Natural Occurrence of MgSiO ₃ -Ilmenite and Evidence for MgSiO ₃ -Perovskite in a Shocked L Chondrite

Abstract

Shock-induced melt veins in the Acfer 040 L5-6 (S6) chondrite contain a previously unknown set of high pressure phases consisting of amorphous grains similar in composition to majorite, MgSiO 3 -ilmenite, and ringwoodite. The amorphous grains have compositions that are similar to those of synthetic MgSiO 3 -perovskites from chemically complex systems and are inferred to be MgSiO 3 -perovskite that crystallized from the melt at high pressure and temperature and subsequently amorphized after pressure release. The ilmenite represents a natural occurrence of a potentially important mineral in Earth's mantle. The MgSiO 3 -perovskite–MgSiO 3 -ilmenite–ringwoodite assemblage is not predicted by phase equilibria studies, but appears to result from crystallization of a melt at pressures above 26 gigapascals.