Complementary insights from orogenic and xenolithic eclogites as recorders of mass transfer in subduction zones: A unique case study from the Farallon plate

大洋俯冲带可以说是地球上唯一最重要的界面，在那里发生了重大的壳幔相互作用，挥发分和其他元素的转移--这些过程是行星演化的基础。一些传质是由流体和熔体作为组成、压力、温度和氧逸度的函数进行的。在这种复杂的环境中，这些参数对元素活动性的影响仍然是模糊的，这也是因为任何一个俯冲带都有不完整的地质记录。由于一种独特的地质偶然性，白垩纪Farallon板块的洋壳在一个异常大的压力区间(~2.0-4.7 Gpa)内被采样，通过方济西杂岩(美国西部)中的构造折返榴辉岩，以及出现在以东700公里的纳瓦霍火山田中的榴辉岩包体。利用具有代表性的样品组，利用块体岩石Li-B-Mg-Fe同位素对法拉隆板块深部质量传递过程中与榴辉岩相互作用的流体来源进行指纹分析，并利用原位Li-Mg-Fe同位素探测短暂的流体流动事件。将地热-氧压法与新型石榴石U-Pb测年工具相结合，将绘制出压力-温度-时间-氧逸度路径。在硫化物δ34S、石榴石Fe3+/ΣFe和石榴石δ56Fe的强化下，耦合的整体碳和硫的浓度、形态和同位素组成将被用来评估氧化还原敏感元素Fe、C和S之间的相互作用，并确定它们被动员或保留在板坯中的反应。结合在一起，这些数据将允许在单个平板中以前所未有的细节追踪对质量传递过程和挥发分循环的物理化学控制，从弧前到弧下深度。

Oceanic subduction zones arguably constitute the single most important interface on Earth where significant crust-mantle interaction, with transfer of volatiles and other elements, occurs - processes that are fundamental to planetary evolution. Some mass transfer is mediated by fluids and melts as a function of composition, pressure, temperature and oxygen fugacity. The effects of these parameters on element mobility in this complex environment remain fuzzy, also owing to the incomplete geological record available for any single subduction zone. Due to a unique geological happenstance, the oceanic crust of the Cretaceous Farallon plate has been sampled over an exceptionally large pressure interval (~2.0-4.7 GPa), by tectonically exhumed eclogites in the Franciscan Complex (western United States), and by eclogite xenoliths occurring in the Navajo Volcanic Field, 700 km to the east. Using a representative sample suite, this study aims to fingerprint the sources of the fluids that interacted with the eclogites during deep mass transfer processes in the Farallon plate with bulk-rock Li-B-Mg-Fe isotopes, and detect short-lived fluid flow events with in situ Li-Mg-Fe isotopes. A pressure-temperature-time-oxygen fugacity path will be delineated by combining geothermo-oxybarometry with a novel garnet U-Pb dating tool. Coupled bulk carbon and sulphur concentrations, speciations and isotopic compositions, enhanced by sulphide δ34S, garnet Fe3+/ΣFe and garnet δ56Fe, will be used to assess the interplay between the redox-sensitive elements Fe, C and S, and identify the reactions by which they are mobilised or retained in the slab. Combined, these data will allow to trace the physicochemical controls on mass transfer processes and volatile cycling from forearc to subarc depth, in a single slab, with unprecedented detail.