ON THE ELECTRICAL NATURE OF THE AXIAL MELT ZONE AT 13-DEGREES-N ON THE EAST PACIFIC RISE

The first controlled source electromagnetic experiment directly on a r idge, with the potential to identify the presence of an axial melt bod y beneath a fast-spreading center, was conducted at 13-degrees-N on th e East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achie ved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the so urce were recorded by seven seafloor electric field receivers position ed 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 km. The results of modeling observed electric field amplitudes r eveal that resistivities in the uppermost crust are very low (approxim ately 1 OMEGAm), indicating a heavily fractured, high-porosity surfici al layer. Below this topmost layer, the upper 2 km of crust is found t o be moderately resistive (approximately 100 OMEGAm). We find no evide nce for a large conductive axial melt body with dimensions on the orde r of kilometers in the middle or upper crust. If a partial melt body i s present, which is continuous along strike and which comprises a conn ected, and therefore conductive, melt texture, it must be of very limi ted volumetric extent. This picture is consistent with recently propos ed 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 la rger region below the sill, characterized by low seismic velocities, m ust contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13-degrees-N is c urrently in a state of relative magmatic quiescence.