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

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Yunhua Wu, 徐伟彪(Weibiao Hsu), Qiu-Li Li, Xiaochao Che, Shiyong Liao

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 20 May 2021

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“Highlights

• Geochemical characteristics of NWA 8653 suggest that it was derived from enriched mantle through mixing of heterogeneous sources instead of crustal assimilation.
• The U-Pb and Pb-Pb analyses of baddeleyite in NWA 8653 indicate a crystallization age of ∼ 190 Ma.
• The Pb-Pb correlation of maskelynite, pyroxene and the majority of phosphate in NWA 8653 result from mixing of Pb isotopes from mantle reservoirs with different μ2 values.”

“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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