HR: 11:10h
AN: V22A-04 INVITED     [Abstracts]
TI: Structure and Composition of Oceanic Crust: What do we know; What do we need to know?
AU: * Carlson, R L
EM: carlson@geo.tamu.edu
AF: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115
AB: The seismic structure of "normal" crust is broadly consistent with the ophiolite model. P- and S-wave velocities are consistent with a crust of "gabbroic" composition; basalts that comprise the uppermost crust have been widely sampled by dredging, drilling, and submersible surveys, and velocities in the lower crust (layer 3) are in good agreement with the measured properties of hydrothermally altered gabbros containing 5-15%\ alteration products (amphiboles and phyllosilicates). However, diabase dikes have been sampled in only two localities, and the dike section is not well resolved in the seismic structure, possibly because the properties of the dike rocks are controlled by cracks. The nature of the layer 2/3 transition is not well resolved, but lab velocities in gabbros agree with layer 3 velocities, while velocities in dike rocks are lower, indicating that the top of layer 3 is within or at the top of the gabbros. While the seismic structure of normal crust is most consistent with the ophiolite model, there are regions where sampling is dominated by partially serpentinized peridotite. In the vicinity of the 15ø 20' Fracture Zone, the ratio of peridotite samples to gabbros recovered by drilling and diving is approximately 3:1. The seismic structure of the crust in this region differs from the structure of "normal" crust in that P-wave velocities increase gradually, with no indication of a transition from layer 2 to layer 3. This structure could indicate either a gradual increase of the gabbro component or a gradual reduction in the degree of serpentinization with depth; profiles of both P- and S-wave velocities are needed to test these hypotheses. Compositional models of the uppermost mantle have not been considered in detail. Low average Mg numbers of crustal rocks and glass suggest that melts rising within the mantle may begin to crystallize as they enter the thermal boundary layer. Thus gabbros may be sequestered in the upper mantle. A comparison of upper mantle P- and S-wave velocities with a Voigt-Reuss-Hill model of the properties of mixed peridotite and gabbro suggests that the uppermost mantle may contain 30 to 60%\ gabbro.
DE: 3035 Midocean ridge processes
DE: 1020 Composition of the crust
DE: 1050 Marine geochemistry (4835, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting