Dynamics, crustal thicknesses, seismic anomalies, and electrical conductivities in dry and hydrous ridge-centered plumes

3D convection and melting models for ridge-centered plumes in the upper 410 km of the mantle have been computed for dry and hydrous mantles, taking into account the effects of water on melting and viscosity. Dry and hydrous models have fundamentally different flow and melting styles in the melting regime: in the dry model, active upwelling occurs to shallow depth and results in a very thick, narrow plume-generated crust, whereas dehydration stiffening in the hydrous models enforces passive upwelling in much of the melting region and causes the plume crust to be much less thick, but broader in along-ridge direction. The output of the numerical models, i.e. temperature, melting degree, water content, and porosity, is used to compute synthetic models of seismic velocity anomalies and electrical conductivity. For this purpose, combinations of theoretical and empirical relations with recent experimental data are developped. The models predict that the seismic velocity anomaly of the deep, unmolten plume stem is mostly thermal in both water-bearing and water-free plumes, but that the two velocity minima in the anomaly in the melting zone of the plume are much more clearly distinguishable in a water-bearing plume. If one assumes a contribution to the electrical conductivity of hydrous mantle from free protons, and if the water reservoir features large-scale contiguity, one would also expect a strong high-conductivity anomaly in the plume stem which should be visible to magnetotelluric observations. However, comparison with observations suggests that a simple free-proton model may not be applicable.