Bonanza: Isotopic Anatomy of a Large Pre- Solar Sic Grain of Type X

SOLAR SIC GRAIN OF TYPE X. E. Zinner, M. Jadhav, F. Gyngard, and L. R. Nittler. Laboratory for Space Sciences and Physics Deparment, Washington University, St. Louis, MO, USA. E-mail: [email protected] Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC, USA. Introduction: Among presolar SiC grains from the LU fraction of the Murchison L separation series [1, 2], we identified an unusually large (30μm, see Fig. 1a) type X grain [3]. Because of its enormous size, this grain, named Bonanza, offers the possibility of analyzing a large number of elements for their isotopic ratios. By isotopic imaging we can determine the distribution of isotopic ratios in the grain and check for isotopic heterogeneities. Results: With the NanoSIMS, we measured C, Si, N, S, Li, B, Al-Mg, Ca and Ti. The first four elements were measured as negative (N as CN), the others as positive secondary ions. The average C, N, and Si isotopic ratios of the whole grain are C/C = 190, N/N = 28, δSi = –282‰, δSi = –442‰. In the isotopic images we observed variations in the C and Si isotopic ratios over the grain. The variations in the Si ratios can be explained by contamination with small mainstream grains [3], but not those of the C ratios when combined with the Si ratios. Magnesium in the grain is completely dominated by radiogenic Mg (δMg up to 4.5×10 ‰). Aluminum is heterogeneous over the grain and so is the inferred Al/Al ratio, which ranges from 0.4 to 0.9 (Fig. 1b).