HR: 0800h
AN: V41A-1434    [Abstracts]
TI: Experimental study of the effect of water on olivine-ringwoodite transformation rate
AU: Diedrich, T
EM: Tamara.Diedrich@asu.edu
AF: Arizona State University, Department of Geosciences, Tempe, AZ 85287-1404 United States
AU: * Sharp, T R
EM: Thomas.Sharp@asu.edu
AF: Arizona State University, Department of Geosciences, Tempe, AZ 85287-1404 United States
AU: Leinenweber, K
EM: kurtl@asu.edu
AF: Arizona State University, Department of Geosciences, Tempe, AZ 85287-1404 United States
AB: The persistence of metastable olivine in subducting slabs as they pass through the mantle transition zone would impact subduction zone geodynamics by decreasing negative buoyancy of the slab and, therefore, subduction rates. In addition, its subsequent transformation has been proposed as the origin of deep-focus earthquakes originating in the transition zone. In light of this, many previous studies have been conducted to determine kinetic parameters for the olivine-wadsleyite and olivine-ringwoodite transformations using both (Mg,Fe)₂SiO₄ and analog compositions. The qualitative enhancement of transformation rates by H₂O in this system has also been well established. More recently, experimental work has quantitatively confirmed this effect using olivine containing 750-4900 wt ppm H₂O. However, several lines of evidence suggest the presence of lower water contents in the Earth's mantle. Presented here are results showing the effect of minor, yet geologically relevant, amounts of water on the olivine-ringwoodite transformation. Multi-anvil experiments are being conducted on 500-micron diameter San Carlos olivine spheroids. These spheroids are first deuterated in piston-cylinder experiments in which liquid D₂O and San Carlos olivine, along with silica activity and oxygen fugacity buffers, are pressure sealed in a thick-walled silver capsule. D₂O is used as a proxy for H₂O, because the low deuterium background enables precise quantification by SIMS. The separate deuteration procedure provides for greater control over deuterium content than in previous experiments and allows for pre-transformation characterization of the olivine. The olivine used in this study contains approximately 300 wt ppm D₂O. Transformation experiments have been conducted at 18 GPa and 1100, 900, 800 and 700^oC. At 900^oC, the ringwoodite growth rates are over an order of magnitude faster than experimentally derived rates for dry olivine. In addition, transformation is observed at 700^oC on an experimental time scale, 200^oC lower than in the dry system. Ringwoodite growth rates in the wet system appear to be less time dependant, possibly due to weakening of the ringwoodite by D₂O, thereby reducing the build-up of elastic strain. These results strongly suggest that the persistence of metastable olivine in subduction zones will be greatly decreased by the presence of even modest amounts of water.
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005