Presolar Graphite from the Murchison Meteorite: Noble Gases Revisited

Introduction: Ne-E(L) is one of the anomalous noble gas components that ultimately led to the discovery of presolar grains in meteorites. The component is extremely enriched in 22 Ne and this fact prompted the idea that the 22 Ne is from the decay of 22 Na [1]. The carrier of Ne-E(L) is presolar graphite [2], which has been extracted from Murchison [3] and Orgueil [4]. There are four graphite-rich Murchison separates with a range of density (1.6–2.2 g/cm 3 ): KE1 (1.6–2.05 g/cm 3 ), KFA1 (2.05–2.10 g/cm 3 ), KFB1 (2.10–2.15 g/cm 3 ) and KFC1 (2.15–2.20 g/cm 3 ) [3]. KE1 was further separated to improve purity, yielding KE3 (1.65-1.72 g/cm 3 ) [5]. Thus, isotopic features of KE1 and KE3 can be regarded as the same. One of the most interesting characteristics of presolar graphite is that isotopic features depend on density. Low-density graphite grains, which are characterized by 18 O excesses, Si isotopic anomalies (mainly in the form of 28 Si excesses) and high inferred 26 Al/ 27 Al ratios (up to 0.1), are believed to have formed in Type II supernovae [6]. On the other hand, high-density graphite grains from KFC1, with their high 12 C/ 13 C ratios and 30 Si excesses, most likely formed in low-metallicity AGB (asymptotic giant branch) stars [7]. In this paper, we will reexamine noble gas data previously obtained on the Murchison separates. Data Sources: Two noble gas analyses were performed: Amari et al. [8] analyzed Ne, Ar, Kr and Xe in bulk ( aggregates) samples from KE1, KFA1, KFB1 and KFC1 by stepwise heating. Nichols et al. [9] analyzed 4 He and 20,22 Ne in single grains from KE3, KFB1 and KFC1. Discussion: We will mainly focus on the data on KE1 and KE3. Neon-22. Amari et al. [8] concluded that the 22 Ne in presolar graphite is dominantly from 22 Na, but portion of it originated from the He-shell and that the proportion of 22 Ne of the two origins depend on density. Nichols et al. [9] found 9 22 Ne-rich grains out of 21 KE3 grains and were able to obtain an upper limit of the 20 Ne/ 22 Ne ratio of 0.01 for grain KE3a-573. Of the 9 grains, 8 grains were previously analyzed for their C, O, and Si isotopic ratios with ion probe. Seven grains show 18 O/ 16 O ratios higher than the solar ratio (2 10 -3 ), ranging from 6.83 10 -3