HR: 08:35h
AN: T51B-03    [PDF]
TI: The Origin of the Low Velocity Zone: Laboratory Constraints
AU: * Faul, U
EM: uli.faul@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Jackson, I
EM: ian.jackson@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: One of the most prominent features of oceanic upper mantle is a minimum in shear wave velocity usually observed at a depth between 100 and 200 km. Partial melting is most commonly invoked as an explanation, but a change in deformation mechanism and the presence of water have also been suggested. An important observation is that the depth of the low velocity zone varies with age of the oceanic lithosphere, implying a relationship with the thermal structure rather than a process occurring at a fixed depth. We have measured shear modulus and attenuation of polycrystalline olivine as a function of temperature at seismic periods. The data from five samples ranging in mean grain size by more than an order of magnitude have been fitted to an extended Burgers model that allows extrapolation of our measurements to the larger grain sizes expected in the upper mantle. An important experimental observation is that polycrystalline olivine behaves nearly elastically at temperatures below $\sim900\deg$C and seismic periods. Above this temperature anelastic processes cause attenuation and shear modulus reduction, resulting in much larger and period-dependent modulus variation relative to lower temperatures. We have calculated shear wave velocities for oceanic upper mantle from the fit to our experimental data and a conductively cooling half-space model commonly invoked to explain the age dependent structure of oceanic lithosphere. In addition to the temperature sensitivity, pressure dependence also is taken into account by an activation volume and anharmonic pressure derivative of the shear modulus. When the shear wave velocity is calculated as a function of depth, a pronounced velocity reduction occurs where the temperature first exceeds $\sim900\deg$C, corresponding to the onset of anelastic behavior. As long as the temperature increases rapidly along the conductive part of the geotherm, the velocity continues to decrease. Once the geotherm joins the adiabat, the increase in temperature with depth is much more modest and the pressure dependence becomes dominant, causing an increase in velocity. This combination of temperature and pressure dependence therefore results in a velocity minimum that is age dependent for oceanic lithosphere less then 100 Myr old. When comparing our calculated velocity profiles with those observed seismologically we find that our calculations match the observations fairly well-both with respect to the magnitude of the calculated velocities and the age dependent depth of the velocity minimum. The calculated attenuation is also consistent with seismic observations, with the least well constrained parameter being the activation volume. Comparison of calculated zero-age velocities with velocities determined by the MELT Seismic Team suggests the presence of melt beneath the East Pacific Rise.
DE: 5144 Wave attenuation
DE: 7218 Lithosphere and upper mantle
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting