Evaluating the Role of Fluid and Melts in Mediating Element Recycling and Exhumation During Retrograde Metamorphism Following UHP Metamorphism

非技术摘要板块构造引入了一个反馈循环，其中会聚板块边缘的地壳物质俯冲不断将表面形成的密度较低的岩石注入地球致密的地幔中。这些地壳物质在俯冲过程中与地幔相互作用，进行元素交换，不断推动地球地球化学演化。为了确定俯冲岩石折返过程中元素再循环的影响，有必要研究地壳岩石折返回地球表面过程中发生的化学反应。超高压（UHP）地幔岩包含通过俯冲带至地幔深处然后返回地表的岩石。超高压火山岩中的一些岩石保留了在地幔深处平衡的“原始”矿物组合。通常，这些岩石的最外层含有新的或“退化”的矿物组合，这些矿物组合是在岩石被挖掘到地表时形成的。通过比较化学的“新鲜”的“倒退”的超高压岩石，所引发的折返的化学变化，将被检查和地球的地球化学演化后，他们的影响将被assessed.Technical AbstractMass传输俯冲带内仍然知之甚少。特别是，有有限的知识的P-T条件下发生元素再循环和什么运输代理驱动元素再循环过程。许多基于实地的研究都集中在高压和超高压（UHP）岩石的折返，以更好地了解脱挥发分反应和产品发生沿着的地壳岩石俯冲在地球深处的俯冲路径。相反，这些深俯冲岩石折返过程中发生的退变质反应在很大程度上被忽略了，尽管峰值温度通常沿着退变质路径发生，并且脱挥发分和熔融反应很常见。因此，退变质反应和相关的质量转移可能对俯冲带的元素再循环过程产生很大影响，也可能决定折返过程的性质，使地壳物质从超高压深度到表面。西部片麻岩地区的综合实地和实验室研究将揭示在超高压地壳岩石的退变质过程中是否发生大规模的元素再循环，以及这些过程是否影响超高压岩石被挖出到地球表面的速率和机制。

Nontechnical AbstractPlate tectonics introduces a feedback loop in which subduction of crustal material at convergent plate margins continually injects less dense rocks formed at the surface back into Earth's dense mantle. As this crustal material is subducted, it interacts with the mantle, exchanges elements, and continuously drives the geochemical evolution of Earth. In order to determine the effect of element recycling during the exhumation of subducted rocks, it is necessary to investigate the chemical reactions that occur during the exhumation of crustal rocks back to Earth's surface. Ultrahigh-pressure (UHP) terranes contain rocks that were carried down to mantle depths via subduction and then returned to the surface. Some of the rocks within UHP terranes preserve "pristine" mineral assemblages that equilibrated at mantle depths. Typically the outermost rind of these rocks contain new or "retrogressed" mineral assemblages that formed as the rocks were being exhumed to the surface. By comparing the chemistry of the "fresh" to the "retrogressed" UHP rocks, the chemical changes that are triggered by exhumation will be examined and their impact upon the geochemical evolution of Earth will be assessed.Technical AbstractMass transfer within subduction zones remains poorly understood. In particular, there is limited knowledge of the P-T conditions at which elemental recycling occurs and of what transporting agents drive the elemental-recycling processes. Many field-based studies have focused upon exhumed high-pressure and ultrahigh-pressure (UHP) terranes to better understand the devolatilization reactions and products that occur along the prograde path as crustal rocks are subducted deep within Earth. In contrast, the retrograde reactions that transpire during the exhumation of these deeply subducted rocks have largely been ignored despite the fact that peak temperatures generally occur along the retrograde path and devolatilization and melting reactions are common. Retrograde metamorphic reactions and associated mass transfer may thus have a great influence on elemental-recycling processes in subduction zones and may also dictate the nature of the exhumation processes that bring crustal material from UHP depths to the surface. This integrated field and laboratory study of the Western Gneiss Region will reveal whether large-scale elemental recycling occurs during retrogression of UHP crustal rocks and whether these processes influence the rates and mechanisms by which UHP terranes are exhumed back to Earth's surface.