Heterogeneous martian mantle: Evidence from petrology, mineral chemistry, and in situ U-Pb chronology of the basaltic shergottite Northwest Africa 8653

1YunhuavWu,2徐伟彪(Weibiao Hsu),3Qiu-Li Li,4Xiaochao Che,2Shiyong Liao
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.011]
1Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai 519082, China
2CAS Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Nanjing 210034, China
3State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
4Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 102206, China
Copyright Elsevier

Shergottites were derived from mantle reservoirs through various magmatic processes, recording geochemical signatures of the martian mantle. But the U-Pb isotopic system of shergottites remains obscured including the geological significance of Pb isotope composition, the role of martian Pb contamination, and other factors. Here we present in situ U-Pb and/or Pb-Pb analyses on minerals of the basaltic shergottite Northwest Africa (NWA) 8653 after detailed petrological and mineralogical studies. The aims of the project are to evaluate the formation process, the crystallization age as well as the characteristic Pb isotope composition of NWA 8653. Texture, major and trace element composition plus geochemical modeling suggest that NWA 8653 is an enriched shergottite derived from mixing of depleted (e.g., fractionated Yamato 980459) and enriched components (e.g., NWA 1068) in the mantle, instead of crustal assimilation. U-Pb and Pb-Pb isotopes of baddeleyite reveal a young crystallization age (187.6 ± 8.0 Ma). Pb isotope compositions of maskelynite, feldspathic intergrowth, and the majority of phosphate cluster near the predicted initial Pb and a 4.1 Ga isochron. For these minerals, calculations suggest that mixing of Pb from different reservoirs with μ (238U/204Pb) varying from 1.4 to 4.7 could explain the apparent 4.1 Ga isochron in young shergottites. Variable extents of mixing among mantle sources could further increase the isotopic heterogeneity of shergottites. Our results demonstrate that NWA 8653 was derived from a heterogeneous mantle source in terms of trace element and isotope composition. Mixing of Pb from different reservoirs in the mantle plays an important role in shaping Pb in minerals with negligible U. This study provides additional geochemical evidence for a highly heterogeneous martian mantle.

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