马里亚纳弧前蛇纹岩泥火山烟囱状自生沉积的成因及深源蛇纹石化流体渗漏活动记录

Mariana forearc serpentinite mud volcanoes, formed by rising of serpentinite muds from serpentinized mantle wedge along faults to the seafloor, are direct windows into certain subduction processes. They are getting focused also owing to the fluid seepage activities and chemosynthetic biocommunity occurring at the summit seafloors, which may be related with abiogenic methane processes and the origin of life on the Earth. The seepage fluids show composition variations with distance from the trench reflecting slab subducting depths and serpentinization processes beneath the seamounts. However, little is know about their evolution over time at present. Fortunately, authigenic chimneys were found near the seepage sites, which should be profiles of the past seepage activities. Therefore, this proposal intends to study the petrography, minerology, both inorganic and organic geochemistry and geochronology of authigenic carbonate chimneys collected from the Mariana serpentinite mud volcanoes, to constrain their formation mechanisms, mainly concerning their carbon sources and contribution of anoxic oxidation of abiogenic methane, and to trace date and evolution of seepage activities. This proposal will expand our knowledge on the evolution of the slab-derived serpentinization seepage fluids from Mariana forearc and their influence factors. Furthermore, it will bring insight into driving forces of serpentinite mud volcanos and the subducting processes.

马里亚纳弧前蛇纹岩海山是深部地幔楔蛇纹石化产物以泥火山形式喷发至海底形成，它是认识板块俯冲过程的窗口；且泥火山顶部发育流体渗漏和化能自养生物群，与无机成因甲烷和生命起源等重大科学问题相关，使蛇纹岩泥火山成为地学界新的研究热点。最近研究表明蛇纹岩泥火山渗漏流体的地球化学组成反映俯冲深度和蛇纹石化过程，受泥火山与海沟空间距离控制，但随时间演化特征及控制因素仍不清楚。而泥火山顶部发育的烟囱状自生沉积，为解决这一科学问题提供了绝佳样本。本项目拟对马里亚纳弧前蛇纹岩泥火山顶部烟囱状自生碳酸盐岩开展研究，通过矿物、岩石、无机和有机地球化学，确定这些自生碳酸盐岩碳的来源及其是否为无机成因甲烷缺氧氧化作用产物，结合年代学揭示所记录的渗漏流体特征及渗漏活动时间，探讨马里亚纳弧前深源蛇纹石化渗漏流体随时间演化特征及其控制因素，为蛇纹岩泥火山驱动因素研究提供新的启示，为板块俯冲过程研究提供新的思考。

弧前蛇纹岩泥火山被认为是观察俯冲过程的直接窗口，是联系地球深部与浅层的重要场所，自发现以来受到持续关注。蛇纹岩泥火山顶部发育烟囱状自生沉积，记录了深部来源富无机成因甲烷流体的渗漏活动，对于生命起源和全球碳循环有重要研究意义。马里亚纳弧前蛇纹岩泥火山发育的烟囱状自生沉积可分为三种类型，幼年烟囱由方解石及以水镁石为主的富镁碱性矿物组成，成熟烟囱富镁矿物含量降低，文石含量增加；死亡烟囱不含富镁矿物，文石含量最高，微观形貌观察可见文石交代富镁矿物，说明渗漏流体导致方解石和水镁石沉淀，文石为交代水镁石产物。同一烟囱横截面由内向外富镁组分含量依次降低，钙质含量依次升高，说明烟囱内侧较新，外侧较老，烟囱生长模式为由外向内生长。尽管蛇纹岩泥火山发育的自生碳酸盐岩样品具有偏负的δ13C值（-11.4‰～1.0‰，VPDB），但其微量硫酸根同位素组成（δ34SCAS为22‰，δ18OCAS为8.8‰）与海水硫酸根值类似，说明自生碳酸盐的形成可能缺乏无机成因甲烷缺氧氧化作用（AOM）的参与或AOM作用对此种自生碳酸盐形成的贡献有限，蛇纹岩泥火山发育的自生沉积可能为较高pH值渗漏流体与海水混合所致的化学沉积。自生沉积U系定年结果显示，Conical蛇纹岩泥火山烟囱状自生碳酸盐岩形成于3070~1623 yBP，Quaker蛇纹岩泥火山自生碳酸盐岩烟囱主要形成于11081~10542yBP、5857~5583yBP、781~164yBP三个时间段，两座泥火山蛇纹岩化渗漏流体均具有活跃—减弱—再活跃的过程间歇性渗漏特征，二个泥火山顶部流体活动活跃时间的不同可能反映了局部区域性因素对流体活动的控制。