The significance of recycled oceanic mantle lithosphere beneath the Arctic Gakkel Ridge

Abstract

Subduction of oceanic crust has long been considered a major cause of mantle heterogeneity. The oceanic lithospheric mantle, the largest volume of recycled plates, however, is often inferred but rarely observed. Here we report a collection of evidence suggesting that the Gakkel Ridge sampled recycled refractory ocean lithosphere. (1) A gradient in composition in the western volcanic zone (WVZ) approaching the sparsely magmatic zone (SMZ) suggests lower extents of melting of more depleted sources. (2) The SMZ itself contains substantial portions of sea floor formed by mantle emplaced at the surface. (3) Sparse enriched basalts from the SMZ are consistent with melting of refractory but metasomatized lithosphere. (4) Unique high-Ti basalts at the WVZ-SMZ boundary originated by deep melting of oxide-gabbro-bearing ocean lithosphere mantle. The volume of recycled ocean lithospheric mantle is enormous– it equals roughly 25 % of the entire mantle per gigayear. However, it is almost never observed as an isolated source. If the crustal portion of the lithosphere is isolated to form discrete sources in the mantle, ten to fifteen times as much depleted mantle lithospheric sources would be formed. The rarity of this source material thus suggests that ordinarily recycled ocean crust and mantle lithosphere become well mixed during mantle convection. The enclosed basin and ultra-slow spreading rate of the Gakkel Ridge may have provided inhibited mixing conditions where recycled oceanic mantle lithosphere was able to be preserved.