A NanoSIMS study of Si- and Ca-Ti-isotopic compositions of presolar silicon carbide grains from supernovae

We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in Si-28 (up to 5X solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal Ca-42/Ca-40 ratios. Seven X grains show enhanced Ca-44/Ca-40 ratios of up to 6X the solar ratio. Spatial distributions of Ca-44 excesses and Ti are positively correlated, giving strong support to the view that excesses in Ca-44 are due to the decay of radioactive Ti-44. Inferred initial Ti-44/Ti-48 ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic Ca-44 is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic Ca-44 and Ti-48 as well as in Ti-44/Ti-48, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with Si-30/Si-29 = 2.2X the solar ratio and C- and N-isotopic compositions as typically observed in X grains. Copyright (C) 2003 Elsevier Ltd