HR: 10:55h
AN: T51H-03    [PDF]
TI: Melt Generation and Mantle Dynamics Beneath the Southern East Pacific Rise: Insights From the Mantle ELectromagnetic and Tomography (MELT) Experiment EM Data
AU: * Baba, K
EM: kiyoshib@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 2-15, Natsushima, Yokosuka, Kanagawa, 237-0061 Japan
AU: Chave, A D
EM: alan@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#7, Woods Hole, MA 02543 United States
AU: Evans, R L
EM: revans@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#24, Woods Hole, MA 02543 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543 United States
AU: Mackie, R L
EM: randy@gsy-usa.com
AF: GSY-USA, Inc., PMB#643, 2261 Market St., San Francisco, CA 94114 United States
AB: Electrical conductivity structure models allow inference of mantle dynamics based on melt and water distribution beneath a mid-ocean ridge system. The electromagnetic data from the Mantle ELectromagnetic and Tomography (MELT) experiment were collected on two survey lines. The major line crosses the East Pacific Rise at $17\deg$S where the ridge segment is inflated, suggesting an abundant magma supply. The northern line crosses at $15\deg$45'S to the north of an overlapping spreading center on a magma-starved ridge segment. The data are inverted for a two-dimensional anisotropic conductivity structure that incorporates correction for three-dimensional topographic effects on the magnetotelluric responses. The model space allows for different conductivity values in the along-strike, cross-strike, and vertical directions along with imposed constraints that the model be smooth and that the three conductivities be as close together as possible. The strength of these constraints is variable, and hence a range of models from isotropic to anisotropic can be explored. The anisotropic models are more consistent with other geophysical and laboratory data, and hence are preferred. Best fitting anisotropic models display three remarkable features. First, the mantle is more conductive in the cross-strike direction to the east of the rise at depths of 60-150 km and is quite resistive above 60 km. Second, the resistive-conductive boundary is not dependent on the crustal age. Third, there is a narrow, highly conductive pipe in the vertical conductivity located immediately beneath the ridge axis at $17\deg$S which never appears beneath the ridge at $15\deg$45'S. The first two features are well resolved by the data. However, the conductive pipe is not strongly required by the data. These results suggest that melt exists over a wide region, but is more highly concentrated and connected in the vertical at ridge axis. The deep ($>$60 km) conductive region to the east of the ridge crest is consistent with the preferred orientation of olivine under wet conditions. The flat resistive-to-conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of olivine. These results suggest that the extraction of water from olivine due to partial melting substantially reduces mantle conductivity.
DE: 1515 Geomagnetic induction
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3914 Electrical properties
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting