HR: 0830h
AN: T41C-0236    [PDF]
TI: Deriving seismic anomalies and electrical conductivities from mantle convection models: effects of melting and water content
AU: * Ruedas, T
EM: tr@dlc.ku.dk
AF: Institute for Meteorology and Geophysics, J.W. Goethe University, Frankfurt/M., Feldbergstrasse 47, Frankfurt/M., D-60323 Germany
AU: * Ruedas, T
EM: tr@dlc.ku.dk
AF: Danish Lithosphere Center, {\O}ster Voldgade 10, L, Copenhagen, 1350 Denmark
AU: Schmeling, H
EM: schmeling@geophysik.uni-frankfurt.de
AF: Institute for Meteorology and Geophysics, J.W. Goethe University, Frankfurt/M., Feldbergstrasse 47, Frankfurt/M., D-60323 Germany
AB: Physical properties of rocks depend on several physical and compositional quantities which are often taken as homogeneous or neglected for simplicity or because they are not known well enough. We take the temperature, depletion, melt content, and water content distributions of 3D numerical convection/melting models of a ridge-centered plume and apply a combination of theoretical and empirical models for the effects of temperature, depletion, melt content and H$^+$ resp. OH$^-$ content (extended after Kreutzmann et al., submitted paper, 2003) to derive seismic anomalies and electrical conductivities for well-defined water-free and water-bearing systems. We find that for both seismic and electrical observables, several factors apart from temperature play an important role, e.g. the presence of melt, but in particular the water content has a very strong impact on the magnitude and shape of seismic anomaly and conductivity patterns in the deeper, unmolten parts of the models, thus confirming predictions by Karato (1990, 1998). Assuming a higher water content for the plume than for the background mantle, the effect of water leads to a stronger low-velocity anomaly in the unmolten mantle which would be misinterpreted as a very hot plume if the effect of water were neglected altogether. On the other hand, the stiffening effect of dehydration on rheology in the melting zone reduces the upwelling rate of a plume and decreases its melt production, so that nonetheless a strong thermal anomaly is needed to generate the thickened crust of islands located above plumes, such as Iceland. In the shallower parts, the disappearance of water results in a convergence of the anhydrous and hydrous models, and other factors, e.g. porosity or depletion, are important, although to a lesser extent. In parts of the partially molten plumehead the low-velocity and high-conductivity anomalies are drastically reduced, so that they may be invisible for seismic or electromagnetic methods.
DE: 3909 Elasticity and anelasticity
DE: 3914 Electrical properties
DE: 5144 Wave attenuation
DE: 7218 Lithosphere and upper mantle
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting