Northwest Africa 13489: A Strongly Metamorphosed Ungrouped Carbonaceous Chondrite

NWA 13489 is a meteorite that has been classified as a brachinite. Brachinites are olivine-rich primitive achondrites representing residual products after a variable degree of silicate melt extraction on a barely differentiated, noncarbonaceous asteroid. Nevertheless, NWA 13489 displays petrographic and mineralogical characteristics that are anomalous when compared with other meteorites of that group. The petrography and thermometric data of this sample are compatible with a high metamorphic grade origin. NWA 13489 results in intermediate between type 6 and 7 chondrites, with a thermal regime broadly straddling the FeNi-FeS eutectic and the onset of silicate melting, resembling other meteorites defined as primitive achondrites. Evidence from mineral chemistry, bulk trace element geochemistry, and oxygen and chromium isotope systematics shows a “carbonaceous” composition and, therefore, NWA 13489 is not a brachinite. Rather, together with an ungrouped chondrite (the NWA 11961 C3-ungrouped) and other ungrouped achondrites (the paired NWA 10503/10859), NWA 13489 supports the existence of a distinct carbonaceous-like meteorite grouplet.