STELLAR MgSiO3 PEROVSKITE: A SHOCK-TRANSFORMED STARDUST SILICATE FOUND IN A METEORITE

We have discovered an isotopically highly anomalous MgSiO3 grain in the ungrouped carbonaceous chondrite Acfer 094 with a perovskite-like crystal structure resembling the dominant high-pressure mineral of the Earth’s lower mantle. Oxygen isotopic ratios of the silicate grain are O/ O p (4.91 0.36) # 10 (12 times the 3 solar value) and O/O p (1.36 0.19) # 10 (0.4 times the solar value). This signature points to condensation 3 in the ejecta of a ∼2 M , close-to-solar metallicity red giant branch (RGB) or asymptotic giant branch (AGB) , star. Alternatively, the grain could have formed in the ejecta of a nova, in which O is highly overabundant. TEM analysis of the grain revealed a high pressure perovskite-like crystal structure not predicted by equilibrium condensation in low-pressure stellar environments. A possible formation scenario is transformation of a silicate precursor triggered by a shock wave, either in the interstellar medium (ISM) or originating from the grain’s parent star. Shock waves must thus be considered as a potential mechanism to recrystallize silicates, or even convert them into high-pressure structures. Alternatively, nonequilibrium condensation or crystallization by a chemical vapor deposition (CVD)–like process, also invoked for the formation of nanodiamonds, is a distinct possibility, although more speculative. Subject headings: astrochemistry — circumstellar matter — ISM: kinematics and dynamics — shock waves — stars: late-type — stars: winds, outflows