Experimental determination of mineral solubility in coexisting vapor-liquid: application to the chemical evolution of mid-ocean ridge hydrothermal fluids

Abstract

As seawater circulates through a mid-ocean ridge (MOR) hydrothermal system its physical properties and chemical composition change drastically. Seawater is cold (2 ˚C), oxic and metal-poor. Hydrothermal fluids are hot (290–400 ºC), anoxic, and metal-rich. The conversion of seawater into hydrothermal fluid therefore represents a major transfer of heat and mass between the oceanic crust and circulating fluids. This transfer of heat and mass regulates the thermal structure of young oceanic crust and plays an important role in controlling seawater chemistry over geologic time. The chemical composition of seafloor hydrothermal fluids collected at the ocean floor provides information regarding heat and mass transfer processes that occur in an otherwise inaccessible environment. This dissertation compares the results of mineral solubility experiments to the chemical composition of natural fluids to further our understanding of the chemical evolution of seafloor vent fluids. Experiments were conducted in a newly designed fixed volume titanium reactor (Chapter 2) at pressure-temperature (P-T) conditions (400–500 ºC, 25–50 MPa) representative of the magma-hydrothermal interface. At such high temperatures and relatively low pressures, seawater can phase separate into a low-salinity/density vapor and high-salinity/density liquid. The P-T conditions at which phase separation occurs at a given vent site can be estimated by comparing the SiO2(aq) and Cl concentrations of a vent fluid with experimental calibration of the solubility of these two components, both of which are sensitive to changes in P-T. Chapter 2 presents the results of quartz solubility experiments and the calibration of a Si-Cl geothermobarometer, an important tool for estimating MOR heat and mass fluxes. The vapor and liquid that result from phase separation of seawater have substantially different physiochemical properties that can result in isotopic fractionation of dissolved chemical components. Chapter 3 presents experiments that determine Ca isotope systematics during phase separation. The results of these experiments are compared with the Ca isotopic signature of natural vent fluids to understand the exchange of Ca isotopes between vent fluids and the oceanic crust. Due to the extreme P-T conditions at which phase separation occurs, there are currently no theoretical models that can predict mineral solubility in either vapor or liquid. Chapter 4 provides the first attempt to calculate mineral solubility in low-density hydrothermal fluids. The solubility calculations are validated by comparison with experimental measurements of anhydrite solubility. The lack of theoretical solubility models also precludes calculation of the concentration of neutral aqueous species, such as H2(aq). Chapter 5 presents measurements of H2(aq) concentrations in saline hydrothermal fluids. These data constrain the redox state of MOR vent fluids, an important parameter in both fluid-mineral and fluid-fluid reactions. Together, the data and analysis presented here provide a more quantitative understanding of the chemical evolution of MOR vent fluids that undergo phase separation. The high quality experimental data will also allow for future development of theoretical models of mineral solubility in low-density hydrothermal fluids.