Seismic evidence for a magma chamber beneath the slow-spreading Mid-Atlantic Ridge

Abstract

SEISMIC reflections from magma chambers have been observed along the fast-spreading East Pacific Rise1,2 and the intermediate-spreading Valu Fa Ridge3,4; sub-axial reflections also exist beneath the intermediate-spreading Juan de Fuca Ridge5. 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 features6-11. Seismic reflection profiles acquired in 1989 over the Snake Pit hydrothermal area, in the rift valley of the Mid-Atlantic Ridge ~25 km south of the Kane fracture zone, showed no evidence of magmatic activity12, although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1a¤-2 km13,14. By suppressing in these data high-amplitude coherent noise generated at the sea floor, I have obtained images, in an otherwise non-reflective crust, of seismic reflections beneath, and just south of, the Snake Pit hydrothermal area. These reflections define a small, 4-km-wide dome whose apex is ~ 1,200 m beneath the sea floor. As bright reflections from the upper flanks of this dome occur in the depth range suggested by the vent-fluid geochemistry, I interpret the dome to be the seismic expression of a small magma chamber.