NanoSIMS isotopic analysis of small presolar grains:: Search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si3N4 grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for 26 Al-26/(27) Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 mu m) and 153 SiC grains from KJG (diameters 1.8-3.7 mu m), 154 SiC and 23 Si3N4 grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 mu m) for their C and N isotopic compositions, 549 SiC and 142 Si3N4 grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si3N4 grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si3N4 grain, with C-13 and (14) N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from -1% for grains >2 mu m to -5-7% for grains of 0.5 mu m), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred Al-26/Al-27 ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low C-12/C-13 ratios of Z grains, but this process apparently does not lead to increased 26 Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high Al-26/(27) Al ratios. The low Ti-46,Ti-47,Ti-49/Ti-48 ratios found in Z grains and their correlation with low Si-29/Si-28 ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in Si-30 and Ti-50 in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in Ti-50 (and Ti-49) are much larger than those found. Even lowering the strength of the C-13 pocket cannot solve this discrepancy in a consistent way. (C) 2007 Elsevier Ltd. All rights reserved.