために前年度に引き続きタングステン同位体分析法の開発を行った。前年度の結果を踏まえて、天然岩石標準試料であるJG-2を用いてテストを行った。その結果、陰イオン交換樹脂を用いた化学分離では高回収率でWが分離され、182W/184W同位体比はμ182W = +1.7±8.2 (2SD, n = 3)で測定できることが分かった。この結果は参考文献値と誤差の範囲で一致しており、繰り返し再現性にやや改善の余地があるが、問題点の目星はついており、目標である繰り返し再現性± 5 ppmでの測定は可能であると見込んでいる。これらを踏まえて、実際に分析を行うHED隕石の試料選定を行った。試料の分解は2025年4月以降に開始する予定。

  • リュウグウ試料の化学分析によるリュウグウ母天体変成プロセスの解明

    2024  

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    はやぶさ2ミッションはCb型小惑星リュウグウのサンプルを地球に持ち帰り、その帰還試料初期分析により、CIコンドライトとの鉱物学的、化学的、および同位体的な類似性が明らかになった。惑星物質中の元素存在比および同位体組成の変動は、前駆体物質が天体に集積する前に経験した条件やプロセスだけでなく、集積後の天体における水質変質などの二次的なプロセスも反映している。CIコンドライトに関するこれまでの研究では、バルクの元素組成を決定するために典型的には50mgが使用されていたが、詳細な母天体形成プロセスを議論するためには、より小さなスケールでの元素分布に焦点を当てる必要がある。本研究では、初期太陽系での物質進化の過程でリュウグウの始原物質が経験した物理化学的プロセスに関するより深い洞察を得るため、リュウグウ試料、CIコンドライトの小規模な化学的不均一性を調査した。具体的には親鉄性元素（HSE;Re, Os, Ir, Ru, Pt, Rh, Pd, and Au)、揮発性親銅性元素 (VCE; S,Ge, As, Se, and Te)、及びその他元素(Mg, Al, and Ca)の存在度測定をリュウグウ試料、CIコンドライトについて行った。1回の測定に用いたサンプル量は約1 mgである。その結果、リュウグウ、CIサンプルは、ほとんどの元素について均一な相対存在度を示す一方、親鉄性元素絶対存在度はサンプル間でバリエーションを示す（0.5-1.1xCI）ことが明らかになった。測定を行ったリュウグウ試料の表面には金属相は確認されておらず、得られた親鉄性元素の存在度は、均質な相対存在度を持つ集積物質と、その後の母天体における水質変成に対する耐性を反映している可能性があることが示唆される。

  • ベスタ隕石及びメソシデライト隕石の同位体組成からみる天体ベスタのコアマントル分離

    2023  

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    ベスタ隕石及びメソシデライト隕石の元素・同位体分析により、その起源と進化を議論する事を最終目標に今年度はタングステン同位体分析法の確立を目指した。分析は（１）試料分解とイオン交換樹脂を用いたタングステンの分離工程と（２）表面電離型質量分析装置を用いたタングステン同位体測定工程に大別できる。これらの工程でタングステンが高回収率で分離され、精度よく同位体比が測定されることをタングステン標準溶液、鉄隕石試料を用いてテストした。その結果、（１）の化学処理工程においては、数種類の分離方法を試し、メソシデライト隕石のような鉄ニッケルが多く含まれる試料の場合、陽イオン交換樹脂と陰イオン交換樹脂を組み合わせることでタングステンが効率的に分離できることがわかった。（２）の装置分析の工程ではタングステン測定に最適な装置設定を検討した上で、測定に用いるフィラメント、アクチベータの種類を変えながらタングステンビーム強度が一番高く出るための条件を比較した。その結果、タングステン標準溶液を用いた測定で182W/184W同位体比の繰り返し再現性~5 ppm(2SD)を達成する事が出来た。これら結果を踏まえて、天然試料である化学処理工程を行った鉄隕石試料について測定を行ったところ、参考文献値と誤差の範囲で一致した182W/184W比を得ることができた。