On the electrical nature of the axial melt zone at 13° N on the East Pacific Rise

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast-spreading center, was conducted at 13oN on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000-year-old crust. Data above ambient noise levels were obtained at ranges of up to 10 kin. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (,,,1 tim), indicating a heavily fractured, high-porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (,,,100 tim). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill-like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13oN is currently in a state of relative magmatic quiescence.

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