KINETIC AND THERMODYNAMIC CONTROLS ON THE CONCENTRATIONS OF CARBON GASES IN HYDROTHERMAL FLUIDS

Dissolved CO₂, H₂, CH₄ and CO are common species in deep-sea hydrothermal vent fluids. We have observed CO and CH₄ formation using a novel experimental configuration that permits constant high-pressure flow through hydrothermal cells while maintaining uniform concentrations of multiple dissolved gases. Using a source fluid containing geologically relevant concentrations of H₂ and CO₂, CO and CH₄ concentrations were monitored after passing through cells constructed of both titanium and gold, with and without magnetite, between 200-400°C and pressures up to 325 bars. We have calibrated the equilibrium constant for the water-gas shift (WGS) reaction (CO₂ + H₂ = CO + H₂O) in pure water up to 374°C, including the critical point. In this region the log K_WGS is linear with respect to water density. At temperatures below 300°C, attainment of equilibrium was kinetically prohibited for residence times applicable to the up-flow zone for hydrothermal fluids at mid-ocean ridges (~1-24 hrs). This is consistent with observations of natural fluids samples, where CO₂-H₂-CO appear in equilibrium at the measured exit temperature for higher temperature fluids (i.e. T > 300°C) while this is not the case for lower temperature fluids.

Mechanisms of abiogenic CH₄ formation in natural hydrothermal systems are still a matter of much debate as redox constraints make magmatic degassing unlikely. Our experiments show CH₄ generation (e.g. CO₂ + 4H₂ = CH₄ + 2H₂O) is linear with residence time in the cells and synthesis is maximized at temperatures near 300°C. CH₄ values were always far from predicted equilibrium, demonstrating the notoriously sluggish reaction kinetics at thermodynamically favorable temperatures. Natural vent fluids, however, often exhibit excess CH₄ relative to equilibrium, with concentrations changing relatively little despite the large increases in CO₂ and H₂ generally associated with eruptions and subseafloor magmatic events. This suggests CH₄ formation may be occurring during recharge, prior to encountering peak reaction conditions near the base of hydrothermal up-flow zones. Additional experimental results demonstrate that CH₄ synthesis is enhanced as much as 200X in the gold cell relative to the titanium cell, while magnetite appears relatively ineffectual as a catalyst for hydrocarbon formation.