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

大洋中脊的热液循环构成了地球上最大的地热系统。沿着火山活动的全球大洋中脊系统，海底硫化物结构中壮观的含金属流体羽流喷发到海水中，这是洋壳和上覆海洋之间热量和化学转移的最终结果。因此，海底热液喷口流体的组成对海洋化学有很大的控制作用，在整个地球历史上也是如此。海底热液还提供溶解气体和其他化学物质，以维持微生物生态系统，为地球上生命的起源提供线索。海底热液喷口及其伴生微生物群落的发现，不需要阳光或光合作用产生的有机物就能生存，这仍然是现代科学中最重大的发现之一。然而，由于这些过程的极端温度和压力，海水转化为成分不同的热液喷口流体的机制尚不确定。这项研究使用新型实验室反应堆和现场传感器系统，在海底热液喷发的温度和压力下进行首次实验，以确定氧化还原反应和pH值对以前未研究的温度和压力下矿物溶解度的影响。这项工作的目标是提供在高温和高压下水-岩相互作用中涉及铁和氢的基本地球化学、热力学和动力学参数。这项工作对研究和科学教育的更广泛的影响包括：(1)本科理科专业学生的参与；(2)研究生对矿物-流体反应的实验和理论研究的培训，这增加了他们的就业机会；(3)开发新的模型，以更准确地预测天然多组分热液系统中的物理和化学过程之间的联系；(4)通过开发新的设备和技术来加强物质基础设施，以便在水的相分离领域进行实验和收集数据；以及(5)与明尼苏达大学贝尔自然历史博物馆的K-12计划合作进行公众宣传。这项研究将获得数据，以促进控制大洋中脊热液流体化学演化的矿物-流体反应的知识。这些实验专门研究了在足以导致氯化钠-水体系相分离的温度和压力下，与矿物缓冲液共存的流体中溶解的铁和氢的变化。这项研究将考察在矿物缓冲反应中固定溶解的铁和氢是否会增加这些组分在两种液体相中的溶解度。这一策略与以前的纯流体相分离实验形成了鲜明的对比，在以前的实验中，溶解的铁的库存是有限的。在大洋中脊富含蒸汽的喷口流体中，特别是在岩浆活动之后，高Fe/Cl和Fe/Mn比值以及高溶解H2和H2S浓度意味着矿物缓冲。因此，这项实验工作的结果将为目前对海底以下温度接近500°C的热液喷口流体的时间演化的定性观察提供定量的严格依据。这些高温流体具有迄今报告的一些最高的溶解Fe和H2浓度；但在没有实验数据的情况下，很难明确地从海底以下氧化还原和pH控制反应的角度对其进行解释。这项研究的科学意义将有助于开发以理论为基础的预测方法，以模拟氯化钠-水体系中低密度和两相区域的水的形态和矿物的溶解度。这些条件也适用于许多大陆热液系统。这些陆地系统具有巨大的科学、经济和实际意义，对地球的热收支和地球化学循环具有至关重要的意义。

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.