HR: 09:30h
AN: T31F-07    [Abstracts]
TI: Water Transport into the Transition Zone and Lower Mantle by High Pressure Hydrous Phases in the Slabs
AU: * Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Tohoku University, Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Yokoyama, A
EM: yokoyama@ganko.tohoku.ac.jp
AF: Tohoku University, Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Hirao, N
EM: hirao@ganko.tohoku.ac.jp
AF: Tohoku University, Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Kondo, T
EM: tdskondo@mail.tains.tohoku.ac.jp
AF: Tohoku University, Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: Water can be transported into the transition zone and lower mantle by the high pressure hydrous phases in the slabs. The candidates for the carriers of water are superhydrous phase B and phase D (= phase G) which are stable in the peridotite or harzburgite layer of the slabs. Phase Egg (AlSiO3OH) and delta-AlOOH are also possible candidates for water carriers in the crustal components in the slabs. We have determined the stability field of these hydrous phases expected in the transition zone and lower mantle. The stability fields of superhydrous phase B and phase Egg were studied up to the pressure of 30 GPa and the temperatures form 1000 to 1400 K by using the conventional quenching method together with the in situ X-ray diffraction study at high pressure using synchrotron radiation from PF and SPring 8. Phase Egg decomposes to delta-AlOOH and stishovite at the top of the lower mantle. The decomposition boundary of superhydrous phase B into phase D (G) + perovskite + periclase has a negative slope and can be expressed by the following equation, P(GPa)=33.2-0.0037T(K). We clarified the stability field of hydrous phase D (G) by using the laser heated diamond anvil cell with the synchrotron X-ray radiation at PF up to a pressure of 60 GPa and 1500 K. Natural orthopyroxenes (Mg#=92-93) were used for the starting material. Fine grained Pt powder was mixed with the pyroxene sample for absorption of the laser power for generation of high temperatures up to 1500 C. Phase D(G) is stable up to c.a. 45 GPa and 1500 K and 50 GPa at 1300 K with a negative dT/dP, The phase boundary is expressed as P(GPa)=75.2-0.021T(K) which is consistent with the previous result by Shieh (1998). These results suggest that there are major dehydration regions associated with the subducting slabs; i.e., the bottom of the transition zone and the deep lower mantle. The fluid generated by the decomposition of the hydrous phases might be responsible for the seismic reflectors observed in some depths in the lower mantle.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting