PETROLOGY AND MINERAL CHEMISTRY OF THE MARTIAN METEORITE NORTHWEST AFRICA 10169: CONSTRAINTS ON ENRICHED POIKILITIC SHERGOTTITE PETROGENESIS

The poikilitic shergottites (also called lherzolitic) make up one of the three groups of martian meteorite shergottites, which all show mafic or ultramafic compositions. Shergottites are further divided as enriched, intermediate, and depleted on the basis of their relative light rare earth element (LREE) enrichment. The enriched poikilitic shergottites, hypothesized to have originated from partial melting of an enriched source, have not been extensively studied due to their rarity. The study of these martian rocks will help constrain the origin of this geochemically and petrologically distinct group of meteorites.

Here we present in situ major and trace element compositions, oxygen fugacity (fO₂), and Sm-Nd and Lu-Hf crystallization age determinations of the newly found enriched poikilitic shergottite Northwest Africa (NWA) 10169. We compare our findings from NWA 10169 with other enriched poikilitic and extrusive shergottites in an effort to understand how it relates petrogenetically to these groups.

Similar to other poikilitic shergottites, this meteorite displays poikilitic (early stage) and interstitial non-poikilitic (late stage) textural zones. We have analyzed major, minor and trace element compositions of the major phases, including olivine, pyroxene, maskelynite (shocked plagioclase), Fe-Ti-Cr oxide, and phosphate. Olivine and pyroxene in the poikilitic zone have higher Mg#’s (Mg# = MgO/MgO+FeO) than those in the interstitial areas. fO₂, which was calculated through the olivine-pyroxene-spinel oxygeobarometer, was determined to be fayalite-magnetite-quartz (FMQ)-2.5 for the poikilitic texture and FMQ-1.1 for the interstitial areas. The relatively flat bulk rock REE profile [(La/Yb)_CI=1.18], texture, fO₂ values, and mineral composition of NWA 10169 are consistent with previously studied enriched poikilitic shergottites (e.g. RBT 04262, NWA 7755). The significant difference in fO₂ values and Mg#’s between the poikilitic and non-poikilitic textures supports the idea that these two textures formed under different conditions and at different times, likely representing oxidation during crystallization. This comprehensive study of NWA 10169 classifies it as a LREE enriched poikilitic shergottite, and furthers our insight into the igneous processes and magmatic evolution of Mars.