Accomplishment Based Renewal: An experimental study of phase separation and mineral fluid equilibria on iron and hydrogen transport in mid-ocean ridge hydrothermal systems

点击翻译按钮获取中文摘要

Hydrothermal circulation at mid-ocean ridges constitutes the largest geothermal system on Earth. Venting of spectacular plumes of hot, metal-bearing, fluid into seawater from sulfide structures on the seafloor along the volcanically active global mid-ocean ridge system is the end result of heat and chemical transfer between the ocean crust and the overlying ocean. As a result, the composition of seafloor hydrothermal vent fluids places a major control on ocean chemistry, as it has throughout Earth history. Seafloor hydrothermal fluids also provide dissolved gases and other chemicals to sustain microbial ecosystems that provide clues to the origin of life on Earth. The discovery of submarine hydrothermal venting and their accompanying microbial communities that do not require sunlight or organic matter created by photosynthesis to live, remains one of the most significant discoveries in modern science. The mechanisms, however, by which seawater is transformed to compositionally distinct hydrothermal vent fluids are uncertain, owing to the extreme temperatures and pressures of these processes. This research uses novel laboratory reactor and in-situ sensor systems to carry out first-of-a-kind experiments at the temperatures and pressures of seafloor hydrothermal venting to determine the effect of oxidation-reduction reactions and pH on mineral solubility at temperatures and pressure not previously studied. Goals of the work are to provide fundamental geochemical thermodynamic and kinetic parameters involving iron and hydrogen in water-rock interaction at high temperatures and pressures. The broader impacts of the work to research and science education include (1) Involvement of undergraduate science majors; (2) graduate student training in experimental and theoretical studies of mineral-fluid reaction which enhances their career opportunities; (3) development of new models to predict more accurately the links between the physical and chemical processes in natural multi-component hydrothermal systems; (4) physical infrastructure enhancement with the development of new equipment and techniques to conduct experiments and collect data in the area of phase separation of H2O; and, (5) public outreach in cooperation with K-12 programs at the Bell Museum of Natural History at the University of Minnesota. This research will acquire data to advance knowledge of mineral-fluid reactions that control the chemical evolution of mid-ocean ridge hydrothermal fluids. The experiments specifically address changes in dissolved Fe and H2 in fluids coexisting with mineral buffers at temperatures and pressures sufficient to cause phase separation in the NaCl-H2O system. The research will examine the possibility that fixing dissolved Fe and H2 in mineral-buffered reactions increases the solubility of these components in both fluid phases. This strategy contrasts sharply with previous fluid-only phase separation experiments where the inventory of dissolved Fe was limited. High Fe/Cl and Fe/Mn ratios and high dissolved H2 and H2S concentrations in vapor-rich vent fluids at mid-ocean ridges, especially in the aftermath of magmatic activity, implicate mineral buffering. Thus, the results of this experimental work will provide quantitative rigor to the presently qualitative observations of the temporal evolution of hydrothermal vent fluids at subseafloor temperatures approaching 500°C. These high temperature fluids have some of the highest dissolved Fe and H2 concentrations yet reported; but, in the absence of the experimental data, are difficult to interpret unambiguously in terms of subseafloor redox and pH controlling reactions. Scientific implications of the research will enable the development of theoretically-based predictive approaches for modeling aqueous speciation and mineral solubility in the low density and two-phase regions in the NaCl-H2O system. These conditions also apply to many continental hydrothermal systems. These terrestrial systems have immense scientific, economic, and practical significance and have critical importance to the Earth's thermal budget and geochemical cycles.