2012ApJ...745...38L

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.