Presolar SiC abundances in primitive meteorites by NanoSIMS raster ion imaging of insoluble organic matter

Davidson, J.; Busemann, H.; Alexander, C. M. O'D.; Nittler, L. R.; Schrader, D. L.; Orthous-Daunay, F. R.; Quirico, E.; Franchi, I. and Grady, M. M. (2009). Presolar SiC abundances in primitive meteorites by NanoSIMS raster ion imaging of insoluble organic matter. In: 40th Lunar and Planetary Science Conference, 23-27 Mar 2009, Houston, Texas.



Here we present results obtained with NanoSIMS raster ion imaging to determine the abundance of presolar SiC in the insoluble organic matter (IOM) extracted from a number of different classes of chondrites (both carbonaceous and ordinary). This builds on previous work [1] aimed at obtaining SiC abundances in primitive meteorites by
SIMS and comparing them with noble gas analyses.

Both IOM and presolar grains are found in similar
CI-like relative abundances in the matrices of the most
primitive chondrites [2, 3], indicating that a homogeneous
mixture of grains was incorporated in the various parent bodies [3]. Both are then subjected to thermal and hydrothermal processing after parent body formation [4]. However, there are significant variations in the matrix-normalized abundances of SiC grains estimated from noble gases carried by presolar grains, which suggest that the primitive chondrites did not form from a well-mixed reservoir of presolar grains. Variations in the source material were attributed to the destruction of presolar grains by heating in the solar nebula (temperatures that may have exceeded 700°C) and were linked to the volatile element fractionations in chondrites [5].

The CR chondrites have amongst the lowest matrix-normalized SiC abundances, and largest volatile element ractionations, reported in the carbonaceous chondrites [5]. However, they contain the most primitive IOM of any chondrite class [6-7], which has experienced peak temperatures of <300°C [8]. These lowtemperatures could not have affected the SiC grains or their noble gas concentrations, indicating that either the IOM escaped heating (implying that it is not presolar)or SiC was degassed/destroyed at low temperatures,
perhaps during parent body processing [3]. Thus, in order to resolve this contradiction, it is necessary to determine SiC abundances independently of noble gases. Ion imaging of SiC grains is a direct technique that has been shown to successfully identify presolar SiC grains amongst others.

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