CHARACTERIZATION OF PRESOLAR MATERIAL IN THE CR CHONDRITE NORTHWEST AFRICA 852

摘要

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains in the CR2 chondrite Northwest Africa (NWA) 852. Thirty-one O-anomalous grains were detected: 24 were identified as silicates (~78?ppm); the remaining 7 are Al-rich oxides (~38?ppm). NWA 852 is the first C2 chondrite containing O-anomalous presolar dust in concentrations comparable to other more primitive meteorites. Eight presolar SiC grains have been found, representing the highest abundance (~160?ppm) observed so far in primitive meteorites. 15N-enriched matter is also present, although very heterogeneously distributed. Twenty-six of the O-anomalous grains are enriched in 17O, originating from the outflows of low-mass asymptotic giant branch (AGB) stars. We calculate a silicate/oxide abundance ratio of ~2, which indicates a higher degree of aqueous alteration than observed for other presolar-grain-rich meteorites. NWA 852 thus stands between the presolar-grain-rich CR3 chondrites (MET?00426, QUE?99177) and CR2 chondrites with low presolar grain abundances (Renazzo, NWA?530). We calculate an initial presolar silicate abundance of ~800?ppm for NWA?852, if silicate destruction by aqueous alteration is taken into account. Transmission electron microscope (TEM) investigation of one presolar Al-rich grain of an AGB star origin revealed that the grain mainly consists of a single crystal of hibonite with slightly varying orientations. A distinct subgrain (d 100?nm) with a Ca/Ti ratio of ~1 is located in the central region, most likely indicating a perovskite-like phase. Our data suggest this phase to be a primary condensate and not an alteration product.