HR: 17:15h
AN: MR44A-06    [Abstracts]
TI: Imaging the East Asian lower mantle water anomaly
AU: Wysession, M E
EM: michael@mantle.wustl.edu
AF: Washington University, Campus Box 1169 One Brookings Drive, St Louis, MO 63130 United States
AU: * Lawrence, J F
EM: jlawrence@ucsd.edu
AF: Washington University, Campus Box 1169 One Brookings Drive, St Louis, MO 63130 United States
AU: * Lawrence, J F
EM: jlawrence@ucsd.edu
AF: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography University of California, San Diego IGPP 0225 9500 Gilman Drive , La Jolla, CA 92093 United States
AB: Using the first global whole-mantle 3D model of shear quality factor, we identify a large region at the top of Earth's lower mantle with unusually high seismic attenuation. This anomaly, found down-dip of western Pacific subduction zones 700-1500 km beneath eastern Asia, is best explained by a 10-fold increase in water content. The anomaly occupies ~1.8×10¹⁰ km³ with quality factor variations bordering on asthenospheric (Q_μ=101) when compared to typical lower mantle quality factor (Q_μ=311). A slab-like high quality factor anomaly, hypothesized to be subducting lithosphere, bounds the eastern and bottom side of the low quality factor anomaly. The anomaly resides in a region having very high resolution, making it a robust feature. The small amplitudes of low-velocity anomalies found in the same region suggest that the anomaly is not thermal. Additionally, a thermal anomaly of this magnitude would require a source to rise from the core mantle boundary through or around a thick sheet-like slab bounding the east and bottom side. We ruled out chemical and grain size anomalies based on the strength of the attenuation anomaly. It has previously been hypothesized that water, in the form of dense hydrous magnesium phase D, could dehydrate from cold slabs at 1100-1500 km depth. This depth range corresponds with the bottom of the anomaly, where free water would be highly unstable and possibly rise quickly through the lower mantle. Due to the low solubility of the expected mineral assemblages at lower mantle pressures, it only takes a small amount of water to drastically change mineral properties such as melting temperature, attenuation, and viscosity. The observation of water in the lower mantle above subducted oceanic lithosphere has significant implications for the processes of mantle convection, the dynamics of plate tectonics, the mixing of the mantle, and the thermal evolution of the Earth.
UR: http://igpphome.ucsd.edu/~jlawrence/VQM3DA/
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5144 Wave attenuation
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005