Evidence for a smaller magma chamber beneath the East Pacific Rise at 9°30′ N

Abstract

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 Rise⁵.