Petrological and textural investigations of ultramafic clast in serpentine mud volcanoes of the Mariana forearc (IODP Expedition 366): Evolution of water-rock interactions between the décollement and the seafloor with special emphasis on the generation of

蛇纹泥火山（或蛇纹海山）是大型建筑物，位于马里亚纳外弧海沟轴线后方50-100公里处。这些特征形成于向下延伸至蛇纹化地幔楔的断层顶部。构造位移粉碎了蛇纹岩，细粒断层泥物质与上升的板状水混合，成为富含液体的泥浆，并迅速上升到海底，形成泥火山。细粒基质中嵌埋着方辉橄榄岩和纯橄榄岩的蛇纹石化碎屑，大小可达米，沿着有少量变质镁铁质岩碎屑。这些碎屑代表了俯冲板片和上覆地幔楔内流体-岩石相互作用的丰富档案。IODP Expedition 366将钻探三个泥火山，覆盖大范围的板块深度（13至25公里）和板块界面的假定温度（<80 to >200 °C）。只有锥形海山（在ODP第125航次期间钻探）的深度和温度更高（30公里，300摄氏度以上）。我们建议对这四个蛇纹海山的蛇纹岩碎屑进行岩石学、地球化学和结构研究，以确定滑脱层和海底之间流体-岩石相互作用岩石的性质、范围和后果。我们的研究将解决（1）压力和温度对地幔楔内蛇纹石化的作用和（2）泥火山管道内的破裂和持续的水-岩相互作用。一个具体的重点将是确定流体的特性，特别是在微量元素的签名和溶解的二氢（H2，水）的丰度。后者作为岩石学和生物学之间的联系是重要的，因为居住在泥火山的流体渗漏区的微生物群落以二氢作为主要的代谢能源而茁壮成长。在我们的工作计划中，我们采用了最近项目中开发的方法和手段，旨在了解蛇纹石化反应和相关的制氢。具体而言，我们建议全岩地球化学和薄片微分析工作（EMPA，LA-ICP-MS）沿着相岩石学观察和地球化学反应路径建模，以解卷积水-岩反应序列及其对流体成分演化和矿物学和物理学变化（例如，密度、磁化率）性质。在这一系列方法中，我们使用X射线µ-CT增加了纹理测量，以描绘不同蚀变域的3D分布，并建立与反复影响蛇纹泥火山根区碎屑的断裂和脉纹事件的关系。这些结果将有助于理解地幔楔中的流体-岩石相互作用和上升的蛇纹泥柱，并评估从泥火山中渗出并支持海底及海底以下生命的氢气产生的程度。

Serpentine mud volcanoes (or serpentine seamounts) are large edifices that are abundant 50-100 km behind the trench axis in the outer Mariana forearc. These features form on top of faults reaching down to the serpentinized mantle wedge. Tectonic displacement comminutes the serpentinite and the fine-grained fault gouge material mixes with the rising slab-derived water to become a liquid-rich mud, which rises buoyantly to the seafloor, producing a mud volcano. Embedded within the fine-grained matrix are variably serpentinized clasts of harzburgite and dunite, ranging to meter-size, along with small clasts of metamorphosed mafic rocks. These clasts represent a rich archive of fluidrock interactions within the subducting slab and overlying mantle wedge. IODP Expedition 366 will drill three mud volcanoes that cover a large spectrum of slab depth (13 to 25 km) and presumed temperatures at the slab interface (from <80 to >200 °C). Only Conical seamount (drilled during ODP Leg 125) represents a greater depth and higher temperatures (30 km, >300 °C). We propose to conduct petrological, geochemical, and textural studies of serpentinite clasts from those four serpentine seamounts to determine the nature, extent and consequences of fluid-rock interactions rocks between the décollement and the seafloor. Our study will address the role of (1) pressure and temperature on serpentinization within the mantle wedge and (2) fracturing and continued water-rock interaction within the conduit of the mud volcano. A specific emphasis will be on identifying fluid characteristics, in particular in terms of trace element signatures and abundance of dissolved dihydrogen (H2, aq). The latter is important as a link between petrology and biology, as the microbial communities inhabiting the fluid seepage areas of the mud volcanoes thrive on dihydrogen as the principal metabolic energy source. In our work plan, we adopt approaches and methods developed in recent projects aimed at understanding serpentinization reactions and related hydrogen generation. Specifically, we propose whole-rock geochemical and thin-section microanalytical work (EMPA, LA-ICP-MS) along with phase petrological observations and geochemical reaction path modelling to deconvolute the sequences of water-rock reactions and their consequences for fluid compositional evolution and changes in mineralogical and physical (e.g., density, magnetic susceptibility) properties of serpentinite clasts and muds. To this range of methods, we add textural measurements using X-ray µ-CT to depict the 3D distribution of different alteration domains and establish relations to fracturing and veining events that repeated affect the clasts in the root zone of the serpentine mud volcanoes. The results will help understanding fluid-rock interactions in the mantle wedge and the rising column of serpentine mud and assessing the extent of hydrogen production, which seeps from the mud volcanoes and support life at and below the seabed.