Chronological study of martian meteorites, shergottites

对玄武岩-坏辉岩混合物进行了冲击恢复和退火实验，以评价冲击对坏辉岩U-Pb系统的影响。由于冲击变质和随后的退火，在这里研究的坏榴辉岩中没有观察到完全的放射性成因铅损失。为了确定火星地质活动的时间尺度，我们对罗伯茨地块04261菱辉岩中的坏辉岩进行了U-Pb研究。baddeleyite的原位U-Th-Pb同位素分析表明，其238U-206Pb年龄为~200 Ma。目前的结果表明，火星岩浆仅在200万年前还在形成，而且直到最近火星的地质活动才开始活跃。目前的结果证实了之前的Rb-Sr、Sm-Nd和Lu-Hf分系统，但与里昂组的铅- pb年龄（4.1 Ga）相冲突，这可能是由于火星地壳成分可能在冲击变质过程中被纳入了辉石岩的组成相。

Shock-recovery and annealing experiments on basalt-baddeleyite mixtures were undertaken to evaluate shock effects on U-Pb systematics of baddeleyite. Complete radiogenic lead loss due to shock metamorphism and subsequent annealing was not observed in the baddeleyites studied here. In order to address the timescale over which Mars was geologically active, we have undertaken U-Pb studies on baddeleyite in the Roberts Massif 04261 shergottite. In situ U-Th-Pb isotopic analyses of baddeleyite yield a young 238U-206Pb age of ~200 Ma. The present results imply that Martian magma was still forming only 200 Ma ago, and that Mars had been geologically active until the recent past. The present results confirm the previous Rb-Sr, Sm-Nd and Lu-Hf systematics but conflict with the Pb-Pb ages of 4.1 Ga for shergottites by the Lyon group, possibly due to Martian crustal component(s) which might have been incorporated into constituent phases of shergottites during shock metamorphism.