HR: 11:35h
AN: T32B-06    [Abstracts]
TI: Water controls fields of metastable olivine in subducting slabs
AU: Hosoya, T
EM: hosoya@ganko.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Aza Aoba Aramaki Aoba, Sendai, 980-8578 Japan
AU: * Kubo, T
EM: kubotomo@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences Faculty of Sciences, Kyushu University, 1-10-6 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Aza Aoba Aramaki Aoba, Sendai, 980-8578 Japan
AU: Sano, A
EM: sano@ganko.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Aza Aoba Aramaki Aoba, Sendai, 980-8578 Japan
AU: Funakoshi, K
EM: funakosi@sp8sun.spring8.or.jp
AF: Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Mikazuki-cho Sayo-gun, Hyogo, 679-5198 Japan
AB: Presence of metastable olivine in subducting slabs has important implications for buoyancy force against subduction, stress state and strength of the slabs, and origin of deep earthquakes. We have examined pressure, temperature, and water content dependences of the $\alpha$-$\beta$ transformation rate in Mg$_{2}$SiO$_{4}$ using in-situ X-ray diffraction method at SPring-8 to discuss fields of metastable olivine in subducting slabs. Graphite and Ag-Pd alloy were used as the sample capsule in nominally dry and wet runs, respectively. Time-resolved X-ray diffraction patterns were successfully obtained during the transformation. Water contents in recovered samples were measured using a micro Fourier-transform infrared spectroscopy. We have determined growth rates in the $\alpha$-$\beta$ transformation at 13.4-15.8 GPa, 730-1100$\deg$C, and 750-4900 wt. ppm H$_{2}$O. In case that the water content is about 750 wt. ppm H$_{2}$O, the activation energy and activation volume for growth were estimated to be 348 kJ/mol and 1.7 cm$^{3}$/mol (Kubo et al., 2004). This growth kinetics is almost consistent with the previous study of Mosenfelder et al. (2001), in which the olivine can survive to a depth of around 600 km in the slab for the coldest thermal model. We observed that the growth in the $\alpha$-$\beta$ transformation of olivine is significantly enhanced by water. The growth rate is roughly proportional to water content to the 3 power. The enhancement associated with an increase of an order of magnitude in water content is identical to that with a 200$\deg$C increase in temperature. Thus, water can control fields of metastable olivine in slabs. When the water content of cold slabs is less than about 750 wt. ppm H$_{2}$O, the depth of the olivine transformation would be greater than 600 km, whereas fields of metastable olivine become much smaller if cold slabs contain much water. References: Kubo et al. (2004) Am. Mineral., 89, 285-293; Mosenfelder et al. (2001) Phys. Earth Planet. Inter., 127, 165-180.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 5139 Transport properties
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting