Aqueous alteration age ofMartian nakhlites

1.Chassigny陨石的同位素分析结果表明，该陨石的Rb-Srage为1406±14Ma，初始ε <87>Sr/^<86>Sr比值为0.702251±0.000034，Sm-Nd年龄为1386±28 Ma，初始ε <143> nd值为+16.9±0.3,Ar- 40>Ar年龄为1360+40-20 Ma。nwa 2737 WR样品的Sm-Nd数据在Chassigny等时线上显示，Chassigny等时线可能具有几乎相同的~ 138Ga结晶年龄。这些年龄与Rb-Sr和Sm-Nd初始同位素特征的一致性表明，chassignites是在1390 Ma以前由低Rb/Sr、轻稀土元素贫源物质结晶而成的。chassignites的年龄和ε 143nd值与Govemador Valadares和Lafayette的nakhite的年龄和ε 143nd值重叠，表明它们可能来自非常相似的地幔源。Nakhla，西北非洲998和大和000593似乎来自相似但不同的来源。迄今为止研究的恰西尼岩和所有的纳克利岩都经历了相似的演化历史。更多的闪辉岩/钠辉岩来自于火山活动持续至少50 Ma的地区，并由不同的熔岩流或地下岩结晶。它们很可能是由一次撞击事件从火星发射出来的。yamto -793605的Sm-Nd内部等时线年龄为185±16 Ma，初始ε^<143>Nd值为+9.7±02。所得的Sm-Nd年龄在分析误差范围内，与该陨石的Rb-Sr年龄以及先前的Allan Hills-77005和Lewis cliff - 88516的Rb-Sr和Sm-Nd年龄相同，尽管ε^<143>Nd值彼此不相同。亲石微量元素的元素丰度几乎不受火星表面含水蚀变的影响。因此，虽然可以清楚地观察到激波变质作用的干扰，但赫尔长石辉长岩的同位素系统被认为是原生的。“~ 180 Ma的年轻年龄”已经从这个和以前的Rb-Sr， Sm-Nd和U-Pb同位素研究中一致得到，并且似乎代表了结晶事件。少

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 … More 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