The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4 over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of kr of sec−1 [mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur.
Collecting gas-tight fluid samples from deep-sea hydrothermal vents is challenging due to the high pressure, temperature, and corrosiveness of hydrothermal fluids. The most critical part of a gas-tight sampler is the sampling valve because it must have reliable bidirectional sealing capability under such harsh conditions. A new titanium sampling valve has been designed to address these issues. Key features of the new sampling valve include the application of a titanium metal-to-metal seating to achieve high temperature and corrosion resistance, a novel self-tightening design that makes the valve a more reliable closure under high pressure, and a hydraulic actuating mechanism which enhances the convenience and reliability of valve's operation under deep sea. Currently, the sampling valve can be used for vent fluids at depths of 6000 m and temperatures up to 400°C. The new sampling valve has been applied on a gas-tight sampler and tested at the hydrothermal vent sites along the Mid-Atlantic Ridge during the cruise KNOX18RR in 2008. Fluid samples were collected from hydrothermal vents with the depths from 744 to 3622 m and temperatures from 94°C to 370°C. Results of the field tests indicated the usefulness of the new sampling valve to collect deep-sea hydrothermal fluids.
