Ancient continental mantle beneath oceanic ridges

The petrology, bulk chemistry, and Sr-isotopic chemistry of ultramafic rocks from the equatorial mid-Atlantic ridge indicate that: (1) the ultramafics are derived from the upper mantle; (2) they are not genetically related to oceanic gabbros and basalts; (3) they are residual and were depleted of lithophile elements at some early stage in the history of the earth; (4) they are similar to alpine-type peridotites from the continents. These results suggest that a zone of alpine-type residual peridotitic material exists in the upper mantle beneath the mid-Atlantic ridge. This zone is tentatively identified with the anomalous, low-density mantle body observed beneath the ridge in seismic and gravity profiles. The alpine-type residual peridotitic upper mantle beneath the ridge was originally part of a layer of ‘continental’ mantle located below a pre-Atlantic rift super-continent (Pangea); it constituted the material left over from the differentiation of the sialic body from the primitive mantle. Upon rifting of the sialic body, the upper mantle ‘continental’ layer is carried along by convection currents except for a central block caught in a stagnant zone at the divergence of the two rising limbs; such block is left behind and is presently beneath the mid-Atlantic ridge; fragments of it become exposed by upward intrusion. The stagnant ‘continental’ mantle should be present below the Atlantic and Indian ridges, which originated by rifting of continental blocks, but not below the East Pacific rise, because the latter probably initiated in a ‘protooceanic’ area. This hypothesis may help explain contrasting features of the two types of ridges; for instance, why a larger heat-flow anomaly is associated with the East Pacific rise than with the mid-Atlantic and Indian ridges.