New geochemical constraints on hydrothermal fluid fluxes at mid-ocean ridge axes and ridge flanks

This research project addresses a very fundamental issue of geochemistry - how much water is processed through and expelled from the sub-seafloor hydrothermal systems that are found throughout the world's oceans? This is an important question because hydrothermal processes have a profound effect on the global geochemical budgets of numerous elements (including the greenhouse gas carbon dioxide) and they thus exert a strong influence on the composition of the oceans, the atmosphere and, through subduction of altered ocean crust, the upper mantle. Estimates of hydrothermal power outputs that are derived from thermal models of the oceanic crust are generally thought to be more reliable than direct geochemical estimates of hydrothermal fluid fluxes, many of which are based on the poorly constrained marine mass balances of individual elements and isotope systems (e.g., Mg and 87Sr/86Sr). The conversion of power outputs into hydrothermal water fluxes is fraught with difficulties, however, primarily due to the unknown partitioning of heat between fluids of different temperature (and hence different chemical reactivity) and uncertainties about the extent to which the available heat is used to drive hydrothermal circulation at mid-ocean ridge axes. This is unfortunate because the geochemical impact of hydrothermal systems is directly related to the water fluxes rather than the power outputs. In this study, we will use a novel geochemical approach to obtain precise constraints on the global water fluxes that are expelled into the oceans from both the hot sub-seafloor hydrothermal systems of mid-ocean ridge axes and the cooler hydrothermal systems that exist at ridge flanks. The method utilizes concentration and isotope composition data for the element thallium (Tl) in relevant samples. A recent pilot study demonstrated that the new approach is significantly more robust than other geochemical methods. The results of the pilot study are only of limited value, however, because they are based on only a small number of analytical data. The present investigation will address this shortcoming, through a thorough characterization of the Tl concentrations and isotopic compositions of altered oceanic basement rocks, using representative samples from different tectonic settings and spreading rates, and a suite of globally representative hydrothermal fluids. Due to the unique properties of the Tl isotope system, these analyses will provide the most reliable geochemical estimates of hydrothermal fluid fluxes available to date.