Abstract Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (238U/204Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding 147Sm/144Nd, 176Lu/177Hf, ɛ182W, and e142Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

中文翻译：

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从贫化的辉长岩的铅同位素系统学推断出地球化学异质的火星地幔

摘要 Shergottite 陨石的放射性同位素组成表明，与火星岩浆海洋 (MMO) 的结晶有关的早期行星分化过程导致了地球化学异质的火星地幔。为了了解火星的早期地球化学演化，我们在对两个地球化学贫化的 Shergottites Dar al Gani (DaG) 476 和 Yamato 980459 (Y-980459) 进行分析的基础上，研究了贫化火星地幔中 Pb 同位素系统。它们的初始铅同位素组成是通过对高度浸出的酸残留物的地球化学分析和使用参考结晶年龄的年龄校正计算来估计的。DaG 476 和 Y-980459 源储层的 μ 值 (238U/204Pb) 分别为 2.33 ± 0.07 和 2.32 ± 0.06。这些 μ 值与其他耗竭的 Shergottite 源储层的值不同（例如，对于 Tissint 陨石为 1.4 ± 0.1），并且与相应的 147Sm/144Nd、176Lu/177Hf、ɛ182W 和 e142Nd 组成呈负相关。长寿命和短寿命同位素特征之间的这种相关性表明，在早期火星上形成了地球化学异质耗尽的 Shergottite 源地幔。这种地球化学异质性是由最初由 MMO 中的分步结晶形成的耗尽和富集的端元组分的可变混合形成的。MMO结晶后地球化学贫化的火星地幔的局部重熔是地球化学异质贫化辉长岩源地幔形成的另一种可能解释。