Introduction The RAMESSES Experiment III: The RAMESSES Experiment III:

Summary

A controlled source electromagnetic sounding survey centred on an axial volcanic ridge (AVR) segment of the Reykjanes Ridge at 57°45'N was performed as part of the RAMESSES experiment. Low frequency (0.35 Hz-11 Hz) electromagnetic energy was transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source, a 100 m long horizontal electric dipole towed at heights of 50-80 m from the seafloor. Coincident seismic and magnetotelluric (MT) studies were conducted in the rest of the RAMESSES experiment.

Data were interpreted using a combination of 1-dimensional forward modelling and inversion, and iterative forward modelling in 2-dimensions. On the axis of the AVR, the resistivity at the seafloor is 1 . There is a steep resistivity gradient in the upper few hundred metres of the crust, with the resistivity reaching approximately 10  at a depth of 500 m. In order to explain the low resistivities, the upper layer of the crust must be heavily fractured and saturated with seawater. The resistivity increases with distance from the axis as the porosity decreases with increasing crustal age.

The most intriguing feature in the data is the large difference in amplitude between fields transmitted along and across the AVR axis. A significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this difference, and is consistent with the low velocity zone in the seismic model. This region has a total electrical conductance in excellent agreement with the results of the MT study, and can be explained by the presence of a body of partially molten basalt in the crust. Taken together these results 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%, with a melt volume sufficient to feed crustal accretion at this segment of the ridge for on the order of 20,000 years. Since this body would freeze in the order of 1500 years, this finding lends support to the hypothesis that at slow spreading rates, crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle to a crustal melt reservoir.

Introduction The RAMESSES Experiment III: The RAMESSES Experiment III: \

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