Evidence for phosphate metasomatism in an olivine-rich achondrites

Brachinites, brachinite-like achondrites (BLA), and other similar primitive achondrites offer important constraints on differentiation processes of the earliest formed planetesimals, as they quenched amidst early differentiation processes on their parent body. Geochemical data for all major mineral phases in two previously poorly characterized meteorites, El Medano (EM) 395 and Northwest Africa (NWA) 12532, show that while EM 395 is a typical brachinite, NWA 12532 is more unusual, containing a high abundance of non-equilibrated apatite (1.26%) likely formed by a late-stage metasomatic event. These new data demonstrate that metasomatism by a P-Cl-Ca-rich fluid probably occurred on the brachinite parent body. This metasomatism may have occurred either during normal cooling of the asteroid or during later impact-related heating, consistent with the late formation of apatite in the paired andesitic achondrites Graves Nunatak (GRA) 06128 and 06129. These conclusions highlight that, while magmatism on small parent bodies ceased shortly after solar system formation, subsolidus processes may have continued much longer, and that metasomatism must be considered when interpreting bulk rock geochemical signatures of primitive achondrites.