FE-SILICATE STABILITY, REACTIVE TRANSPORT PROCESSES, AND FLUXES OF H2 FROM HYDROTHERMAL VENTS THROUGH EARTH HISTORY

Hydrothermal fluxes of H₂ through Earth history played a critical role in shaping the early evolution of life and the redox state of the early atmosphere. Seafloor measurements demonstrate that hydrothermal oxidation of Fe-bearing igneous minerals by convecting seawater is a primary source of H₂ today, but how this process has changed through time is not well understood. Primarily, constraints on hydrothermal H₂ fluxes throughout Earth history have been estimated using geochemical calculations of mineral-fluid equilibria. These calculations have provided important estimates of the potential H₂ fluxes from ultramafic hydrothermal systems, but much of the thermodynamic data they employ is based upon preliminary estimates, and several key reactions tend to be overlooked. Additionally, the predicted H₂ fluxes generally exceed observations from modern hydrothermal systems, mostly because they tend to focus on rock-buffered equilibria rather than the reactive transport processes that inevitably characterize venting fluids. Furthermore, recent experimental and seafloor observations show that SiO₂ cycling through ultramafic-hosted hydrothermal systems and its influence on the thermodynamic stability of Fe-silicates is a potentially important yet overlooked factor. Here, we produce revised models of hydrothermal vent fluxes of H₂ through Earth history by merging recent experimental and field observations, numerical calculations, and estimates of paleo-seawater composition. In general, our results show that the thermodynamic drive for Fe to remain in the Fe(II) state at elevated temperatures, combined with considerations of the high fluid fluxes passing through these systems, tend to predict venting H₂ concentrations considerably lower than those estimated in previous studies.