HR: 16:45h
AN: V24A-04    [Abstracts]
TI: The Effect of ~290 ppmw H2O on the Rate of Olivine-Ringwoodite Transformation at 18 GPa
AU: * Diedrich, T
EM: tdiedric@nrri.umn.edu
AF: Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442, United States
AU: Sharp, T G
EM: tom.sharp@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, P.O. Box 871404, Tempe, AZ 85287-1404, United States
AU: Leinenweber, K
EM: kurtl@asu.edu
AF: Arizona State University, Dept. of Chemistry & Biochemistry, P.O. Box 871604, Tempe, AZ 85287-1604, United States
AB: 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. In the coldest subduction zones, olivine is predicted to transform directly to ringwoodite. Converging lines of evidence from seismology, mineral physics, and petrology suggest that subducting mantle olivine may be partially hydrated. This research investigates the effect of H₂O on the kinetics of the olivine to ringwoodite transformation. We used a two-part methodology to study the transformation of hydrated olivine crystals to ringwoodite: 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. Transformation of hydrated olivine was performed in the ringwoodite stability field at 18 GPa and temperatures of 700^oC, 900^oC, and 1100^oC for various run durations. The addition of approximately 290 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 ringwoodite to allow for dissipation of elastic strain energy sufficient to maintain constant growth rates throughout transformation. SIMS analyses of partially transformed samples indicate that hydrogen diffuses into the ringwoodite rim during transformation, progressively depleting the olivine center of hydrogen during transformation. Olivine in partially transformed samples contain as little as 50 ppmw H₂O without a corresponding decrease in transformation rate, indicating that between approximately 50 and 290 ppmw H₂O, transformation rate is independent of hydrogen content. This suggests that extrapolations of kinetic data from more hydrated samples underestimate the effect of 50 ppmw H₂O and that hydrogen contents consistent with values observed in mantle-derived olivine are sufficient to promote olivine to ringwoodite transformation in the transition zone.
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting