Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164

Northwest Africa 3164 is a coarse‐grained angrite that shows reaction coronas, a unique character among achondrites. Olivine (Fo₅₇; 1.2 wt% CaO), fassaitic clinopyroxene, anorthite, and spinel account for 46–47, 28–29, 8–13, and 4–8 vol%, respectively; kamacite is an accessory phase. The spinel grains in contact with clinopyroxene are bounded by discontinuous 20 μm thick coronas of anorthite and olivine, indicating the reaction Cpx + Spl → Ol + An (R1). In addition, irregular coronas of clinopyroxene and spinel developed around the primary anorthite in contact with primary olivine, during the reaction Ol + An → Cpx + Spl (R2). R2 also generated clinopyroxene and spinel films between the secondary olivine and anorthite coronas produced during R1, implying that R1 preceded R2. Both are metamorphic reactions that developed in the solid state. Finally, the coronas are cross cut by μm‐thick veinlets due to a late shock. A mass‐balance study shows that R2 is almost the reverse of R1. The P–T metamorphic evolution of the rock, modeled by calculating a P–T isochemical diagram, indicates an equilibrium T of 940 ± 120 °C at P < 0.9 GPa for the initial assemblage, followed by an increase of T up to approximately 1000–1200 °C during reaction R1 and a subsequent cooling during R2. Several causes are envisaged to account for this metamorphic evolution. Contact metamorphism due to a hot magmatic intrusion in the angrite parent body is favored, as similar metamorphic coronas are well known in metamorphic terrestrial rocks. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which would have been a large asteroid or a planetary‐sized body.