Geochemically heterogeneous Martian mantle inferred from Pb isotope systematics of depleted shergottites
Geochemically heterogeneous Martian mantle inferred from Pb isotope systematics of depleted shergottites
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DOI：
10.1016/j.gca.2020.01.014
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发表时间：
2020-04
影响因子：
5
通讯作者：
R. Moriwaki;T. Usui;M. Tobita;T. Yokoyama
中科院分区：
文献类型：
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作者：
R. Moriwaki;T. Usui;M. Tobita;T. Yokoyama
辉长岩陨石的放射性成因同位素组成表明，与火星岩浆海洋（MMO）结晶有关的早期行星分异过程导致了火星地幔的地球化学不均匀性。为了解火星早期地球化学演化，在分析Dar al Gani (DaG) 476和Yamato 980459 (Y-980459) 2个地球化学贫辉长岩的基础上，研究了枯竭火星地幔Pb同位素系统。根据高浸酸渣的地球化学分析和参考结晶年龄的年龄校正计算，估算了它们的初始Pb同位素组成。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值不同于其他贫辉长岩源储层（如Tissint陨石为1.4 ± 0.1），与相应的147sm /144Nd、176Lu/177Hf、182W和ε142组成呈负相关。长期和短期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上不均匀的枯竭的辉长岩源地幔。这种地球化学非均质性可能是由MMO中最初由分离结晶形成的贫和富端元组分的可变混合形成的。地球化学上枯竭的火星地幔在MMO结晶后的局部重熔是地球化学上非均质枯竭的辉长岩源地幔形成的另一个可能的解释。
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 corresponding147Sm/144Nd,176Lu/177Hf, ɛ182W, and ε142Nd 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.