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

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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.