Hydrogen isotope exchange between n-alkanes and water under hydrothermal conditions

To investigate the extent of hydrogen isotope (H-2 and H-1) exchange between hydrocarbons and water under hydrothermal conditions, we performed experiments heating C-1-C-5 n-alkanes in aqueous solutions of varying initial H-2/H-1 ratios in the presence of a pyrite-pyrrhotite-magnetite redox buffer at 323 degrees C and 35-36 MPa. Extensive and reversible incorporation of water-derived hydrogen into C-2-C-5 n-alkanes was observed on timescales of months. In contrast, comparatively minor exchange was observed for CH4. Isotopic exchange is facilitated by reversible equilibration of n-alkanes and their corresponding n-alkenes with H-2 derived from the disproportionation of water. Rates of delta H-2 variation in C3+ n-alkanes decreased with time, a trend that is consistent with an asymptotic approach to steady state isotopic compositions regulated by alkane-water isotopic equilibrium. Substantially slower delta H-2 variation was observed for ethane relative to C-3-C-5 n-alkanes, suggesting that the greater stability of C3+ alkenes and isomerization reactions may dramatically enhance rates of H-2/H-1 exchange in C3+ n-alkanes. Thus, in reducing aqueous environments, reversible reaction of alkanes and their corresponding alkenes facilitates rapid H-2/H-1 exchange between water and alkyl-bound hydrogen on relatively short geological timescales at elevated temperatures and pressures. The proximity of some thermogenic and purported abiogenic alkane delta H-2 values to those predicted for equilibrium H-2/H-1 fractionation with ambient water suggests that this process may regulate the delta H-2 signatures of some naturally occurring hydrocarbons. (C) 2011 Elsevier Ltd. All rights reserved.