Upper mantle heterogeneity below the Mid‐Atlantic Ridge, 0°–15°N

Abstract

Small‐scale variations in composition of mantle‐derived peridotites have been investigated in the 0°–15°N portion of the Mid‐Atlantic Ridge (MAR), thanks to a relatively close‐spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle‐equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter‐Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid‐ocean ridge basalt showing a “hot spot”‐type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero‐age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O‐rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.