SULFUR ISOTOPIC COMPOSITIONS OF SUBMICROMETER SiC GRAINS FROM THE MURCHISON METEORITE

Abstract

We report C, Si, N, S, Mg-Al, and Ca-Ti isotopic compositions of presolar silicon carbide (SiC) grains from the SiC-rich KJE size fraction (0.5-0.8 µm) of the Murchison meteorite. One thousand one hundred thirteen SiC grains were identified based on their C and Si isotopic ratios. Mainstream, AB, C, X, Y, and Z subtypes of SiC, and X-type silicon nitride (Si 3 N 4) account for 81.4%, 5.7%, 0.1%, 1.5%, 5.8%, 4.9%, and 0.4%, respectively. Twenty-five grains with unusual Si isotopic ratios, including one C grain, 16 X grains, 1 Y grain, 5 Z grains, and 2 X-type Si 3 N 4 grains were selected for N, S, Mg-Al, and Ca-Ti isotopic analysis. The C grain is highly enriched in 29 Si and 30 Si (δ 29 Si = 1345‰ ± 19‰, δ 30 Si = 1272‰ ± 19‰). It has a huge 32 S excess, larger than any seen before, and larger than that predicted for the Si/S supernova (SN) zone, providing evidence against the elemental fractionation model by Hoppe et al. Two SN models investigated here present a more satisfying explanation in terms of a radiogenic origin of 32 S from the decay of short-lived 32 Si (τ 1/2 = 153 yr). Silicon-32 as well as 29 Si and 30 Si can be produced in SNe by short neutron bursts; evidence for initial 44 Ti (τ 1/2 = 60 yr) in the C grain is additional evidence for an SN origin. The X grains have marginal 32 S excesses, much smaller than expected from their large 28 Si excesses. Similarly, the Y and Z grains do not show the S-isotopic anomalies expected from their large Si isotopic anomalies. Low intrinsic S contents and contamination with isotopically normal S are the most likely explanations.

Sulphur isotope mass-independent fractionation observed in comet 67P/Churyumov–Gerasimenko by Rosetta/ROSINA

“…Besides the SiC grains, there are seven other types of grains with presolar origin identified: diamond, graphite, oxides, silicon nitride, Ti-/Te-/Zr-/Mo-carbides, kamacite/iron, and olivine (Zinner 2007). Before pre-solar grains were embedded in Solar system bodies like meteorites, interplanetary dust particles, Antarctica meteorites, and cometary matter (Xu et al 2015), they travelled through interstellar matter and were exposed to cosmic galactic rays, could experience sputtering by stellar wind and could be evaporated by SN shocks (Zinner 1998). At some point, they became part of the dense molecular cloud from which our Solar system emerged.…”

“…Since then, only during the last five years extensive studies of single grains could be done due to improvements of the analysis techniques (Hoppe et al 2010(Hoppe et al , 2015Xu et al 2015). Here we concentrate on sulphur isotopic ratios in those grains.…”

“…Recently, two studies examined SiC grains from Murchison meteorite for their isotopic composition: C, Si, N, S, Mg-Al, and Ca-Ti isotopic composition by Xu et al (2015), and Si, C, and S isotopic composition by Hoppe et al (2015). For their analysis, Xu et al (2015) used SiC grains of the separate KJE [described by Amari et al (1995)] which have a typical diameter between 0.5 and 0.8 μm.…”

“…For their analysis, Xu et al (2015) used SiC grains of the separate KJE [described by Amari et al (1995)] which have a typical diameter between 0.5 and 0.8 μm.…”

“…They conclude that the large 32 S excess comes more likely from decay of short-lived 32 Si than from the fractionation model by Hoppe et al (2012) because the excess is larger than what is predicted for the Si/S zone of core-collapse SNe. Besides comparing models of SNe, Xu et al (2015) addressed two main problems for S isotope studies: first, low intrinsic concentration since S does not readily condense into SiC, and secondly some contamination with isotopically terrestrial S during the separation process. So for the C grain in the negative ion image, most of the S was located at the rim and the depletion was less there than for the centre part (for more details, see Xu et al 2015).…”