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

Abstract

To investigate the extent of hydrogen isotope (2H and 1H) exchange between hydrocarbons and water under hydrothermal conditions, we performed experiments heating C1–C5n-alkanes in aqueous solutions of varying initial 2H/1H ratios in the presence of a pyrite–pyrrhotite–magnetite redox buffer at 323°C and 35–36MPa. Extensive and reversible incorporation of water-derived hydrogen into C2–C5n-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 H2 derived from the disproportionation of water. Rates of δ2H 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 δ2H variation was observed for ethane relative to C3–C5n-alkanes, suggesting that the greater stability of C3+ alkenes and isomerization reactions may dramatically enhance rates of 2H/1H exchange in C3+n-alkanes. Thus, in reducing aqueous environments, reversible reaction of alkanes and their corresponding alkenes facilitates rapid 2H/1H 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 δ2H values to those predicted for equilibrium 2H/1H fractionation with ambient water suggests that this process may regulate the δ2H signatures of some naturally occurring hydrocarbons.