Presolar Grain Inventories of the Ungrouped C3 Adelaide and the Cv3 Rbt 04133

Introduction: Presolar grains are particularly abundant in primitive extraterrestrial materials such as IDPs [e.g. 1-3] and the matrices of the least altered chondrites [4-6]. To date, the presolar silicate and oxide inventories of the primitive chondrites Acfer 094 [4-7] and ALHA 77307 [7] have been reported extensively. More recently, the search for presolar silicates and oxides has been extended to the CR [8,9] and ungrouped chondrites [10]. Here we report the presolar grain inventories (silicates, oxides, SiC, and other C-anomalous phases) determined for Adelaide and RBT 04133 by NanoSIMS raster ion imaging. Adelaide (an ungrouped C3) was chosen for this study because (i) it is primitive and (ii) it was previously reported to have a low presolar silicate abundance of ~41 ppm [11] in comparison to Acfer 094 and ALHA 77307; 90-175 [7,12] and 95 ppm respectively [7]. RBT 04133 (a mildly thermally altered CV3) was also analysed as part of an ongoing study [13]. Experimental: NanoSIMS raster ion imaging was used to map pressed fragments of matrix material from Adelaide and RBT 04133 to locate presolar grains and other isotopic anomalies. Individual fragments varied from 10x10 to 20x20 μm in size. Two sets of measurements were performed: (i) to locate C-anomalous phases such as SiC and graphite and N-anomalies (C, C-, CN-, CN-, O-, Si-, and MgO-) and (ii) to locate O-anomalous phases such as oxides and silicates (O-, O-, O-, Si-, Si-, Si-, and MgO-). Total areas of 2600 and 2700 μm were analysed in Adelaide and RBT 04133 respectively. C and N isotopic compositions were normalized using insoluble organic matter (IOM) standards [14]. O isotopic compositions were normalized to the average composition of matrix within each meteorite. C-anomalous Grains: Four C-anomalous grains (one SiC and three other C-anomalous phases) were located in each of Adelaide and RBT 04133. On the basis of C and N-isotopic compositions both SiC grains appear to be Y grains but could potentially be X grains within error (Fig. 1). Si isotopic compositions of both SiC grains (determined during subsequent analyses) are solar within 3σ error. O-anomalous Grains: Seven and four Oanomalous grains were located in Adelaide and RBT 04133 respectively (Fig. 2). The presolar silicate/oxide nature of each O-anomalous grain was determined on the basis of their Si, MgO compositions, and Si/O ratio [16]. Five O-anomalous grains from Adelaide were determined to be presolar silicates whilst the remaining two grains appear to be presolar oxides. Three O-anomalous grains from RBT 04133 were determined to be presolar silicates whilst the remaining grain appears to be a presolar oxide. Si isotopic compositions of all presolar silicates grains are solar within 3σ error.