HR: 08:00h
AN: DI41B-01 INVITED    [Abstracts]
TI: The Strong Effect of H2O On Olivine Transformation Kinetics Suggests That Some Subducting Slabs Are Dry
AU: * Sharp, T G
EM: tom.sharp@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
AU: Diedrich, T
EM: Tamara.Diedrich@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
AU: Du Frane, W L
EM: wdufrane@asu.ed
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, United States
AU: Marton, F C
EM: fmarton@alumni.uchicago.edu
AF: Natural Sceinces Department, Bergen Community College, Paramus, NJ 07652-1595, United States
AU: Leinenweber, K
EM: KURTL@asu.edu
AF: Department of Chemistry, Arizona State University, Tempe, AZ 85287, United States
AB: Because of the high solubilities of hydrogen in nominally anhydrous mantle and transition-zone minerals, it is possible that much of Earth's H2O resides in the upper mantle and transition zone. If Earth's mantle is part of its water cycle, subduction zones would provide the vehicle for transporting H2O from Earth's surface to the transition zone. Seismic and petrologic evidence has been used to argue that the mantle portion of subducting oceanic lithosphere can be hydrated, which would allow hydrous olivine to be transported to the transition zone. In fast and cold subducting slabs, where temperatures may be low enough to inhibit equilibrium transformation of olivine, a wedge of metastable olivine is thought to extend deep into Earth's transition zone. Olivine transformational faulting in such a metastable wedge provides an explanation for deep focus earthquakes. Previous kinetic experiments show that large amounts of H2O in olivine enhance growth rates in the olivine-wadsleyite and olivine-ringwoodite phase transformations. Here we present new experimental results on the transformation of hydrous (100 and 300 wt-ppm H2O) San Carlos olivine to ringwoodite at 18 GPa. Our experiments show that small amounts of H2O enhance ringwoodite growth rates and greatly decrease activation enthalpies relative to anhydrous samples. Even at temperatures as low as 700 °C, we obtain measurable growth rates, implying rapid ringwoodite growth at temperatures corresponding to the interiors of cold subducting slabs. Combining our kinetic data with thermal modeling of subduction zones shows that even for very old and fast subduction zones, small amounts of H2O would eliminate the metastable olivine wedge. Seismic evidence for a metastable olivine wedge in the Mariana subduction zone suggest that the olivine in this slab has less than 100 wt-ppm H2O. If deep-focus earthquakes are caused by olivine transformational faulting, then subduction zones that exhibit these earthquakes must be nearly dry.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
DE: 3924 High-pressure behavior
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting