Accessory silicate mineral assemblages in the Bilanga diogenite: A petrographic study

The petrographic relationships in diogenites between orthopyroxene and minor phases such as chromite, troilite, diopside, plagioclase, and silica are often obscured by the intense brecciation that characterizes these meteorites. Although brecciated, Bilanga preserves numerous clasts displaying primary textural relations between orthopyroxene and these minor phases that are large enough to analyze by electron microprobe. In this study, we focus on the distribution, composition, and origin of the minor phases in Bilanga to provide new insights into the crystallization and metamorphic history of these rocks. The samples examined consist mainly of orthopyroxene grains plus five types of assemblages containing diopside + a Fe-rich phase (chromite, troilite, and/or Fe-Ni metal) +/- plagioclase +/- silica. We interpret type 1 assemblages as being remnants of intercumulus melt trapped in the interstices between orthopyroxene grains after crystal settling in a magma chamber. Type 2 assemblages appear to have formed by heterogeneous exsolution during thermal metamorphism. Type 3 assemblages are believed to be remnants of other assemblages that have been shocked, melted, and rapidly recrystallized by impact events. Type 4 assemblages consist of veins that also appear to have formed from trapped intercumulus melt. Regions of silica-rich mesostasis (type 5) appear to be larger patches of more evolved intercumulus melt that have been significantly affected by late-stage impact melting. Finally, large clasts containing plagioclase diopside are interpreted to be exotic fragments of a different but possibly related rock type incorporated in the Bilanga breccia.