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

板块构造学引入了一个反馈循环，在这个循环中，汇聚的板块边缘的地壳物质的俯冲作用不断地将表面形成的密度较低的岩石注入到致密的地幔中。这些地壳物质在俯冲过程中与地幔相互作用，交换元素，不断推动地球的地球化学演化。为了确定俯冲岩石掘出过程中元素再循环的作用，有必要研究地壳岩石掘出回地表过程中发生的化学反应。超高压（UHP）地体包含通过俯冲作用被带到地幔深处然后返回地表的岩石。超高压地体中的一些岩石保存着“原始”矿物组合，这些矿物组合在地幔深处达到平衡。通常，这些岩石的最外层含有新的或“倒退”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“新鲜”和“倒退”的超高压岩石的化学性质，将检查挖掘引发的化学变化，并评估它们对地球地球化学演化的影响。技术摘要：对俯冲带内的质量传递仍然知之甚少。特别是，人们对元素循环发生的P-T条件和什么运输剂驱动元素循环过程的了解有限。许多基于野外的研究都集中在挖掘出的高压和超高压（UHP）地层上，以更好地了解地壳岩石在地球深处俯冲时沿进积路径发生的脱挥发反应和产物。相比之下，在这些深俯冲岩石的挖掘过程中发生的逆行反应在很大程度上被忽视了，尽管事实上峰值温度通常发生在逆行路径上，并且脱挥发和熔化反应很常见。因此，逆行变质反应和相关的质量传递可能对俯冲带的元素再循环过程有很大的影响，也可能决定将地壳物质从超高压深处带到地表的掘出过程的性质。这项对西部片麻岩地区的综合野外和实验室研究将揭示在超高压地壳岩石退退过程中是否发生大规模元素再循环，以及这些过程是否影响超高压地层被挖掘回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.