HR: 08:15h
AN: DI41B-02 INVITED    [Abstracts]
TI: Melting in the Deep Upper Mantle Oceanward of the Honshu Slab
AU: * Bagley, B
EM: bagl0025@umn.edu
AF: University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Courtier, A M
EM: cour0090@umn.edu
AF: University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Revenaugh, J
EM: justinr@umn.edu
AF: University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AB: Melting in the Deep Upper Mantle Oceanward of the Honshu Slab Utilizing P-wave tomography, Obayashi et al. (2006) discovered a low velocity zone (LVZ) oceanward of the northern Honshu slab. They concluded that the LVZ was due to a 200 °C thermal/chemical anomaly in combination with either a localized increase in iron content or a 50 km thick layer of partial melt above the 410-km discontinuity. We re-examine the region studied by Obayashi et al. (2006) using multiple ScS reverberations, SH- polarized shear phases that are reflected off of the core-mantle boundary and discontinuities within the mantle. Shear waves are more sensitive to changes in shear strength than compressional waves, making them well suited for distinguishing between diffuse thermal/chemical anomalies and regions of partial melt. Our paths sample the region studied by Obayashi et al. (2006) as closely as allowed by the differing source-receiver geometries required by the two different methods. The region where Obayashi et al. (2006) observed the LVZ is well sampled by our data and our results are similar to theirs, with the exception of the region directly east of Taiwan. The Hales, Gutenberg, Lehmann, and 520-km discontinuities are found in many of the paths at various depths. All of the paths contain the 410-km and 660-km discontinuities. Three of the paths on the oceanward side of the Honshu slab contain a LVZ above the 410-km discontinuity, with an average depth of 356 km and a thickness that ranges from 50-75 km. The top of the LVZ is marked by an impedance decrease of -1.2 to -2.0 %. We interpret the LVZ as a layer of negatively buoyant partial melt produced by the combined effects of water and temperature, and estimate the temperature anomaly to be 155 °C with a corresponding maximum water content of 0.100 wt %. If water content varies with depth, some melts may be buoyant and others dense, helping to explain both the LVZ at depth and the surface volcanism in the area. Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters. 243, 149-158.
DE: 3619 Magma genesis and partial melting (1037)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting