EVIDENCE FOR A SMALLER MAGMA CHAMBER BENEATH THE EAST PACIFIC RISE AT 9-DEGREES-30' N

THE size and shape of magma chambers beneath mid-ocean ridges are fundamental features that control the availability of melt, composition of magmas and formation of oceanic crust. Models derived from the study of ophiolites1, and from thermal considerations2, include a very large magma chamber, which can exceed 10-20 km in width. Cross-axis seismic reflection profiles from the East Pacific Rise, however, constrain the width of the axial magma chamber to be < 3-4 km (ref. 3). Even this may be an over-estimate, arising from the under-migration of diffracted energy generated at the edges of a smaller magma chamber. Here we show that forward modelling of these diffraction hyperbolae yields a distance between the best-fitting point diffractors, and by inference a magma chamber width, of only 800-1,200 m. Reflectivity modelling also suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt. A narrow and thin axial magma chamber would inhibit along-axis mixing4, and might thereby account for variations in magma composition along the East Pacific Rise 5. © 1990 Nature Publishing Group.