The Effect of Trace Amounts of H2O on the Rates and Mechanisms of Olivine and Enstatite Phase Transformations in Earth's Mantle Transition Zone

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"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Through plate tectonics and subduction, Earth recycles oceanic crust and lithosphere into its mantle. The down going oceanic crust carries H2O, which, when released into the overlying mantle wedge, causes melting that results in arc volcanism. If the mantle component of the subducting oceanic plate is also hydrated, then some of the subducted H2O is likely to be carried deep into Earth's mantle. High-pressure experiments have shown that minerals from the upper mantle and especially the transition zone (410 to 660 km depth) are capable of storing many oceans worth of H2O. However, we do not yet know how much H2O is actually stored in Earth's mantle or recycled through by subduction. Deep and dangerous earthquakes associated with subduction (deep-focus earthquakes) occur from 400 km up to 700 km where conventional earthquake mechanisms are not possible. The mostly widely accepted hypothesis for the origin of deep-focus earthquakes is that they are triggered by the mineral olivine, which is subducted into the transition zone and beyond where it is stable. This hypothesis requires very slow reaction rates in the relatively cool interiors of subducting plates. The purpose of this research is to determine how trace amounts of H2O enhance olivine and enstatite transformation rates at high-pressure. These data will provide a test of the transformational faulting model and constrain the maximum H2O content compatible with deep focus earthquakes by transformational faulting. This research will provide a better understanding of deep earthquakes and the potential deep-Earth hydrologic cycle. Previous studies on the olivine-wadsleyite and the olivine-ringwoodite phase transformations have shown that H2O greatly increases transformation rates. Preliminary experimental studies have also shown that 290 ppm D2O in olivine increases ringwoodite growth rates by two orders of magnitude at 1100°C and reduces the activation enthalpy from around 392 kJ/mol for anhydrous olivine to 274 kJ/mol. Based on their kinetic data and thermo-kinetic models of subduction zones, 290 ppm H2O is enough to eliminate the survival of metastable olivine as a mechanism for deep focus earthquakes. Additional results for 30 ppm H2O in olivine indicate similarly fast growth rates and low activation enthalpy. It is proposed to continue these investigation of the olivine-ringwoodite transformation with 30 ppm H2O, using San Carlos olivine hydrated in the piston-cylinder apparatus. These samples will be partially transformed at 18, 19, 20ad 21 GPa and 700, 900 and 1100 °C, using a multi-anvil apparatus, to determine the pressure dependence of ringwoodite growth for 30 ppm H2O. Transformation rates and mechanisms will also be investigated for enstatite, the second most abundant mineral in the mantle, hydrated under the same conditions as our olivine. The team will also use scanning and transmission electron microcopy to characterize transformation mechanisms in their samples. The results will be used in thermo-kinetic models of subduction zones to determine the maximum H2O contents of olivine and enstatite that are compatible with deep focus earthquakes by transformational faulting.