MANTLE CONTROL OF A DYNAMICALLY EVOLVING SPREADING CENTER - MID-ATLANTIC RIDGE 31-34-DEGREES-S

A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33-degrees -S changes dramatically as its center is approached. Towards the cente r of the segment, the axis shoals from 3900 to 2400 m and a deep media n valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower de nsity mantle or both. Incompatible element and radiogenic isotope rati os 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 Tris tan da Cunha, Gough and Discovery Islands. The average extent of mantl e melting for the entire segment, as determined from mid-ocean ridge b asalt (MORB) major element chemistry, is slightly greater than for adj acent segments. The segment has lengthened to 100 km by ridge propagat ion at both ends during the past 3.5 m.y., and is presently the longes t and shallowest segment in the region. Although the ridge crest anoma lies of this ridge segment strongly resemble those caused by the inter action of mid-ocean ridges with mantle hotspots, the geochemical and g eophysical evidence suggests that they may instead be related to inter action of the ridge with a passively embedded chemical heterogeneity i n the mantle.