HR: 09:00h
AN: T31F-05 INVITED     [Abstracts]
TI: Water contents in the mantle transition zone beneath the north Pacific derived from the electrical conductivity anomaly
AU: * Koyama, T
EM: tkoyama@jamstec.go.jp
AF: IFREE, Japan Agency for Marine-Earth Science and Technology, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Shimizu, H
EM: shimizu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Utada, H
EM: utada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AU: Hae, R
EM: hae_ryo@ganko.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AB: In the former work, we estimated the three-dimensional electrical conductivity anomaly in the mantle transition zone beneath the north Pacific by inversion of semi-global electromagnetic network data. In this paper we try to convert the result to anomalies of temperature or water contents by combining it with results of recent seismic tomography (Fukao et al., 2004), and with reference to physical parameters determined by laboratory experiments of mantle materials. First we assumed that both electrical conductivity and seismic P-wave velocity anomalies are simply corresponding to thermal anomaly. Under this simple assumption, conductivity and velocity models were independently converted to temperature field by using the physical relationship derived from the laboratory measurements (Karato, 1993; Xu et al., 2000). Temperature field models from electrical and seismic data show similarities as a whole. For instance, high temperature anomaly of about 200K is seen beneath Hawaii. However, significant discrepancy is found beneath Mariana islands where the seismic tomography indicates low temperature anomaly, while electromagnetic data shows high temperature anomaly. Although there still remains a problem of spatial resolution, this result indicates that this particular feature may not be explained only by thermal effect. Taking into consideration that this region is surrounded by subducted slabs, we further assumed that this discrepancy is caused by the existence of water dehydrated from those slabs. Under this assumption, by combining the Nernst-Einstein relationship (e.g. Karato, 1990) and the recent result of laboratory measurements of hydrogen diffusivity in wadsleyite (Ohtani et al., in preparation), the water contents anomaly was estimated from the electrical conductivity anomalies. As a result, the mantle transition zone beneath Mariana islands turned out to contain about 0.2 weight % more water than surrounding mantle which is expressed by a reference structure.
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 1025 Composition of the mantle
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 0600 ELECTROMAGNETICS
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting