2006 Fiscal Year Final Research Report Summary
Aqueous alteration age ofMartian nakhlites
| Project/Area Number |
17540464
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Geochemistry/Astrochemistry
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| Research Institution | National Institute of Polar Research |
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Principal Investigator |
MISAWA Keiji National Institute of Polar Research, Division for Research and Education, Associate Professor, 研究教育系, 助教授 (70212230)
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| Co-Investigator(Kenkyū-buntansha) |
NOGUCHI Takaaki Ibaraki University, Faculty of Science, Associate Professor, 理学部, 助教授 (40222195)
KAIDEN Hiroshi Division for Research and Education, Research Associate, 研究教育系, 助手 (10302811)
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| Project Period (FY) |
2005 – 2006
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| Keywords | Martian meteorite / aqueous alteration / chronology trace element / Rb-Sr / Sm-Nd |
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| Research Abstract |
1.Isotopic analysis of the Martian meteorite Chassigny yields a Rb-Srage of 1406±14Ma with an initial ^<87>Sr/^<86>Sr ratio of 0.702251±0.000034, a Sm-Nd age of 1386±28 Ma with an initial ε^<143>Nd-value of+16.9±0.3 and an ^<39>Ar-^<40>Ar age of 1360+40-20 Ma. The Sm-Nd data for WR samples ofNWA 2737 plots on the Chassigny isochron, suggesting that the chassignites may have almost identical crystallization age of〜138Ga. The concordance of these ages and the Rb-Sr and Sm-Nd initial isotopic signatures suggest that chassignites crystallized from low Rb/Sr, light rare earth element depleted source materials〜1390 Ma ago. The ages and ε143Nd-values of chassignites and the nakhlites Govemador Valadares and Lafayette overlap, suggesting that they could have come ＆urn very similar mantle sources. Nakhla, Northwest Africa 998 and Yamato 000593 appear to be from similar but distinct sources. Chassignites and all nakhlites so far studied have undergone similar evolution histories. That is, chassi
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gnites/nakhlites were derived from a region where volcanism lasted at least 50 Ma and crystallized from different lava flows or subsurface sills. They probably were launched from Mars by a single impact event.
2.The Sm-Nd internal isochron obtained for Yamato-793605 yields an age of 185±16 Ma with an initial ε^<143>Nd value of+9.7±02. The obtained Sm-Nd age is, within analytical errors, identical to the Rb-Sr age ofthis meteorite as well as to the previous Rb-Sr and Sm-Nd ages of Allan Hills-77005 and Lewis Cliff88516, although the ε^<143>Nd values are not identical to each other. Elemental abundances of lithophile trace elements remain nearly unaffected by aqueous alteration on the Martian surface. The isotopic systems of lherzolitic shergottites, thus, are considered to be indigenous, although disturbances by shock metamorphism are clearly observed. "Young ages of〜180 Ma" have been consistently obtained from this and previous Rb-Sr, Sm-Nd and U-Pb isotopic studies and appear to represent crystallization events. Less
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