Northwest Africa 16788: The Largest Known Individual Martian Meteorite—A New Olivine Microgabbroic Shergottite and Its Implications for Martian Magmatism

Abstract

We present a comprehensive study of a new Martian meteorite, Northwest Africa (NWA) 16788, found in the Sahara Desert in July 2023. This specimen constitutes the largest known individual meteorite of Martian origin, with a total weight of ⁓25 kg. A detailed mineralogical and geochemical investigation identifies NWA 16788 as an enriched olivine microgabbroic shergottite. Petrographic analysis reveals a millimeter-sized cumulate texture, which is intermediate between poikilitic and gabbroic shergottites, primarily composed of pyroxene (⁓61 vol.%), maskelynite (shocked plagioclase, ⁓21 vol.%), and olivine (⁓15 vol.%). Pyroxene grains exhibit a distinctive zoning pattern, placing NWA 16788 within a small group of Martian meteorites and lunar samples that record unique cooling histories, suggesting that similar samples may be more prevalent within the Martian igneous rocks than previously recognized. High-precision analyses of bulk ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr isotopic composition, combined with trace element abundance, indicate that NWA 16788 originated from the partial melting of an enriched Martian mantle source. Electron backscatter diffraction analysis reveals a unimodal distribution of olivine grain misorientation, averaging ⁓4.6°, consistent with a high-energy single-impact event. Moreover, visible and near-infrared spectroscopy data acquired on NWA 16788 could aid in identifying potential locations of analogous igneous rocks on the Martian surface. Finally, this study proposes refinements to the current classification scheme of Martian meteorites, aiming to reduce taxonomic ambiguity and improve the alignment between Martian meteorites and Martian igneous rocks, based on data from Mars rovers.