Origin and implications of troilite-orthopyroxene intergrowths in the brecciated diogenite Northwest Africa 7183

1,2Ai-Cheng Zhang, 1Yi-Fan Bu, 1Run-Lian Pang, 3Naoya Sakamoto, 3,4Hisayoshi Yurimoto, 1Li-Hui Chen, 5Jian-Feng Gao, 1De-Hong Du, 1Xiao-Lei Wang, 1Ru-Cheng Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.051]
1State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
2Lunar and Planetary Science Institute, Nanjing University, Nanjing 210046, China
3Isotope Imaging Laboratory, Creative Research Institution Sousei, Hokkaido University, Sapporo 001-0021, Japan
4Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
5State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Copyright Elsevier

Troilite-orthopyroxene intergrowths are present as a common material in the brecciated diogenite Northwest Africa (NWA) 7183. In this study, we report on the petrographic, mineralogical, and rare earth element abundances of the troilite-orthopyroxene intergrowths to constrain their origin and assess their implications for the diverse petrogenesis of diogenites.

Two groups of troilite-orthopyroxene intergrowths with various grain sizes and mineral chemistry have been observed in NWA 7183. One group of intergrowths contains fine-grained (<5 μm) olivine and chromite as inclusions in orthopyroxene (10–20 μm in size). The other group, in which orthopyroxene is more fine-grained (<10 μm in size), is closely associated with coarse irregular olivine grains. The orthopyroxene grains in both groups of troilite-orthopyroxene intergrowths are depleted in Cr, Al, Ti, and Ca compared with diogenitic orthopyroxene. Based on the texture and mineral chemistry, we suggest that the two groups of troilite-orthopyroxene intergrowths formed via reactions between diogenitic olivine and S-rich vapors, probably at different temperatures. The fact that some of the intergrowths are included in diogenitic lithic clasts indicates that the formation of the host diogenite should postdate the formation of the majority of troilite-orthopyroxene intergrowths. This relationship further implies that not all of the diogenites are cumulates that directly crystallized from the Vestan magma ocean. Instead, they probably originated from partial melting and recrystallization of magma ocean cumulates. The replacement of olivine by troilite and orthopyroxene intergrowths can partly explain why the expected olivine-rich lithologies were not detected at the two south pole impact basins on Vesta.

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