Perspectives on Ocean Ridge Basalts from the Segment to the Global Scale

This study addresses the influences on ridge basalt chemistry, through analysis of their major and trace element and isotopic composition at scales ranging from individual ridge segments to the entire length of the ridge system. Local-scale studies of basalts along the Mid-Atlantic Ridge shed light on crustal accretion at slow-spreading ridges, and on the nature of plume-ridge interaction in this region. We show that segments must have multiple supplies of magma delivered along their length, but with preferential delivery of magma to segment centers. Plume-ridge interaction near the Azores is not simple two-component mixing between "plume" mantle and "depleted" mantle as previously argued. The elevated highly incompatible trace element ratios possessed by basalts well south of the plume are the definitive sign of a low-degree melt, which can fractionate highly incompatible element ratios. We show that a low-degree melt of plume mantle acts to metasomatize ambient depleted mantle, creating a mixed source that melts to produce the enriched basalts south of the Azores. This metasomatized source is the enriched mixing component that produces the observed geochemical gradient, rather than bulk plume mantle.