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

地壳之下是地幔，由一种名为橄榄岩的超镁铁质岩石组成。橄榄岩通过大洋中脊的断层暴露在海底，在那里它们与流体反应形成蛇纹岩。反应流体可以是冷海水或热(350摄氏度)的热液流体，这些热液源自海水，由深部玄武岩岩浆侵入产生的热量驱动。橄榄岩的水化作用形成蛇纹岩，产生氢，这为海底热液喷口周围的微生物群落提供了支持。这些海底热液环境可能是地球上生命起源的地方，发现的微生物群落与其他星球上的生命类似。然而，关于这个地下生物圈的范围仍然存在重大问题，例如这个生物圈距离海底喷口有多远，以及微生物在海底下面生活的距离有多远。除了生命，在海底的橄榄岩中，炽热的热液从海底涌入海洋，就形成了金属矿床。尽管人们对这些矿床及其伴生流体进行了很好的研究，但我们对下伏基底中的流体流动路径、对寄主橄榄岩的流体渗透和蛇纹岩作用的控制以及它们与地下微生物活动的关系的了解还不够深入。这项研究考察了流体穿透并与橄榄岩反应形成蛇纹岩的地球化学，以及由此产生的水-岩石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热液的相对作用；(2)确定对海底生物圈的控制及其在海底橄榄岩中的横向和深度范围。这项工作的更广泛影响包括增进对海洋块状硫化物矿藏的产生和地热能源行业感兴趣的过程的了解。这项工作还包括通过培训对采矿业感兴趣的学生来整合研究和教育。这项研究涉及了解流体路径、蛇纹岩作用以及海底超镁铁质热液系统地下微生物硫酸盐还原的分布和程度。这项工作涉及对暴露在(意大利)亚平宁北部蛇绿岩中的这些海底系统的模拟进行深入的实地和实验室研究。通过常量元素、微量元素和硫同位素的分析，将岩石中的热液作用与海水的热液作用区分开来。结合岩相学、对蛇纹石中微量元素的激光感应耦合质谱仪进行的微尺度斑点分析以及单个硫化物颗粒的硫同位素组成，将有可能绘制流体流动路径图；确定它们是如何发展的；以及了解微生物过程如何影响和(或)催化高温超镁铁质海底热液系统中的反应。块状岩石分析将能够评估热液流体和海水以及基底中微生物硫酸盐还原的总体影响。

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.