The stepwise oxidation of the Allende meteorite reported here has shown that there are at least three isotopically distinct carbon phases, each combustible over different temperature ranges. The temperature of combustion is dependent both on grain size of the fragments studied and/or on the nature of the carbonaceous material and its availability to the oxygen. Thus we attempt here to correlate the isotopic signatures recognized with phases whose existence has been postulated to explain noble gas systematics1,2 and to provide some insight into the nature and location of these phases. Most of the carbon (δ13C=-20‰), possibly a highly cross-linked polymer, exists predominantly in the matrix and has been reported by others 3 to be the host of most of the normal planetary noble gases. Carbon protected in mineral grains may be isotopically heavier at -13 to -7‰, but is still more easily oxidized than graphite when exposed by HF/HCI demoralization. A very easily oxidized phase, as yet unidentified, has a δ13C value typically ∼ -23 to -27‰.
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