Abstract Shergottite Northwest Africa (NWA) 8159 is a basaltic rock derived from a mantle source with chemical characteristics that are unique in the martian meteorite suite. To further investigate this source reservoir, the Pb isotope compositions of plagioclase/maskelynite, pyroxene, phosphates, and shock melt-glass in NWA 8159 have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Due to the limited spread in Pb isotope data, these Pb isotope compositions have been used to calculate an imprecise Pb Pb isochron age of 3.4 ± 2.1 Ga (2σ), which is broadly consistent with the crystallization age of 2.37 ± 0.25 Ga determined previously by 147Sm 143Nd. The lack of radiogenic in-growth within individual minerals since 2.4 Ga means that this sample is depleted in U, which is in agreement with NWA 8159's positive initial e143Nd. An initial Pb composition was calculated using an x-y weighted average of the least radiogenic Pb isotope population measured in the sample. This initial Pb composition is not consistent with the model for Pb growth in the shergottite mantle at 2.4 Ga. This composition is, however, consistent with the model for the Nakhla-Chassigny mantle. Using the latter model, a source μ (238U/204Pb) of 2.6 ± 0.6 has been calculated. This μ-value is in contrast with the other depleted shergottites (1.4-1.5) and falls significantly off the array of source e143Nd vs. μ defined by the rest of the martian meteorite suite and thus, necessitates a differentiation history distinct from the other martian meteorites. Sequestering Pb in sulphides during differentiation is the only mechanism to fractionate U from Pb and create a low-μ reservoir. Consequently, the relatively high μ-value of the source of NWA 8159 is in contrast with the positive initial e143Nd and indicates that its mantle source region likely lacked significant sulphur. This is consistent with the lack of sulphides in the sample itself and could have played a role in its complicated oxidation history.

中文翻译：

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从西非西非 8159 陨石的 Pb 同位素系统学洞察火星地幔的化学多样性

摘要 Shergottite Northwest Africa (NWA) 8159 是一种玄武岩，源自地幔源，具有火星陨石套件中独一无二的化学特性。为了进一步研究该源储层，NWA 8159 中斜长石/硅长石、辉石、磷酸盐和冲击熔体玻璃的 Pb 同位素组成已通过二次离子质谱法 (SIMS) 进行原位测量。由于 Pb 同位素数据的传播有限，这些 Pb 同位素组成已被用于计算 3.4 ± 2.1 Ga (2σ) 的不精确 Pb Pb 等时线年龄，这与之前确定的 2.37 ± 0.25 Ga 结晶年龄大致一致147Sm 143Nd。自 2.4 Ga 以来，单个矿物内缺乏放射成因生长意味着该样品中 U 耗尽，这与 NWA 8159 的正初始 e143Nd 一致。使用样品中测得的放射性最低的 Pb 同位素群体的 xy 加权平均值计算初始 Pb 组成。这种初始铅成分与 2.4 Ga 的 Shergottite 地幔中铅生长的模型不一致。然而，这种成分与 Nakhla-Chassigny 地幔的模型一致。使用后一种模型，已计算出 2.6 ± 0.6 的源 μ (238U/204Pb)。这个 μ 值与其他耗尽的 Shergottites (1.4-1.5) 形成对比，并且显着脱离了由火星陨石套件的其余部分定义的源 e143Nd 与 μ 的阵列，因此，需要与其他火星陨石不同的分化历史陨石。在分化过程中将 Pb 隔离在硫化物中是从 Pb 中分离 U 并创建低 μ 储层的唯一机制。最后，NWA 8159 源的相对较高的 μ 值与正初始 e143Nd 形成对比，表明其地幔源区可能缺乏显着的硫。这与样品本身缺乏硫化物是一致的，并且可能在其复杂的氧化历史中起作用。