Structural and isotopic microanalysis of presolar SiC from supernovae
Abstract
We report on the microstructure, crystallography, chemistry, and isotopic compositions of seven SiC X grains and two mainstream grains from the Murchison meteorite. TEM crystallographic analysis revealed that the X grains (approximately 3μm) are composed of many small crystals (24-457nm), while the similarly sized mainstream grains are composed of only a few crystals (0.5-1.7μm). The difference in crystal size likely results from differences in their formation environments: the X grain crystals evidently formed under conditions of greater supersaturation and rapid growth compared to their mainstream counterparts. However, the same polytypes are observed in both mainstream and X grains. Six X grains and both mainstream grains are entirely the 3C-SiC polytype and one X grain is an intergrowth of the 3C-SiC and 2H-SiC polytypes. EDXS measurements indicate relatively high Mg content in the X grains (<~5 atomic%), while Mg was undetectable in the mainstream grains. The high Mg content is probably from the decay of 26Al into 26Mg. Estimates of the 26Al/27Al ratios, which range from 0.44-0.67, were made from elemental Mg/Al ratios. This range is consistent with the 26Al/27Al ratios inferred from previous isotopic measurements of X grains. We also report the first direct observations of subgrains in X grains, including the first silicides [(Fe,Ni)nSim]. Diffraction data do not match any previously observed presolar phases, but are a good fit to silicides, which are predicted stable SN condensates. Eight subgrains with highly variable Ni/Fe ratios (0.12-1.60) were observed in two X grains.
- Publication:
-
Meteoritics and Planetary Science
- Pub Date:
- April 2010
- DOI:
- 10.1111/j.1945-5100.2010.01045.x
- Bibcode:
- 2010M&PS...45..596M