Crustal magma plumbing within a segment of the Mid-Atlantic Ridge, 35 degrees N

We report the results of a seismic tomography experiment which images the t hree-dimensional nature of the crustal melt delivery system beneath a segme nt of the slow-spreading Mid-Atlantic Ridge. In the lower crust ( > 3.5 km depth) near the segment center, inversion of first-arriving crustal P-waves reveals a pair of vertical pipe-like ( < 10-km-diameter) low-velocity anom alies (-0.4 km/s), in the upper crust, these two features, which are physic ally isolated from each other below 3 km, both connect to a l0-km-wide, 45- km-long, axis-parallel, low-velocity zone (-0.2 km/s). Three higher-amplitu de low-velocity anomalies (-0.6 km/s) are observed in the upper crust ( < 2 km depth), and are located directly beneath seafloor volcanic features. We interpret the overall image to represent the thermal/melt signature of a m agma feeding system in which focused injections of magma from the mantle tr avel upward until they intersect the brittle-ductile transition, where they are then diverted along-axis to supply shallow intrusive bodies and seaflo or eruptions along much of the ridge segment. (C) 2000 Elsevier Science B.V . All rights reserved.