CHARACTERIZATION OF PRESOLAR MATERIAL IN THE CR CHONDRITE NORTHWEST AFRICA 852

Abstract

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 ({approx}78 ppm); the remaining 7 are Al-rich oxides ({approx}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 ({approx}160 ppm) observed so far in primitive meteorites. {sup 15}N-enriched matter is also present, although very heterogeneously distributed. Twenty-six of the O-anomalous grains are enriched in {sup 17}O, originating from the outflows of low-mass asymptotic giant branch (AGB) stars. We calculate a silicate/oxide abundance ratio of {approx}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 {approx}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 consistsmore » of a single crystal of hibonite with slightly varying orientations. A distinct subgrain (d < 100 nm) with a Ca/Ti ratio of {approx}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.« less