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

蛇纹状泥火山(或蛇纹状海山)是位于马里亚纳岛外弧前海沟轴线50-100公里后的大型建筑物。这些特征形成于向下延伸到蛇纹状地幔楔体的断层之上。构造置换作用粉碎了蛇纹岩和细粒断层泥物质，与上升的板岩衍生水混合成为富含液体的泥浆，浮升到海底，产生泥火山。镶嵌在细粒基质中的是方辉橄榄岩和纯橄榄岩的蛇纹岩碎屑，大小可达一米，还有变质的镁铁质岩石的小碎屑。这些碎屑代表了俯冲板块和上覆地幔楔体内流体岩石相互作用的丰富档案。IODP探险366将钻探三座泥火山，覆盖板岩深度(13至25公里)和板岩界面假定温度(从&lt；80到&gt；200°C)的很大范围。只有圆锥形海山(在ODP125航段期间钻探)代表更深的深度和更高的温度(30公里；300°C)。我们建议对这四个蛇纹岩海山的蛇纹岩碎屑进行岩石学、地球化学和结构研究，以确定冰川和海底之间流体-岩石相互作用岩石的性质、程度和后果。我们的研究将讨论(1)压力和温度对地幔楔体内蛇纹岩作用的作用，以及(2)泥火山管道内的破裂和持续的水-岩相互作用。一个特别的重点将是确定流体特征，特别是在痕量元素特征和溶解二氢(H2，AQ)丰度方面。后者作为岩石学和生物学之间的纽带很重要，因为居住在泥火山流体渗漏区的微生物群落以二氢为主要代谢能源而茁壮成长。在我们的工作计划中，我们采用了在最近的项目中开发的旨在了解蛇纹石化反应和相关氢气产生的途径和方法。具体地说，我们提出了全岩地球化学和薄片显微分析工作(EMPA、LA-ICPMS)以及相岩石学观察和地球化学反应路径模拟，以揭示水-岩反应序列及其对流体成分演化的影响，以及蛇纹岩碎屑和泥浆的矿物学和物理性质(如密度、磁化率)的变化。在这一范围的方法中，我们增加了使用X射线µ-CT的纹理测量来描述不同蚀变区域的三维分布，并建立了与反复影响蛇纹岩泥火山根区碎屑的破裂和脉化事件的关系。这一结果将有助于理解地幔楔体和上升的蛇纹石泥浆柱中的流体-岩石相互作用，并评估从泥火山渗出并支持海底和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.