Evidence for accumulated melt beneath the slow–spreading Mid–Atlantic Ridge

The analysis of data from a multi–component geophysical experiment conducted on a segment of the slow–spreading (20 mm yr-1) Mid–Atlantic Ridge shows compelling evidence for a significant crustal magma body beneath the ridge axis. The role played by a crustal magma chamber beneath the axis in determining both the chemical and physical architecture of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the last decade subsurface geophysical techniques have successfully imaged such magma chambers beneath a number of intermediate and fast spreading (60-140 mm yr-1 full rate) ridges. However, many similar geophysical studies of slow–spreading ridges have, to date, found little or no evidence for such a magma chamber beneath them.The experiment described here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57° 43′ N. It consisted of four major components: wide–angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; controlled source electromagnetic sounding; and magneto–telluric sounding. Interpretation and modelling of the first three of these datasets shows that an anomalous body lies at a depth of between 2 and 3 km below the seafloor beneath the axis of the AVR. This body is characterized by anomalously low seismic P–wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of this melt body shows a number of similarities with the axial magma chambers observed beneath ridges spreading at much higher spreading rates. Magneto–telluric soundings confirm the existence of very low electrical resistivities in the crust beneath the AVR and also indicate a deeper zone of low resistivity within the upper mantle beneath the ridge.