Does Overstepping of Garnet-producing Reactions Delay Devolatilization in Subduction zones? Insights From the Cycladic Blueschist Belt, Greece

最大的地震和最重大的火山灾害是构造板块俯冲的结果。人们普遍认为，这些大地震和俯冲地壳和上覆地幔熔融的触发因素以某种方式涉及到通过脱挥发反应从俯冲沉积物和地壳中释放出流体，这些反应导致结构中含有水的矿物分解。然而，负责流体释放的特定矿物和反应的身份一直难以捉摸，因为根据热力学计算，据信在俯冲通道中常见的矿物中没有一种具有适当的压力和温度稳定性。这项研究将支持一名研究生的博士论文研究，该研究生来自地球科学中代表性较低的群体，并将涉及阿默斯特学院的两名本科生，该学院正在合作开展一些分析活动。该项目旨在检验另一种假设，即在俯冲过程中产生流体的石榴石反应相对于其在特定压力和温度下的标称平衡位置显著超前。也就是说，在俯冲带中发现的常见矿物可能会持续到更深的地方(地震记录的深度)，因为启动反应所需的新相的成核被抑制了。这一假设将通过应用一种新的方法来验证，该方法利用矿物石榴石内部的石英包裹体的拉曼光谱来确定石榴石成核的压力-温度条件，从而确定发生主要脱挥发的条件。这项研究的结果应该有助于解决一个长期存在的悖论，并将提供对触发地震和地幔融化的过程的更清晰的理解，从而有助于预测这些危险的发生。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The largest earthquakes and the most significant volcanic hazards occur as a consequence of the subduction of tectonic plates. It is generally accepted that the triggers to both these large earthquakes and the melting of the subducted crust and overlying mantle somehow involve the release of fluids from the subducted sediments and crust via devolatilization reactions that cause the breakdown of minerals that contain water in their structure. However, the identity of the specific minerals and reactions responsible for the release of fluids has been elusive because none of the minerals that are believed to be common in subduction channels have the appropriate stabilities with respect to pressure and temperature based on thermodynamic calculations. This study will support the Ph.D. dissertation research of a graduate student from an underrepresented group in geosciences, and will involve two undergraduate students from Amherst College that is collaborating on some of the analytical activities. This project is to test the alternative hypothesis that garnet-bearing reactions that produce fluids during the subduction process are significantly overstepped with respect to their nominal equilibrium location at a specific pressure and temperature. That is, the common minerals found in subduction zones may persist to significantly greater depths (the depths recorded by earthquakes) because the nucleation of the new phases necessary to initiate the reaction is suppressed. This hypothesis will be tested through application of a novel approach that utilizes Raman spectroscopy on inclusions of quartz inside of the mineral garnet to determine the pressure-temperature conditions at which the garnet nucleated and, thus, the conditions at which major devolatilization occurs. The results of this study should help resolve a long-standing paradox and will provide a much clearer understanding of the processes responsible for triggering earthquakes and mantle melting thus aiding in the ability to predict the occurrence of these hazards.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.