SEISMIC EVIDENCE FOR A MAGMA CHAMBER BENEATH THE SLOW-SPREADING MID-ATLANTIC RIDGE

SEISMIC reflections from magma chambers have been observed along the f ast-spreading East Pacific Rise(1,2) and the intermediate-spreading Va lu Fa Ridge(3,4); sub-axial reflections also exist beneath the interme diate-spreading Juan de Fuca Ridge(5). But no magma chambers have been identified beneath the slow-spreading Mid-Atlantic Ridge, suggesting that here magma chambers lie unusually deep or are transient features( 6-11). Seismic reflection profiles acquired in 1989 over the Snake Pit hydrothermal area, in the rift valley of the Mid-Atlantic Ridge simil ar to 25 km south of the Kane fracture zone, showed no evidence of mag matic activity(12), although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1-2 km (13,14). By suppressing in these data high-amplitude coherent noise ge nerated at the sea floor, I have obtained images, in an otherwise non- reflective crust, of seismic reflections beneath, and just south of, t he Snake Pit hydrothermal area. These reflections define a small, 4-km -wide dome whose apex is similar to 1,200 m beneath the sea floor. As bright reflections from the upper flanks of this dome occur in the dep th range suggested by the vent-fluid geochemistry, I interpret the dom e to be the seismic expression of a small magma chamber.