Unique achondritic impact debris in the CH3 chondrite Acfer 182

Abstract

The metal-rich CH carbonaceous chondrites contain abundant xenolithic clasts originating from different regions of the Solar System. In the CH3 chondrite Acfer 182, we identified two phosphide spherules (one 95-μm in diameter and the other $50\mathrm{\mu m}\times 60\mathrm{\mu m}$) of schreibersite ((Fe,Ni)₃P) and barringerite ((Fe,Ni)₂P) with kamacite eutectic structures. These objects are likely to have formed during an impact between planetesimals during the debris-disk phase of the protoplanetary disk before being incorporated into the CH chondrite parent body. In the same sample we identified a $130\mathrm{\mu m}\times 60\mathrm{\mu m}$ heideite grain (iron‑titanium sulfide: (Fe,Cr)_1.15(Ti,Fe)₂S₄) with exsolution lamellae of calcium-rich titanium oxide. Thin veins of shock-induced kamacite cross-cut the oxide lamellae, suggesting that it was ejected into the protoplanetary debris disk during an impact event before eventually being accreted by the CH chondrite parent body. This assemblage is distinct from heideite grains found in enstatite chondrites, aubrites, and the Kaidun meteorite. We propose that this object originated from a highly-reduced planetesimal in the inner Solar System that may have been similar to proto-Mercury.