Speaker: Naoji Sugiura, Mizuho Koike
Place: Room 839 Faculty of Science Bldg.1, UTokyo
Time: May 9th 2016, 17:30-19:00
Title: Mesosiderites: a probe of early interplanetary environment?
(メソシデライトAsuka-882023ジルコンのU-Pb・Hf-W年代分析)
Abstract:
Mesosiderites were located near the surface of a differentiated parent body. They were reheated to ~ 1000 C at about 4520 Ma. Subsequently, they cooled rapidly as recorded by the silicates. However, their cooling rates below ~ 400 C as recorded by the Fe-Ni metal are the slowest among all the meteorites. In this talk, I suggest that electromagnetic induction due to changing solar-wind magnetic field could be the heat source for the reheating event and also during the very slow cooling period. I note, however, quantitative evaluation of this model is not yet fully successful.
メソシデライトは石鉄隕石の主要なグループの１つで, Fe-Ni合金とHED隕石に似た組成のケイ酸塩鉱物から構成される。この隕石グループは, 分化天体の地殻とコアの混合で形成されたと考えられるが, その形成史はよく分かっていない。一方で, メソシデライトケイ酸塩中のジルコンは, （１）母天体の地殻形成時に結晶化し, （２）メタル混合イベントで過成長（または再結晶）を経験したと考えられ, メソシデライト隕石の形成プロセスと母天体の熱変成史についての情報を与えると期待される。
本研究では, メソシデライトA-882023中の比較的大きなジルコンに対し, NanoSIMSにてU-Pb年代分析・Hf-W年代分析を行った。セミナーでは, これまでの結果を報告し, 先行研究にて示されている他の年代値と比較して議論したい。
Speaker: Haruka Ono
Place: Room 839 Faculty of Science Bldg.1, UTokyo
Time: October 3rd 2016, 17:00-18:00
Title: ユークライト中のシリカ多形について
Abstract:
シリカ鉱物は地球上では地殻を構成する主要な造岩鉱物の一つとして存在しており、様々な温度圧力条件によって23種以上の多形を持つことが知られている（Sosman R. B. 1965）。しかし地球外物質中でのシリカ鉱物の報告例はあまり多くはない。なぜなら、シリカ鉱物は部分溶融によって晶出しやすいため、未分化では出にくく、また分化隕石はシリカ成分の少ない玄武岩質のものが多いからである。分化した隕石であるエコンドライト中でのシリカ鉱物としては、主に輝石や斜長石を含む玄武岩での結晶化末期の副成分鉱物としての報告がある（Leroux H. and Cordier P. 2006）。それらのシリカ鉱物は隕石ごとに幾つかの異なる多形（α-quartz, tridymite,cristbalite etc.）として存在することが知られている（e.g. Kimura. et al. 2005）。また、ユークライト中では水により沈殿したと考えられているQuartzの報告(Treiman A. et al. 2004)や、TridymiteからQuartzへの転移から、熱水活動があったことを示唆する報告(Kanemaru et al., 2016)も存在する。しかし、多くの場合、それらは"silica"と記載されるのみであり、詳細な分析や、シリカ鉱物に着目した議論はほとんど行われていない。このため、隕石中のシリカ鉱物は、その隕石が含まれていた岩体の温度圧力履歴の条件や熱水活動などの形成環境を推測する一つの指標になり得ると考えられるものの、あまり注目されていないのが実状である。本研究では、月、火星、HED隕石、地球のそれぞれの玄武岩中に存在するシリカ多形を比較することによって、各天体ごとの玄武岩形成時における温度圧力履歴の条件および冷却過程の差と形成環境について検討していくということを目標に研究を行ってきた。冷却速度の差による違いを考慮するために、それぞれの天体起源の隕石から、粗粒な玄武岩と細粒な玄武岩の少なくとも２種類を用いて研究を行う予定である。今回の発表では、シリカ鉱物の存在が多く報告されてきたEucrite隕石を用いたこれまでの研究成果を報告したい。その隕石の冷却過程や形成環境とSilicamineralsの転移速度や周辺の鉱物組み合わせを考慮し、母天体と考えられているVesta地殻の形成について考察していきたい。
Speaker: Geneviève Hublet (NIPR)
Place: Room 839 Faculty of Science Bldg.1, UTokyo
Time: 2016-10-17, 17:00-18:00 (Japan standard time)
Title: Early Solar System chronology: a study of 26Al-26Mg isotopic system on achondrite
Abstract:
The main goal of this PhD was the development of 26Al-26Mg short-lived chronometer at Laboratoire G-Time in Université Libre de Bruxelles to obtain high resolution dating of planetary differentiation. In this study, two different groups of differentiated meteorites (achondrites) were investigated: Howardite-Eucrite-Diogenite (HED) group and ureilites.
The HED group is igneous meteorite series well known to originate from asteroid Vest. Dating these meteorites with 26Al-26Mg can give us information about the formation and evolution of Vesta during the first millions years (Ma) of our Solar System. Whole rock and internal isochrons have been obtained on 10 eucrite (basaltic and cumulate) and diogenites.
Crystallization ages obtained from internal 26Al-26Mg systematic in basaltic eucrites show that Vesta’s upper crust was formed during a short period of magmatic activity at 2.66 ± 0.73 Ma after the Solar System formation (assf). In opposition, cumulate eucrites crystallized progressively, deeper in the Vesta’s crust from 5.48 ± 0.77 to >8 Ma assf. The 26Al-26Mg systematic in diogenite show that they likely formed after the complete decay of 26Al and are younger than both type of eucrites. Model isochrons were also performed considering a chondritic composition of Vesta parent body. Model ages obtained for both basaltic and cumulate eucrites are similar and suggest a common eucrite source differentiated from a chondritic reservoir at 2.88 ± 0.13 Ma assf.
Ureilite are the second largest group of differentiated meteorites after the HED meteorite groups. There are ultramafic achondrites usually known to come from a single parent body (UPB) that was completely destroyed early in the Solar System history. These achondrites are highly fractionated igneous rock but have also some primitive characteristics, such as heterogeneous Δ17O values, suggesting a complex history for their formation. Only whole rock isochron can be performed due to the mineralogy homogeneity of ureilites. Results obtained show that ureilites do not have the same crystallization ages or do not share the same source. Some other stable isotopes (Δ17O, δ25Mg, δ57Fe and δ66Zn) were also investigated in our samples to answer at two major questions: Is there one or more UPB; what are the asteroid processes that formed ureilites? Based on our results, we suggest that ureilites are formed by smelting process (evidenced by δ66Zn isotopic signatures) from a precursor with a composition similar to CI type chondrite. Aluminium-26-26Mg model age obtained suggest also that the UPB differentiated at 1.09 ± 0.75 Ma assf which is ~1.8 Ma before Vesta differentiation.
Speaker: Sachiko Amari (Research Professor) - Washington University in St. Louis
Place: Room 839 Faculty of Science Bldg.1, UTokyo
Time: 2016-12-05, 17:00-18:30 (Japan standard time)
Title: The development of the presolar grain study
Abstract:
プレソーラー粒子と呼ばれる太陽系生成以前に星でできた粒子の研究は１９８０年代の終わりから始まった。プレソーラー粒子とは何か、それらを調べるとどのようなことがわかるか、といった事と共に、どのような技術や装置の発展・開発がプレソーラー粒子研究の研究の発展に寄与してきたかということを述べる。また私見であるが今後のプレソーラー粒子研究がどの方向に向かうと予想されるか、ということを述べる。
Speaker: Minato Tobita（Tetsuya Yokoyama lab., M2） - Tokyo Tech.
Place: Room 839 Faculty of Science Bldg.1, UTokyo
Time: 2016-12-12, 17:00-18:30 (Japan standard time)
Title: New Constraints on Shergottite Petrogenesis from Analysis of the Pb
Isotopic Compositional Space: Implications for Mantle Heterogeneity and
Crustal Assimilation on Mars
Abstract:
Geochemical studies of shergottites (Martian basalts) based on Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided clues to understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs. However, U?Pb isotopic systematics has been used to a limited extent for shergottite petrogenesis, because it is generally difficult to discriminate indigenous magmatic Pb components from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic data of shergottites, as well as their Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics.
The Sr-Nd-Hf isotopic systematics suggests that the geochemical variability of the shergottite suite (i.e., enriched, intermediate, and depleted shergottites) reflects a mixture of two distinct source reservoirs. In contrast, the Pb isotopic systematics does not support the two-component mixing model for shergottites, because the geochemically enriched, intermediate, and depleted shergottites do not participate in a two-component mixing array in Pb isotopic space. To reconcile the isotopic signatures of the Sr-Nd-Hf and Pb systems, we propose a new mixing model in which the geochemically enriched, intermediate, and depleted shergottites were derived from compositionally distinct mantle sources that had different μ (238U/204Pb) values. Moreover, a linear mixing trend defined by the enriched shergottites in Pb isotopic spaceis interpreted as the incorporation of a high-μ Martian crustal component into a parental magma derived from a fertilized Martian mantle source. Our model implies that the geochemical diversity of shergottites reflects heterogeneous mantle sources and an assimilated high-μ crustal component on Mars.
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