The bulk abundances and isotopic compositions of H in CM and CI carbonaceous chondrites
Conel M. O’D. Alexander, Dionysis I. Foustoukos
Geochimica et Cosmochimica Acta
Available online 28 May 2026
“The H abundances and isotopic compositions of primitive chondritic meteorites are important for understanding: (i) the accretion environments of and the internal processes in their asteroidal parent bodies, and (ii) the potential sources of volatile elements in the terrestrial planets. However, ubiquitous terrestrial contamination complicates the analysis and interpretation of H measurements. Two important groups of chondrites are the CMs and CIs that formed in the outer Solar System, are abundant in the asteroid belt, and are strong candidates for being major sources of Earth’s volatiles. Here we report the results of bulk H abundance and isotope analyses of (1) heating experiments conducted on two CMs and a CI, and (2) a suite of 48 CMs and two CIs. The analyses used a protocol designed to minimize contributions from terrestrial contamination. Prior to the analyses, the samples were stored in a dry Ar-flushed glovebox and preheated at either 120 °C or 200 °C for 48 h. For the CMs, the 120 °C preheating removes most or all terrestrial contamination, but the preheating to 200 °C also removes some indigenous OH/H2O. For the 120 °C preheated CMs, there is a strong correlation between their bulk H isotopic compositions and C/H ratios that is interpreted to result from variable mixing of initially isotopically light H2O (δD≈-440 ‰) and isotopically heavy organic material (δD≈3200 ‰). The current H isotopic compositions of the OH/H2O and organic material in the CMs are less extreme than their initial compositions at the time of accretion due to isotopic exchange during aqueous alteration. The results of heating experiments on bulk samples of Orgueil (CI1) show that the exchangeable interlayer H2O of Orgueil’s saponite is entirely of terrestrial origin, and that using our protocol it is only removed by heating to 200 °C. It is not known to what extent the interlayer sites in Orgueil’s saponite were hydrated in its original parent body. However, if its saponite was fully hydrated with indigenous H2O, Orgueil’s bulk composition falls close to the CM δD vs. C/H trend, implying that the parent bodies of both chondrite groups accreted H2O and organics with similar initial H isotopic compositions”
