¹Peter Hoppe, ²Katharina Lodders, ¹Wataru Fujiya
¹Max Planck Institute for Chemistry, Mainz, Germany
²Department of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri, 63130, USA

We studied 14 presolar SiC mainstream grains for C-, Si-, and S-isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite-normalized S/Si ratios between 2 × 10−5 and 2 × 10−4. All grains have S-isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low-mass asymptotic giant branch (AGB) stars. In a δ33S versus δ34S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess 32S found in some Type AB SiC grains are the result of in situ decay of radioactive 32Si from born-again AGB stars that condensed into AB grains.

Reference
Hoppe P, Lodders K, Fujiya W (2015) Sulfur in presolar silicon carbide grains from asymptotic giant branch stars. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12449]

Published by arrangement with John Wiley&Sons

¹Steven B. Simon, ^1,2Lawrence Grossman
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
²The Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA

The Antarctic carbonaceous chondrites DOM 08004 and DOM 08006 have been paired and classified as CO3.0s. There is some uncertainty as to whether they should be paired and whether they are best classified as CO chondrites, but they provide an opportunity for the study of refractory inclusions that have not been modified by parent body processes. In this work, refractory inclusions in thin sections of DOM 08004 and 08006 are studied and compared with inclusions in ALHA77307 (CO3.0) and Acfer 094 (C3.0, ungrouped). Results show that the DOM samples have refractory inclusion populations that are similar to each other but not typical of CO3 chondrites; main differences are that the DOM samples are slightly richer in inclusions in general and, more specifically, in the proportions of grossite-bearing inclusions. In DOM 08004 and DOM 08006, 12.4% and 6.6%, respectively, of the inclusions are grossite-bearing. This is higher than the proportion found in Acfer 094 (5.2%), whereas none were found in ALHA77307. Like those in Acfer 094, DOM inclusions are small (mostly

Reference
Simon SB, Grossman L (2015) Refractory inclusions in the pristine carbonaceous chondrites DOM 08004 and DOM 08006. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12452]

Published by arrangement with John Wiley&Sons

¹A. A. Shiryaev,²A. V. Fisenko,³L. F. Semjonova,⁴A. A. Khomich,⁵I. I. Vlasov
¹Institute of Physical Chemistry and Electrochemistry RAS, Moscow, Russia
²Institute of Ore Deposits, Petrography, Geochemistry and Mineralogy RAS, Moscow, Russia
³Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
⁴General Physics Institute RAS, Moscow, Russia
⁵National Research Nuclear University MEPhI, Moscow, Russia

Photoluminescence spectra show that silicon impurity is present in lattice of some nanodiamond grains (ND) of various chondrites as a silicon-vacancy (SiV) defect. The relative intensity of the SiV band in the diamond-rich separates depends on chemical composition of meteorites and on size of ND grains. The strongest signal is found for the size separates enriched in small grains; thus, confirming our earlier conclusion that the SiV defects preferentially reside in the smallest (≤2 nm) grains. The difference in relative intensities of the SiV luminescence in the diamond-rich separates of individual meteorites are due to variable conditions of thermal metamorphism of their parent bodies and/or uneven sampling of nanodiamond populations. Annealing of separates in air eliminates surface sp2-carbon; consequently, the SiV luminescence is enhanced. Strong and well-defined luminescence and absorption of the SiV defect is a promising feature to locate cold (<250 °C) nanodiamonds in space.

Reference
Shiryaev AA, Fisenko AV, Semjonova LF, Khomich AA, Vlasov II (2015) Photoluminescence of silicon-vacancy defects in nanodiamonds of different chondrites. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12450]

Published by arrangement with John Wiley&Sons