Olivine-Ringwoodite Transformation Kinetics Suggest that the Deep Marianas and Pacific Slabs have less than 90 ppm H2O
Abstract
Hydrogen increases the growth rates of the metastable olivine to ringwoodite transformation. These ringwoodite growth rates determine the likelihood of a metastable olivine wedge persisting into the Earth's mantle transition zone (410 to 660 km depth) in subduction zones. Seismological evidence for metastable olivine coincides with the observed locations of deep earthquake hypocenters in subducted slabs [Kaneshima, et al., 2007; Iidaka and Suetsuga, 1992], and metastable olivine is likely associated with mechanisms causing deep earthquakes. Additionally the presence of a metastable olivine wedge adds negative buoyancy to the slab which must be accounted for in precise determination of subduction forces. Furthermore the presence or absence of a metastable olivine wedge may constrain the water content of subducting lithosphere in the Earth's transition zone. Previous results have indicated that olivine containing as little as 290 wt- ppm D2O (an H2O proxy) will transform too quickly for a metastable wedge of olivine to survive into the Earth's transition zone [Diedrich, et al., 2007]. In this study, we have investigated the olivine-ringwoodite transformation kinetics using olivine hydrated with approximately 90 wt-ppm H2O. At 18 GPa and 900 celcius, this olivine transforms quickly, with a ringwoodite growth rate of 2.4e-9 m/s, which is nearly identical to growth rates for olivine with 290 wt-ppm D2O at the same P and T. The activation enthalpy for ringwoodite growth in these samples (140 kJ/mol) is also approximately the same as in the 290 ppm D2O samples. Based on the thermo-kinetic models in Diedrich et al. [2007], approximately 90 ppm H2O in olivine within old, cold, and fast subducting slabs would eliminate the metastable wedge of olivine. Conversely, these results imply that slabs exhibiting seismological evidence of a metastable olivine wedge, such as the subducted Mariana slab [Kaneshima, et al., 2007] and the subducted Pacific slab [Iidaka and Suetsuga, 1992], have less than 90 ppm H2O at transition zone depths.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMDI53A1104D
- Keywords:
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- 3612 Reactions and phase equilibria (1012;
- 8412);
- 3621 Mantle processes (1038);
- 3994 Instruments and techniques;
- 8124 Earth's interior: composition and state (1212;
- 7207;
- 7208;
- 8105);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)