Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior

1Logan M.Combs,1Arya Udry,2Geoffrey H.Howarth,3Minako Righter,3Thomas J.Lapen,4,5Juliane Gross,6Daniel K.Ross,1Rachel R.Rahib,7James M.D.Day
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.001]
1Department of Geoscience, University of Nevada Las Vegas, Las Vegas NV 89154, USA
2Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
3Department of Earth and Atmospheric Sciences, University of Houston, Houston TX 77204, USA
4Department of Earth and Planetary Sciences, Rutgers University, Piscataway NJ 08854, USA
5Department of Earth and Planetary Sciences, The American Museum of Natural History, New York, NY10024, USA
6Jacobs-JETS/NASA Johnson Space Center, Houston TX 77058, USA
7Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093, USA
Copyright Elsevier

The martian meteorite Northwest Africa (NWA) 10169 is classified as a member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf and Sm-Nd isotope systematics, and rare earth element (REE) concentrations. Similar to other enriched and intermediate poikilitic shergottites, NWA 10169 is a cumulate rock that exhibits a bimodal texture characterized by large pyroxene oikocrysts (poikilitic texture) surrounded by olivine-rich interstitial material (non-poikilitic texture). Olivine chadacrysts and pyroxene oikocrysts have higher Mg#s (molar Mg/Mg+Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization and accumulation, as opposed to late-stage non-poikilitic (i.e., interstitial material) crystallization. Calculated oxygen fugacity values are more reduced (FMQ -2.3 ± 0.2) within the poikilitic regions, and more oxidized (FMQ -1.1 ± 0.1) within the interstitial areas, likely representing auto-oxidation and degassing during magma crystallization. Calculated parental melt compositions using olivine-hosted melt inclusions display a dichotomy between K-poor and K-rich melts, thus possibly indicating mixing of parental melt with K-rich melt. The 176Lu-176Hf crystallization age for NWA 10169 is 167 ± 31 Ma, consistent with the ages reported for other enriched shergottites. Based on the isochron initial 176Hf/177Hf value, the modeled source 176Lu/177Hf composition for NWA 10169 is 0.02748 ± 0.00037, identical within uncertainty to the source compositions of the enriched shergottites Shergotty, Zagami, LAR 06319, NWA 4468, and Roberts Massif (RBT) 04262, suggesting a shared, long-lived geochemical source, and distinct from the source of other enriched shergottites Los Angeles, NWA 856, and NWA 7320. This study reveals that at least two sources are responsible for the enriched shergottites, and that the martian mantle is more heterogeneous than previously thought. Additionally, the enriched shergottites, which share a source with NWA 10169, have consistent crystallization ages and magmatic histories, indicating that a common magmatic system on Mars is likely responsible for the formation of this group.

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