The RAMESSES experiment - III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57 degrees 45 ' N

A controlled-source electromagnetic sounding survey centred on an axial vol canic ridge (AVR) segment of the Reykjanes Ridge at 57 degrees 45'N was per formed as part of the RAMESSES experiment. Low-frequency (0.35-11 Hz) elect romagnetic signals were transmitted through the crust to an array of horizo ntal electric field recorders at the seafloor to ranges of 15 km from the s ource, which was a 100 m long horizontal electric dipole towed at heights o f 50-80 m from the seafloor. Coincident seismic and magnetotelluric studies were conducted during the rest of the RAMESSES experiment. Data were interpreted using a combination of 1-D forward modelling and inve rsion, and iterative forward modelling in two dimensions. On the axis of th e AVR, the resistivity at the seafloor is 1 Ohm m. There is a steep resisti vity gradient in the upper few hundred metres of the crust, with the resist ivity reaching approximately 10 Ohm m at a depth of 500 m. In order to expl ain the low resistivities, the upper layer of the crust must be heavily fra ctured and saturated with sea water. The resistivity increases with distanc e from the axis as the porosity decreases with increasing crustal age. The most intriguing feature in the data is the large difference in amplitud e between fields transmitted along and across the AVR axis. A significant z one of low-resistivity material is required at approximately 2 km depth ben eath the ridge crest in order to explain this difference. It is coincident with the low-velocity zone required by the seismic data, and has a total el ectrical conductance in excellent agreement with the results of the magneto telluric study. The low-resistivity zone can be explained by the presence o f a body of partially molten basalt in the crust. Taken together, these res ults provide the first clear evidence for a crustal magma chamber at a slow spreading mid-ocean ridge. The data constrain the melt fraction within the body to be at least 20 per cent, with a melt volume sufficient to feed cru stal accretion at this segment of the ridge for of the order of 20 000 year s. Since this body would freeze in the order of 1500 years, this finding le nds support to the hypothesis that, at slow spreading rates, crustal accret ion is a cyclic process, accompanying periodic influxes of melt from the ma ntle to a crustal melt reservoir.