Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.021]
1Earth & Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington DC 20015, USA
A series of experiments was performed to constrain the chemical and isotope evolution of insoluble organic material (IOM) during hydrothermal alteration at temperatures ranging from 250 °C to 450 °C at 50 MPa. Experiments involved IOM that was extracted from the Murchison (CM2) meteorite or synthesized by aqueous carbonization of dextrose. Flash (dry) pyrolysis experiments at 400 – 1000 °C were also conducted with Murchison-IOM to distinguish between the effects of hydrothermal and thermal degradation. Extended reaction times (up to 3905 h) were employed to establish D/H equilibria between IOM and H2O. The H isotope compositions of the H2O used in the experiments ranged from δD = -447 ‰ to 3259 ‰. Results revealed that the extent of the IOM H isotope evolution strongly depends on the δD composition of the coexisting H2O with minimal temperature effects. The empirical relationship that describes the isotope exchange between IOM and H2O is as follows:
δDIOM (‰) = 0.643 (± 0.007) * δDH2O (‰) – 86 (± 8) (‰)
Based on this empirical relationship, two models are proposed for the H2O-IOM H exchange. The first assumes that all H in IOM is exchangeable and that the redistribution of H-bearing moieties with experiment temperature results in an “apparent” εorganics-H2O= -357 ‰. The second model considers a higher εorganics-H2O (-131 ‰), in accordance with theoretical studies, and assumes the presence of two H reservoirs, one that undergoes H isotope exchange with H2O and one that does not. In this case, 74 % of the H in IOM is exchangeable with H2O.
In our experiments, the hydrothermally altered Murchison-IOM lost labile 15N enriched N-H moieties. Experiments that included 15N-labelled NH3(aq) found that there was only minor N exchange with IOM. Furthermore, the experimental data show that the extent of H and N loss is temperature and process dependent. This results in the decoupling of N/C and H/C atomic ratio systematics between hydrothermal alteration and flash (dry) pyrolysis, with much more limited changes in H/C and N/C after flash pyrolysis.
In the light of the experiments, two models for the range of bulk and IOM H isotope compositions of the aqueously altered CI, CM, and CR chondrites are explored. The very D-rich IOM compositions, relative to the bulk compositions, cannot be explained by a fully exchangeable IOM with a reasonable value for εorganics-H2O (i.e., <0 ‰). Instead, a two-component IOM model is invoked in which the initial bulk and non-exchangeable IOM have δD = 3650 ‰. The estimated ranges of Fexchange, including uncertainties in εorganics-H2O, are 0.59-0.75 and 0.13-0.30 for CMs and CRs, respectively. Most values of Fexchange are significantly lower than in the experiments, perhaps because the alteration temperatures in the chondrites were << 250 °C. An apparent relationship between Fexchange and the IOM δ15N suggests an endmember composition of ∼ 300 ‰. For the CMs, alone, however, the initial δ15N is projected to ∼ 137 ‰.