Mantle control of a dynamically evolving spreading center: Mid-Atlantic Ridge 31–34°S

Abstract

A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33°S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower density mantle or both. Incompatible element and radiogenic isotope ratios in MORB increase, creating a ‘spike high’ centered on the summit of the segment. The basalts' enrichment is confined to this robust ridge segment alone and is geochemically unlike the nearby hotspots at Tristan da Cunha, Gough and Discovery Island. The average extent of mantle melting for the entire segment, as determined from mid-ocean ridge basalt (MORB) major element chemistry, is slightly greater than for adjacent segments. The segment has lengthened to 100 km by ridge propagation at both ends during the past 3.5 m.y., and is presently the longest and shallowest segment in the region. Although the ridge crest anomalies of this ridge segment strongly resemble those caused by the interaction of mid-ocean ridges with mantle hotspots, the geochemical and geophysical evidence suggests that they may instead be related to interaction of the ridge with a passively embedded chemical heterogeneity in the mantle.