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

评估流体和熔体在超高压变质作用后逆行变质过程中介导元素回收和折返中的作用

• 批准号: 1624546
• 负责人: Stacia Gordon
• 金额: $ 32.82万
• 依托单位: Board of Regents, NSHE, obo University of Nevada, Reno
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-01 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1624546&HistoricalAwards=false
• 关键词: Evaluating; Role; Fluid; Melts; Mediating

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.

非技术摘要板块构造引入了一个反馈回路，其中会聚板块边缘的地壳物质俯冲不断地将表面形成的密度较低的岩石注入地球致密的地幔中。当这种地壳物质俯冲时，它与地幔相互作用，交换元素，并不断驱动地球的地球化学演化。为了确定俯冲岩石折返过程中元素循环的影响，有必要研究地壳岩石折返到地球表面过程中发生的化学反应。超高压（UHP）地体含有的岩石通过俯冲作用被带到地幔深处，然后返回地表。超高压地体中的一些岩石保留了在地幔深处达到平衡的“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“新鲜”和“退化”超高压岩石的化学成分，将检查折返引发的化学变化，并评估它们对地球地球化学演化的影响。技术摘要俯冲带内的质量传递仍然知之甚少。特别是，对于元素回收发生的 P-T 条件以及驱动元素回收过程的运输介质的了解有限。许多实地研究都集中在挖出的高压和超高压（UHP）地体上，以更好地了解当地壳岩石在地球深处俯冲时沿前进路径发生的脱挥发分反应和产物。相比之下，尽管峰值温度通常沿着逆行路径发生并且脱挥发分和熔化反应很常见，但在这些深俯冲岩石折返过程中发生的逆行反应在很大程度上被忽略了。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循环过程产生很大影响，也可能决定将地壳物质从超高压深度带到地表的折返过程的性质。这项对西部片麻岩地区的综合现场和实验室研究将揭示超高压地壳岩石退缩期间是否发生大规模元素循环，以及这些过程是否影响超高压地体被挖掘回地球表面的速率和机制。

项目成果

Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

钛矿的形变强化再结晶驱动 U-Pb 与微量元素之间的解耦

• DOI: 10.1016/j.epsl2021.116810
• 发表时间: 2021
• 期刊: Earth and planetary science letters
• 影响因子: 5.3
• 作者: Gordon, S.M.;Kirkland, C.L.;Reddy, S.M.;Blatchford, H.J.;Whitney, D.L.;Teyssier, C.;Evans, N.J.;McDonald, B.J.
• 通讯作者: McDonald, B.J.