Murchison presolar carbon grains of different density fractions: A Raman spectroscopic perspective

Abstract

Raman analyses are reported of μm-sized areas of 103 individual carbonaceous presolar grains (“graphite grains”) from three different density fractions of the Murchison meteorite. Few of the grains (2 or 3 of each density fraction) have Raman spectra typical for non-crystalline sp2-bonded carbon (i.e., “organic carbon”) with extremely wide 1st-order and no (or very subdued) 2nd-order peaks, similar to the ones found for terrestrial kerogens. Based on depth profiles of isotopic ratios measured with the NanoSIMS, it is unlikely that such kerogen-type Raman signatures are caused by contamination of the presolar grains with insoluble organic material from the Murchison matrix that stuck to the surfaces of the grains. Rather, the kerogen-type grains are considered to be a new type of presolar carbon grains, which are made up of organic (PAH-like) sp2-bonded carbon. However, most of the other studied presolar carbon grains (95 of 103) have spectra with very narrow 1st-order peaks (called D and G peaks) and very strong 2nd-order peaks typical for inorganic sp2-bonded carbon. Based on their D/G intensity ratios, those grains were grouped into the following Raman types: (fairly well ordered) “graphite” (D/G<0.5), “disordered graphite” (0.5<D/G<1.1), “glassy carbon” (D/G>1.1), and “unusual sp2-bonded graphitic carbon” (with extremely intense 2nd-order peaks relative to the 1st-order peaks). Grains from the low-density fraction KFA1 (2.05–2.10g/cm3) have predominantly “cauliflower” morphology and Raman spectra characteristic of either very disordered graphite or “glassy carbon” (i.e., the latter is amorphous from the Raman spectroscopic perspective), whereas most grains from the high-density fraction KFC1 (2.15–2.20g/cm3) have “onion” morphology and Raman spectra characteristic of well-crystalline graphite. The KFB1 grains with intermediate density (2.10–2.15g/cm3) are mixed, both in terms of their morphology and their Raman spectra but are closer to KFC1 than to KFA1 grains. The correlation of the Raman results with both morphology and isotopic data show that presolar sp2-bonded carbon grains from different stellar sources differ in their crystalline structure. Grains that dominate the high density fractions and whose isotopic ratios indicate an origin in AGB stars consist of well crystallized graphite, whereas grains that dominate the low-density fraction and whose isotopic ratios indicate a supernova origin consist of very disordered graphite or even of amorphous “glassy carbon”.