A NANOSIMS STUDY OF SI- AND CA-TI-ISOTOPIC COMPOSITIONS OF PRESOLAR SILICON CARBIDE GRAINS FROM SUPERNOVAE

Abstract

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 28Si (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 42Ca/40Ca ratios. Seven X grains show enhanced 44Ca/40Ca ratios of up to 6x the solar ratio. Spatial distributions of 44Ca excesses and Ti are positively correlated, giving strong support to the view that excesses in 44Ca are due to the decay of radioactive 44Ti. Inferred initial 44Ti/48Ti ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic 44Ca is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic 44Ca and 48Ti as well as in 44Ti/48Ti, 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 30Si/29Si = 2.2x the solar ratio and C- and N-isotopic compositions as typically observed in X grains.