HR: 17:15h
AN: T34A-06    [Abstracts]
TI: Compositional Controls on Oceanic Mantle: Geophysical Evidence from the MELT Area
AU: * Evans, R L
EM: revans@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543
AU: Baba, K
EM: kbaba@eri.u_tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1--1--1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Forsyth, D
EM: Donald_Forsyth@brown.edu
AF: Dept of Geology, Brown University, 324 Brook St, Providence, RI 02912
AU: Chave, A
EM: achave@whoi.edu
AF: Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543
AU: Mackie, R
EM: randy@gsy-usa.com
AF: GSY-USA, Inc, PMB#643, 2261 Market St, San Francisco, CA 94144
AB: Geophysical data from the MELT experiment carried out at 17°S on the Southern East Pacific Rise, constrain how oceanic lithosphere and the underlying asthenosphere evolves during its early cooling history. We discuss structure imaged ~~100-350 km east of the ridge crest, corresponding to seafloor ages from ~1.3~Ma to 4.5~Ma. At this distance from the ridge flow patterns in the upper mantle are expected to be largely horizontal, reflecting plate spreading motions rather than upwelling. Electromagnetic data show a transition from a dry uppermost 60~km of mantle to a damp underlying asthenosphere. Electrical conductivity increases in both magnitude and in anisotropy beneath this boundary with the highest conductivity in the direction of plate motion, consistent with enhanced conduction through the diffusion of hydrogen in olivine. Strong seismic anisotropy is found in the upper 60~km and must continue to greater depths, so the onset of electrical anisotropy cannot be due to increased alignment of anisotropic olivine crystals. The expected increase in mantle viscosity under dry conditions suggests that this uppermost 60~km acts as a thicker, more viscous oceanic plate than would be predicted by simple thermal cooling models.
DE: 3006 Marine electromagnetics
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: Fall Meeting 2005