Correlated isotopic and microstructural studies of turbostratic presolar graphites from the Murchison meteorite

Abstract

We present data from TEM and NanoSIMS investigations of Murchison (CM2) KFC1 presolar graphites. TEM examinations of graphite ultramicrotome sections reveal varying degrees of graphite disorder, leading to distinctions between well-graphitized onions, more turbostratic platy graphites, and the most disordered cauliflower graphites. Aside from their larger size, platy graphites are roughly similar in isotopic composition and in internal grain properties to the well-graphitized onions. Most carbide-containing platy graphites exhibit large s-process element enrichments (~200 solar Mo/Ti ratios), suggesting origins predominantly in AGB carbon stars. The C isotopic distribution of platy graphites is similar to onions, with representatives in both 12C-depleted (5 < 12C/ 13C < 40) and 12C-enriched groups (100 < 12C/13C < 350) and a pronounced gap in the 40 < 12C/13C < 75 region that contains 75% of mainstream SiCs. The large 12C enrichments combined with the extreme s-process element enrichments suggest formation in an environment inhomogeneously enriched in the nucleosynthetic products of thermal pulses in AGB stars. In contrast, numerous scaly cauliflower graphites show 18O enrichments and lack s-process-enriched carbides, suggesting a SN origin, as was the case for many Murchison KE3 SN graphites. The more turbostratic graphites (platy and scaly) are on average larger than onions, likely resulting from formation in a gas with higher C number density. Oxygen content increases progressively with increasing degree of graphite disorder, which can stabilize these grains against further graphitization and may be a reflection of higher O/C ratios in their formation environments.