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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Signatures of early differentiation of Mars
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Signatures of early differentiation of Mars

机译：火星早期分化的特征

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The SNC meteorites Chassigny, ALH84001, Nakhla, and the newly discovered nakhlite NWA817 contain high concentrations of xenon isotopes produced by the fission of the extinct radionuclide ~(244)Pu (t_(1/2) = 82 Ma). the fission gas is released at temperatures >900 ℃ together with indigenous solar-type Xe which represents a Martian interior component. Both nakhlites are rich in U, rare earth elements, and fissiogenic ~(136)Xe~*, suggesting that fissiogenic xenon has been enriched through magmatic differentiation in closed system conditions. In Chassigny and ALH84001, fission Xe concentrations are consistent with a chondritic initial abundance of ~(244)Pu in Mars. The ratios (~(129)Xe/~(136)Xe)~* (where ~(129)Xe~* was produced by the decay of extinct ~(129)I (t_(1/2) = 16 Ma) and ~(136)Xe~* by ~(244)Pu fission) observed in these meteorites at high temperature are systematically lower than the value that would be expected from decay in a closed mantle reservoir having a bulk Mars composition, requiring early differentiation of volatile iodine with respect to refractory plutonium. W develop a model in which iodine and xenon are degassed together during large-scale magmatic events (e.g., magma ocean episode) on early Mars. The results show that bulk ~(129)I/~(244)Pu fractionation must have occurred ≤35 Ma after the start of solar system formation and that degassing might have continued during the first 330 Ma for the nakhlite mantle source. After this period, the mantle sources of these meteorites did not experience significant degassing, suggesting that Mars has been a static planet for most of its history. The computed amount of mantle Xe released into the early Martian atmosphere is about three orders of magnitude higher than the Xe abundance observed in the present-day atmosphere. The amount of ~(129)Xe~* produced by the decay of ~(129)I transferred to the Martian surface is also three orders of magnitude higher than the present-day atmospheric ~(129)Xe~*, implying that loss of Martin atmospheric gases must have lasted over the decay interval of several tens of Ma.
机译：SNC陨石Chassigny，Nakhla的ALH84001和新发现的Nakhlite NWA817包含由灭绝的放射性核素〜（244）Pu的裂变产生的高浓度氙同位素（t_（1/2）= 82 Ma）。裂变气体在大于900℃的温度下与代表火星内部成分的固有太阳型Xe一起释放。两种nakhlites均富含U，稀土元素和易裂变〜（136）Xe〜*，这表明易裂变氙已通过封闭系统条件下的岩浆分化而富集。在Chassigny和ALH84001中，裂变Xe浓度与火星中〜（244）Pu的软骨状初始丰度一致。比值（〜（129）Xe /〜（136）Xe）〜*（其中〜（129）Xe〜*是由已消失的〜（129）I（t_（1/2）= 16 Ma）的衰减产生的）和在高温下在这些陨石中观察到的〜（136）Xe〜*（~~ 244Pu裂变）系统地低于由具有火星组成的封闭地幔储层中的腐烂所预期的值，这需要及早区分挥发物耐火p中的碘。 W开发了一个模型，其中在火星早期大规模岩浆事件（例如岩浆海洋事件）中将碘和氙一起脱气。结果表明，在太阳系形成开始后，〜（129）I /〜（244）Pu的分馏必定发生在≤35Ma，并且对于nakhlite地幔源而言，在最初的330 Ma内可能继续进行脱气。在此期间之后，这些陨石的地幔源未经历明显的除气，这表明火星在其大部分历史中一直是静止的行星。计算出的释放到火星早期大气中的氙气量比当今大气中观测到的氙气丰度高出三个数量级。由转移到火星表面的〜（129）I的衰变产生的〜（129）Xe〜*的量也比当今的大气〜（129）Xe〜*高三个数量级，这意味着马丁大气中的气体一定持续了几十Ma的衰减时间。

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