Martian meteorites are only samples that we can handle directly, and can give us the information about the martian interior. Recently, geochemical study on martian meteorites reported that shergottite, which is the largest group of martian meteorites having wide mineralogical variations, could be classified into two groups ("reduced" shergottite group and "oxidized" shergottite group). The source melt of these groups seems to be formed from the magma ocean at 4.5 Ga and has never been mixed with each other. Reduced shergottites have no evidence for assimilation with the martian crustal material although oxidized shergottites have it. Therefore, reduced shergottite group could retain the information about the early-stage of martian formation and evolution. In order to verify this issue, I investigated the crystallization histories, relationships, and the possibility of environmental change of reduced shergottites.

　In this study, three martian meteorites of five reduced shergottites were studied. Y980459 (Y98) is an olivine-phyric shergottite that consists of olivine megacryst with the groundmass composed of olivine, pyroxene and glassy mesostasis, and no plagioclase. The core compositions of olivine (Fo(86)) and pyroxene (En(81)Fs(17)Wo2) are the most mafic compositions among martian meteorites found so far. The equilibrium calculation and crystallization experiment with the bulk composition of Y98 indicated that Y98 crystallized from the source melt having the bulk composition of Y98 in a closed system. This mafic reduced shergottite crystallized at cooling rate of 1 ℃/hr according to the Mg-Fe diffusion calculation considering fractional crystallization on olivine megacryst of Y98.

　Dar al Gani 476 (DaG) is also an olivine-phyric shergottite that consists of olivine megacryst with the groundmass composed of pyroxene and maskelynite. Although the core compositions of olivine (Fo(76)) and pyroxene (En(78)Fs(19)Wo3) are more Fe-rich than those of Y98, the bulk composition and REE pattern of DaG are nearly identical to those of Y98. These facts and other mineralogical features indicate that both meteorites could be derived from the same melt. In order to investigate the DaG crystallization history from the Y98 melt, MELTS calculation was performed with the bulk composition of Y98. According to the calculation result, both olivine and pyroxene having the same compositions as those of core composition of DaG were equilibrium with the melt at 1250 ℃. This result indicated that DaG crystallized at slow cooling rate, which could equilibrate olivine and pyroxene by elemental diffusion. Mg-Fe diffusion calculation on DaG olivine megacryst gave ~0.035 ℃/hr cooling rate for the DaG crystallization.

　QUE 94201 (QUE) is a basaltic shergottite that consists of coarse-grained pyroxene and maskelynite. QUE is one of the most Fe-rich martian meteorites, but the REE pattern is similar to that of Y98. The MELTS calculation was performed to verify the relation between these meteorites, and the result suggested that the similar composition to the bulk composition of QUE could be produced as the residual melt from Y98 melt at 1160 ℃. The temperature of 1160 ℃ was the estimated liquidus temperature of the QUE bulk composition from crystallization experiments. Crystallization experiments also revealed that the major mineral phases such as pyroxene and plagioclase crystallized from the source melt whose composition was similar to the bulk composition of QUE. It is difficult to calculate the cooling rate by diffusion calculation because QUE has no olivine unlike Y98 and DaG. The results from the cooling experiments at various cooling rates gave the upper limit of cooling rate (0.5 ℃/hr) for the QUE crystallization.

　The results about three reduced shergottites reveal that these reduced shergottites were derived from the same melt having the bulk composition of Y98, and that the differences of cooling rates could produce the mineralogical differences among these meteorites. Because Y98 is the most mafic martian meteorite, its bulk composition might be the same as or similar to that of source melt of reduced shergottite group.

　Oxygen fugacity is one of the important factors to control the martian interior conditions. If reduced shergottites have never suffered the secondary events that could change the redox condition, they have the possibility to retain the information about the early martian evolution. In order to verify the influence of the oxygen fugacity on mineral crystallization, isothermal experiments with the QUE bulk composition under various oxygen fugacities were performed. The results from these experiments showed that the liquidus phase changed from oxidized condition (olivine) to reduced condition (pyroxene) from the same melt. One of the plausible reasons for this change is the valence change of iron. Synchrotron XANES analysis was performed on the glass that was formed under various oxygen fugacities to verify this hypothesis, and the shift of pre-edge peaks to higher energy with increasing Fe(3+)/ΣFe ratio was observed.

　Although the oxygen fugacity has large influence on the meteorite formation, the estimated oxygen fugacities have large gaps (1-2.5 log units) between two methods. One method is using the composition of Fe-Ti oxides and the other is using the Eu partitioning into augite. One of the appropriate reasons for these gaps is that the oxygen fugacity changed during crystallization between the early-stage (pyroxene crystallization) and late-stage (Fe-Ti oxide crystallization). There should be large-scale assimilation of crustal materials or hydrous rocks if such change of oxygen fugacity occurred. I verified whether such environmental change really occurred or not by analyzing pyrrhotite. Determining crystal structure of pyrrhotite is useful because it has several superstructures depended upon oxygen fugacity and other environmental functions (e.g., temperature, pressure, hydrous alternation). However, there had been few studies on pyrrhotites in martian meteorites mainly because of their small abundances and sizes. Electron microprobe and EBSD analyses were performed to determine structures of pyrrhotites in reduced shergottites. According to these analyses, all pyrrhotites in reduced shergottites are 1C type, which is stable at high temperature under low pressure, suggesting that these meteorites seem to have experienced no environmental change such as hydrous alternation during and after the meteorite formation.

　This study revealed that reduced shergottites could be derived from the melt of Y98, and Y98 seems to retain the information of primitive reservoir of reduced shergottites that were formed from the martian magma ocean. Therefore, I compared the bulk composition of Y98 with the partial melt composition of the martian mantle. Partial melt compositions of the martian mantle were obtained from the partial melting experiment with estimated martian mantle composition (DW) by Bertka and Holloway (1994). The partial melt composition that was formed at 1500 ℃ is similar to the bulk composition of Y98. This similarity indicated that the source melt of reduced shergottites could be formed in the magma ocean that was produced by partial melting of the martian mantle, and thus shows direct relationship with the martian mantle. Therefore, the melt of Y98 seems to be the first sample that has direct relationship with martian mantle, and the model of the evolution from martian mantle to differentiated rocks was first established by this study.

　本論文は9章からなる。第1章はイントロダクションとして、火星隕石の分類および火星進化と内部構造を探る上での火星隕石の重要性を述べた後、本研究の概要と目的を説明している。第2章は本研究の実験手法を、第3章から第8章までが本論文、第9章が全体のまとめとなっている。

　シャーゴッタイト(shergottite)グループの火星隕石は酸化的および還元的隕石に大別され、火星形成初期に形成された酸化還元状態の違う二種のマントルから結晶化されたとされている。酸化的火星隕石には、火星地殻との同化が頻繁に行われた形跡がある一方、還元的火星隕石にはその形跡が見られず、火星形成初期の情報を残していると考えられている。このため、初期分化以降は還元的環境のまま閉鎖系を保ったマグマから結晶化したシャーゴッタイトは、始原的な火星マグマの情報を残していると期待できる。

　本研究では、還元的な5個のシャーゴッタイトの中から、Mgに富むYamato 980459(Y98)、Dar al Gani 476(DaG)、およびFeに富むQueen Alexandra Range 94201(QUE)を対象とし、その結晶化過程を明らかにすることにより、これらの隕石が同じ始原的なマグマから閉鎖系で形成可能であるかを検証した。さらに、火星隕石結晶化時における酸素分圧の与える影響を調べた。

　本研究を進めるうえで、酸素分圧をコントロールした環境下でのシャーゴッタイト全岩化学組成のメルトを用いた等温・冷却結晶化実験、MELTSによる相平衡計算、結晶成長を考慮に入れた元素拡散計算、カンラン石・輝石に対する分配係数を用いた計算、結晶化実験生成物中の鉄の価数比の酸素分圧依存性の測定(XANES)、及び電子線後方散乱(EBSD)などを行った。

　第3章から5章では、還元的シャーゴッタイトの結晶化過程を3個の隕石Y98、DaG、QUEについて議論している。Y98はカンラン石巨晶とカンラン石・輝石・メソスタシスの石基から成り、斜長石が存在しない。また、カンラン石、輝石共に火星隕石の中で最もMgに富んでいる。この隕石はメルトから閉鎖系にて約1℃/hrの冷却速度で固結したことを示した。DaGはカンラン石斑晶と輝石・斜長石の石基からなり、そのバルク組成はY98のそれとほぼ一致しており、鉱物学的特徴もY98と同じメルトから結晶化したことを示している。等温・冷却実験により、この隕石のカンラン石はY98のバルク組成と1250℃において平衡であり、カンラン石の累帯構造から計算した冷却速度は約0.035℃/hrとなった。QUEは輝石と斜長石からなり、カンラン石を含まずFeに富んでいる。MELTSによる計算によれば、QUEはY98を晶出により分化したメルトから結晶化した可能性が高い。また、QUEの結晶化過程は完全な閉鎖系ではないが、主要鉱物はメルトから0.5℃/hrよりも遅い冷却速度で結晶化したことを結晶化実験により示した。

　第6章と7章では酸素分圧が結晶化過程に及ぼす影響と、還元的環境が変化していないことを検証している。QUEバルク組成を用いて異なる酸素分圧下での結晶化実験を行って、酸素分圧の変化に伴い結晶化する鉱物が変わることを示すとともに、メルト中のFe(2+)/Fe(3+)比を放射光XANES法を用いて求め、酸素分圧の変化によりFeの価数が系統的に変化することを確認した。また、結晶化後期に晶出して酸素分圧の影響を受けるpyrrhotiteの組成及び電子線後方散乱(EBSD)を用いた結晶構造の分析を行い、結晶化後期の段階において低温域における水質変成を受けた可能性が低い事を確認した。

　第8章と第9章では、以上の知見をもとに火星マントルから火星隕石が形成されるまでのモデルを提唱している。火星隕石Y98・DaG・QUEは共にY98メルトに由来しており、鉱物組成等の違いは結晶化時冷却速度の差による事を明らかにした。さらに、火星隕石中で最もMgに富むY98のバルク組成が還元的火星隕石の元となる物質の情報を保持していることを示した点でも大きな意味がある。以上の結果から、軽希土類に欠乏した火星マントルから火星隕石が形成されるまでのモデルを提唱した。

　以上の研究は、還元的火星隕石相互の成因関係を統一的に解明したものであり、今後の惑星科学の発展に寄与するところが大きい。

　なお、Yamato 980459とDar al Gani 476の研究の一部は既に論文として公表されているが、いずれの論文も申請者が筆頭著者として主体的に関わったものである。したがって、博士(理学)の学位を授与出来ると認める。