The classification of CM and CR chondrites using bulk H, C and N abundances and isotopic compositions

Here we show that bulk H, C and N elemental and isotopic analyses can be used to classify CM and CR chondrites. These meteorites in both groups form well-defined trends in plots of H content vs. δD and C/H vs. δD, and these trends appear to primarily reflect varying degrees of aqueous alteration. The subset of samples with evidence for thermal alteration plot well away from these trends. In CMs, both bulk H and N isotopic compositions, in particular, strongly correlate with petrologic indicators of the degree of alteration and have been used to classify 54 unheated or weakly heated meteorites on a scale of 2-3. However, extrapolation of the trends based on this scale to type 3.0 predicts relatively high water contents, and the schemes cannot be used to classify altered meteorite belonging to other chondrite groups. Here we propose a different classification scheme based on the degree of hydration (wt.% H in water and OH) of a meteorite that can be determined straightforwardly from a meteorite's bulk H and C contents. Our estimates of the extent of hydration in CMs correlate well with petrologic estimates of the extent of hydration and with the previously determined phyllosilicate abundances. This is not the case for the CRs, which we suggest is due to cryptic alteration of some CRs at low temperatures.