Petrography and geochemistry of the chassignite Northwest Africa 2737 (NWA 2737)

We report on the petrology and geochemistry of the Northwest Africa 2737 (NWA 2737) meteorite that was recovered from the Morrocan Sahara in 2000. It is the second member of the chassignite subclass of the SNC (Shergotitte-Nakhlite-Chassignite) group of meteorites that are thought to have originated on Mars. It consists of black olivine- and spinel-cumulate crystals (89.7 and 4.6 wt%, respectively), with intercumulus pyroxenes (augite 3.1 wt% and pigeonite-orthopyroxene 1.0 wt%), analbite glass (1.6 wt%) and apatite (0.2 wt%). Unlike Chassigny, plagioclase has not been observed in NWA 2737. Olivine crystals are rich in Mg, and highly equilibrated (Fo = 78.7 ± 0.5 mol%). The black color of olivine grains may be related to the strong shock experienced by the meteorite as revealed by the deformation features observed on the macroscopic to the atomic scale. Chromite is zoned from core to rim from Cr_83.4Uv_3.6Sp_13.0 to Cr_72.0Uv_6.9Sp_21.1. Pyroxene compositional trends are similar to those described for Chassigny except that they are richer in Mg. Compositions range from En_78.5Wo_2.7Fs_18.8 to En_76.6Wo_3.2Fs_20.2 for the orthopyroxene, from En_73.5Wo_8.0Fs_18.5 to En_64.0Wo_22.1Fs_13.9for pigeonite, and from En_54.6Wo_32.8Fs_12.6 to En_46.7Wo_44.1Fs_9.2 for augite. Bulk rock oxygen isotope compositions confirm that NWA 2737 is a new member of the martian meteorite clan (Δ¹⁷O = 0.305 ± 0.02‰, n = 2). REE abundances measured in NWA 2737 mineral phases are similar to those in Chassigny and suggest a genetic relationship between these two rocks. However, the parent melt of NWA 2737 was less evolved and had a lower Al abundance.