EVIDENCE FOR ACCUMULATED MELT BENEATH THE SLOW-SPREADING MID-ATLANTICRIDGE

The analysis of data from a multi-component geophysical experiment con ducted on a segment of the slow-spreading (20 mm yr(-1)) Mid-Atlantic Ridge shows compelling evidence for a significant crustal magma body b eneath the ridge axis. The role played by a crustal magma chamber bene ath the axis in determining both the chemical and physical architectur e of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the la st decade subsurface geophysical techniques have successfully imaged s uch magma chambers beneath a number of intermediate and fast spreading (60-140 mm yr(-1) full rate) ridges. However, many similar geophysica l studies of slow-spreading ridges have, to date, found little or no e vidence for such a magma chamber beneath them. The experiment describe d here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57 degrees 43' N. It consisted of four major components: wide-angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; control led source electromagnetic sounding; and magneto-telluric sounding. In terpretation 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 a nomalously low seismic P-wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of th is melt body shows a number of similarities with the axial magma chamb ers observed beneath ridges spreading at much higher spreading rates. Magneto-telluric soundings confirm the existence of very low electrica l resistivities in the crust beneath the AVR and also indicate a deepe r zone of low resistivity within the upper mantle beneath the ridge.