The relative abundances of resolved l2 CH 2 D 2 and 13 CH 3 D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases

We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant D12CH2D2 and D13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where D12CH2D2 and D13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and D12CH2D2 values are a key tracer of microbial recycling.Published235-2646A. Geochimica per l'ambienteJCR Journa

Lithium isotopic systematics of hydrothermal vent fluids at the Main Endeavour Field, Northern Juan de Fuca Ridge

Vent fluids issuing from the Main Endeavour Field (MEF), Juan de Fuca Ridge, were analyzed for δ7Li to help constrain subseafloor hydrothermal alteration and phase separation processes. Magmatic activity prior to sampling of the fluids in 1999 enhanced heat and mass transfer, as indicated by the large scale, but temporary, changes in vent fluid chemistry. In particular, dissolved chloride concentrations indicate formation of supercritical Cl-poor vapors, which affected alteration throughout the MEF system. δ7Li of fluids, however, ranges from +7.2 to +8.9‰ and reveals no significant correlation with dissolved chloride, being consistent with results of hydrothermal experiments that show no lithium isotope fractionation during supercritical phase separation. On a chloride-normalized basis, Li concentration data indicate relatively short residence times or high fluid/rock mass ratios of vent fluids most impacted by phase separation effects. Reaction path models involving Li isotope data also show elevated fluid/rock mass ratios. Boron data, in contrast, suggest direct input from degassing magma. Enhanced heat flow associated with magmatic injection at depth inhibits penetration of seawater-derived hydrothermal fluid into fresh basalt, particularly in those systems where magmatic volatile input is most active. The inverse correlation between Li/Cl and B/Cl in vapor-rich vent fluids may be a useful indicator of recent subseafloor magmatic activity

Structural features of quench products of melts in the chloride-carbonate-silicate systems revealed by vibrational and X-ray spectroscopy

Quench products of melts synthesized at 5 GPa and 1500°C in model system CaMgSi2O6–Na2CO3(±CaCO3)–KCl, were studied using vibrational (IR and Raman) and X-ray absorption spectroscopy (XANES). Correlations between structural peculiarities of the quenches with chemical composition are established. Increase of the CaMgSi2O6 content of the melts results in gradual substitution of the Са-bearing carbonate groups by Na-bearing, whereas Ca is progressively more bounded with silicate structural units. XANES spectra reveal that chlorine is predominantly present as (KxNa1–x)Cl complexes. XANES spectra also indicate distribution of potassium cations between chloride and silicate groups, although its partial bonding with carbonate groups in the melt is not excluded.</p