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

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 Lu-176-Hf-176 crystallization age for NWA 10169 is 167 +/- 31 Ma, consistent with the ages reported for other enriched shergottites. Based on the isochron initial Hf-176/Hf-177 value, the modeled source Lu-176/Hf-177 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. (C) 2019 Elsevier Ltd. All rights reserved.