Serpentinization processes in the subsurface of a seafloor ultramafic-hosted hydrothermal system

地壳下面是地幔，由一种叫做橄榄岩的超镁铁质岩石组成。橄榄岩是通过大洋中脊的断裂暴露在海底的，在那里它们与流体反应形成蛇纹岩。反应流体可以是寒冷的海水，也可以是由玄武岩岩浆侵入所产生的热量驱动的来自海水的热液（350摄氏度）。橄榄岩水化形成蛇纹岩产生氢气，支持海底热液喷口周围的微生物群落。这些海底热液环境可能是地球上生命起源的地点，在那里发现的微生物群落与其他行星上的生命相似。然而，关于这个地下生物圈的范围，主要的问题仍然存在，比如这个生物圈延伸到离海底喷口多远的地方，以及微生物在海底下生活的距离有多远。除了生命之外，海底橄榄岩的热液流入海洋的地方也形成了金属沉积物。虽然这些矿床及其伴生流体已经得到了很好的研究，但我们对下伏基底的流体流动途径、流体渗透和寄主橄榄岩蛇纹石化的控制以及它们与地下微生物活动的关系的了解还不充分。本研究考察了流体渗透和与橄榄岩反应形成蛇纹岩的地球化学过程，以及由此产生的水岩相互作用对深层生物圈的影响。该项目的两个主要目标是：(1)了解在海底循环的冷海水和热液流体的相对作用；(2)确定对海底橄榄岩中地下生物圈及其横向和深度范围的控制。这项工作的更广泛的影响包括提高对海洋块状硫化物矿床的产生和地热能源工业感兴趣的过程的理解。这项工作还包括通过培训有意从事采矿业的学生，将研究和教育结合起来。本研究包括了解海底超镁铁质热液系统的流体路径、蛇纹石化作用以及微生物硫酸盐还原的分布和程度。工作包括对这些海底系统的模拟物进行密集的实地和实验室研究，该系统暴露在亚平宁山脉北部（意大利）的蛇绿岩中。通过对岩石中主要元素、微量元素和硫同位素的分析，将热液流体的作用与海水的作用区别开来。结合岩石学、蛇纹石中微量元素的激光电感耦合质谱微尺度斑点分析和单个硫化物颗粒的硫同位素组成，可以绘制流体流动路径；确定它们是如何发展的；并了解微生物过程如何影响和/或催化高温超镁铁海底热液系统中的反应。对大块岩石的分析将有助于评价热液流体和海水以及微生物硫酸盐在基底中的还原作用的总体影响。

Beneath Earth's crust lies the mantle, composed of an ultramafic rock called peridotite. Peridotites are exposed on the seafloor by faulting at mid-ocean ridges where they react with fluids to form serpentinites. The reacting fluids can be cold seawater or hot (350 degrees Celsius) hydrothermal fluids derived from seawater and driven by heat generated by intrusions of basaltic magma at depth. The hydration of peridotite to form serpentinite generates hydrogen, which supports microbial communities around hydrothermal vents at the seafloor. These seafloor hydrothermal environments are likely sites for the origin of life on Earth and the microbial communities found there are analogues for life on other planets. Major questions remain, however, about the extent of this subsurface biosphere, such as how far away from seafloor vents this biosphere extends and how far microbes live below the seafloor. In addition to life, metal deposits form where hot hydrothermal fluids vent into the ocean from peridotite on the seafloor. Although these deposits and their associated fluids are well studied, our understanding of the fluid flow pathways in underlying basement and the controls on fluid penetration and serpentinization of the host peridotite and their relationship to microbial activity in the subsurface is not. This research examines the geochemistry of fluid penetration and reaction with peridotite to form serpentinite and the impact of the resulting water-rock interactions on the deep biosphere. Two major goals of the project are: (1) understanding the relative roles of cold seawater versus hot hydrothermal fluids that circulate through the seafloor and (2) determining the controls on the subsurface biosphere and its lateral and depth extent in seafloor peridotites. Broader impacts of the work include improving understanding of the generation of marine massive sulfide ore deposits and processes of interest to the geothermal energy industry. The work also includes integration of research and education by training a student who is interested in a career in the mining industry. This research involves understanding fluid pathways, serpentinization, and the distribution and extent of microbial sulfate reduction in the subsurface of seafloor ultramafic-hosted hydrothermal systems. Work involves an intensive field and laboratory study of an analog of these seafloor systems that is exposed in an ophiolite in the northern Apennines (Italy). The effects of hydrothermal fluids in the rocks will be distinguished from those of seawater through analyses of major and trace elements and sulfur isotopes. Combined with petrography, micro-scale spot analyses by laser inductively coupled mass spectrometry of trace elements in serpentine, and the sulfur isotope compositions of individual sulfide grains, it will be possible to map the fluid flow pathways; determine how they developed; and understand how microbial processes affect and/or catalyze reactions in high-temperature ultramafic-hosted seafloor hydrothermal systems. Bulk rock analyses will enable evaluation of the overall effects of hydrothermal fluids and seawater and of microbial sulfate reduction in the basement.