Electrical properties of the mantle upwelling zone beneath a mid-ocean ridge, an application of vertical gradient sounding

grantor: University of TorontoOn mid-ocean ridges, as adjacent plates move apart, the mantle material rises to fill the void created. During its ascent the solidus of the material is crossed and melting occurs. The melt itself is eventually emplaced at the ridge axis producing new oceanic crust. The understanding of the flow of the solid and molten material is hampered by the lack of knowledge of vital model parameters such as the connectivity of the partial melt. Connectivity is related to the permeability in the upwelling region. It therefore controls the migration pattern of the buoyant melt, the flow of the solid phase material, and the mantle upwelling mechanism. Changes in the geometry of the distribution of melt in the solid material have a large impact on the electrical conductivity. I have measured the conductivity of the upwelling region to constrain possible partial melt geometries. I present results of vertical gradient sounding (VGS) experiments on the Endeavour and Explorer ridge, which are part of the Juan de Fuca and its northern extension, the Explorer ridge, respectively. The VGS method is a natural source EM method based entirely on measurements of the magnetic fields. Electrical responses of the 1D layered normal seafloor combined with a 2D region representing the mantle upwelling zone and proposed upwelling mechanisms are derived. A comparison of the synthetic response of a range of models with data measured on the Endeavour segment shows that the conductivity in the upwelling region is very high (in the order of 1 to 5 ohm m depending on the shape of the upwelling region). The results of this experiment suggest that the pore space containing the conductive melt is well connected. The melt must be able to move freely through the upwelling region. The experiments support so called melt migration models. The data measured on the Explorer segment yielded a different conductivity model. The data do not require the presence of a pronounced 2D conductivity anomaly at depth and therefore precludes the existence of large or well connected melt fractions in an upwelling region underneath Explorer ridge. The data therefore support a study of magnetic anomaly maps that suggests that the Explorer ridge is degenerate, i.e. that the Explorer plate is slowly deforming internally and its movement is governed by the movement. of the surrounding plates.Ph.D