Stellar MgSiO 3 Perovskite: A Shock-transformed Stardust Silicate Found in a Meteorite

Abstract

We have discovered an isotopically highly anomalous MgSiO 3 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 17 O/ 16 O p (4.91 ± 0.36) × 10 -3 (12 times the solar value) and 18 O/ 16 O = (1.36 ± 0.19)× 10 (0.4 times the solar value). This signature points to condensation in the ejecta of a ∼ 2 Mȯ, 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 17 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. © 2007. The American Astronomical Society.