Expanding Mars’ lithologic diversity: discovery of a garnet-bearing clast in NWA 8171

A garnet-bearing clast has been identified in martian breccia meteorite NWA 8171, comprising two distinct domains: an andradite-diopside domain, and a K-feldspar-augite domain. Similar assemblages occur in terrestrial metamorphic/metasomatic settings like skarns, in alkali igneous rocks; and as secondary phases in carbonaceous chondrites. Mineralogical and textural analyses of the clast reveals a complex history, possibly reflecting multiple crystallisation stages and/or alteration events on Mars. However, as NWA 8171 is a regolith breccia, we also consider if the garnet-bearing clast is extra-martian in origin. Analysis of pyroxene Mn/Fe ratios indicate that augites from the K-feldspar-rich domain match martian values, while diopsides in the andradite-bearing domain are more varied in composition. This variability, together with similarities to metasomatic assemblages from chondritic and terrestrial analogues, suggests that the andradite-rich domain may not comprise primary igneous minerals. This could indicate the clast was altered on Mars in an oxidising metasomatic event, although an extra-martian origin cannot be ruled out. Still, the first identification of garnet in a martian meteorite has major implications. The andradite-bearing clast in NWA 8171 may be the first sample of a garnet-bearing lithology from Mars, representing a previously unidentified martian magma source, alteration process, regolith impactor component, or metamorphic event.