Pb-isotopic evidence for an early, enriched crust on Mars
Pb-isotopic evidence for an early, enriched crust on Mars
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DOI：
10.1016/j.epsl.2014.11.018
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发表时间：
2015-01-15
影响因子：
5.3
通讯作者：
Zanda, B.
中科院分区：
文献类型：
--
作者：
Bellucci, J. J.;Nemchin, A. A.;Zanda, B.
火星陨石NWA 7533是一个风化层角砾岩，其成分类似于轨道飞行器和着陆器测量的火星表面。NWA 7533含有锆石U-Pb年龄为4.428 Ga的二长碎屑。用二次离子质谱法（西姆斯）测定了NWA 7533二长岩碎屑中斜长石和碱性长石的Pb同位素组成以及氯磷灰石的U-Pb同位素组成。从NWA 7533中的氯磷灰石中测量的U-Pb同位素组成产生的年龄为1.357 +/- 81 Ga（2 sigma）。斜长石和钾长石中放射成因铅同位素组成与4.428Ga年龄的误差范围内。这些数据表明，NWA 7533中的二长岩碎屑是分异历史的产物，该分异历史包括在4.428 Ga之前形成的储层中的驻留，其中μ值（U-238/Pb-204）至少为13.4 +/- 1.7（2 σ），K值（Th-232/U-238）类似于4.3。这个μ值比任何其他有记录的火星水库高出三倍多。这些结果表明，火星地幔的异质性比以前认为的要大得多（μ值为1-14 vs. 1-5）和/或火星地壳熔化形成的二长岩碎屑的μ值至少为13.4。因此，NWA 7533可能包含了第一个同位素证据，证明火星上有一个富集的、分化的地壳。(C)2014作者由爱思唯尔公司出版
Martian meteorite NWA 7533 is a regolith breccia that compositionally resembles the Martian surface measured by orbiters and landers. NWA 7533 contains monzonitic clasts that have zircon with U-Pb ages of 4.428 Ga. The Pb isotopic compositions of plagioclase and alkali feldspars, as well as U-Pb isotopic compositions of chlorapatite in the monzonitic clasts of NWA 7533 have been measured by Secondary Ion Mass Spectrometry (SIMS). The U-Pb isotopic compositions measured from the chlorapatite in NWA 7533 yield an age of 1.357 +/- 81 Ga (2 sigma). The least radiogenic Pb isotopic compositions measured in plagioclase and K-feldspar lie within error of the 4.428 Ga Geochron. These data indicate that the monzonitic clasts in NWA 7533 are a product of a differentiation history that includes residence in a reservoir that formed prior to 4.428 Ga with a mu-value (U-238/Pb-204,) of at least 13.4 +/- 1.7 (2 sigma) and a K-value (Th-232/U-238) of similar to 4.3. This mu-value is more than three times higher than any other documented Martian reservoir. These results indicate either the Martian mantle is significantly more heterogeneous than previously thought (mu-value of 1-14 vs. 1-5) and/or the monzonitic clasts formed by the melting of Martian crust with a mu-value of at least 13.4. Therefore, NWA 7533 may contain the first isotopic evidence for an enriched, differentiated crust on Mars. (C) 2014 The Authors. Published by Elsevier B.V.