220 Geochemistry: Recent Advances in Geochemistry

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 102BC

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

Paper number: 220-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

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 KWGS 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.