Constraints on the H2O content of subducting slabs from olivine-ringwoodite transformation kinetics

It is possible that much of Earth's H2O resides in the upper mantle and mantle transition-zone in nominally anhydrous minerals (NAMs). If Earth's water cycle includes a deep mantle component, subduction would provide the transport mechanism from Earth's surface to the transition zone. How hydrated is subducting mantle lithosphere? Double seismic zones and petrologic evidence have been used to argue that breakdown of serpentines can cause intermediate-depth earthquakes by dehydration embrittlement. If this model is correct, then NAMs in the mantle portion of subducting oceanic lithosphere would interact with an H2O-rich fluid from 2 to 6 GPa, adding hundreds to a thousand wt ppm H2O subducting olivine. In fast and cold subducting slabs, where temperatures are low enough to inhibit equilibrium transformation of olivine, a wedge of metastable olivine is thought to survive deep into Earth's transition zone. Olivine transformational faulting provides an explanation for deep focus earthquakes if the metastable olivine can persist deep into the transition zone. The likelihood of metastable olivine in the transition zone has been evaluated using experimentally determined kinetic data combined with thermo-kinetic models. Our kinetic experiments show that 300 wt ppm H2O in olivine enhances ringwoodite growth rates so much that metastable olivine is unlikely to survive in cold and fast subduction zones if hydrated with 300 wt ppm H2O. Evidence for metastable olivine in the transition zone, including seismic data and deep focus earthquakes, suggest that olivine in subducting mantle lithosphere has less than 300 wt ppm H2O. Here we present experimental results on the transformation of hydrous (~30 wt-ppm H2O) San Carlos olivine to ringwoodite at 18 GPa. Transformation kinetics for olivine with 30 wt ppm H2O will be discussed to constrain how hydrated a subducting slab can be and still have metastable persistence of olivine in the transition zone.