The effect of hydrogen on olivine to ringwoodite transformation

It has been suggested that in the interior of cold subducting lithosphere, the transformation of olivine to its high-pressure polymorphs, wadsleyite and ringwoodite, may be kinetically inhibited, leading to the subduction of metastable olivine deep into the mantle transition zone. The rapid transformation of this metastable olivine has been identified as a possible origin for deep-focus earthquakes. However, converging lines of evidence from seismology, mineral physics, and petrology suggest that mantle olivine may be partially hydrated. In the coldest subduction zones, olivine is predicted to transform directly to ringwoodite. This dissertation investigates the effect of H2O on olivine to ringwoodite transformation.; A novel two-part methodology was developed to study the transformation of hydrated olivine crystals to ringwoodite. It consists of two distinct phases: a hydration experiment, in which olivine spheroids are hydrated in water during piston-cylinder experiments; and a multi-anvil experiment that partially transforms two of these spheroids in each run. Benefits of this methodology include the ability to consistently and uniformly hydrate olivine for numerous transformation experiments, greater potential to control hydrogen content, and increased opportunity for pre-transformation characterization. A new multi-anvil assembly is designed to produce as ideal as possible conditions for studying the transformation of olivine.; Transformation of hydrated olivine was performed in the ringwoodite stability field at 18 GPa and temperatures of 700°C, 900°C, and 1100°C for various run durations. The addition of 289 ppmw H2O to olivine is shown to promote its transformation to ringwoodite by enhancing growth rates, lowering the activation enthalpy for growth, and weakening the olivine and/or ringwoodite to allow for dissipation of elastic strain energy sufficient to maintain constant growth rates throughout transformation. Thermo-kinetic modeling conducted by collaboration with Dr. Frederic Marton indicates that olivine with as little as 289 ppmw H2O is completely transformed by 440 km depth, even in the coldest subduction zones. This would eliminate the possibility of the transformation of hydrated olivine leading to deep-focus earthquakes. Conversely, if deep-focus earthquakes are due to olivine transformation, then the olivine must contain less than 289 ppmw H2O.