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. By far the largest volume of recycled plates, however, is the lithospheric mantle, which is often inferred but rarely observed. Here we report evidence that the Gakkel ridge sampled a section of recycled refractory ocean lithosphere: (1) a unique group of high-Ti basalts occurs adjacent to a long sparsely magmatic zone (SMZ) that is consistent with melting of oxide gabbro hosted in ancient ocean lithosphere; (2) the SMZ itself which is 300km of ancient mantle emplaced at the surface; (3) sparse enriched basalts from the SMZ with no residual garnet signature suggesting shallow melting of ancient refractory but metasomatized lithosphere with low mantle potential temperature; (4) a gradient in composition approaching the SMZ suggesting lower extents of melting of more depleted sources. The rarity of these occurrences elsewhere shows that ordinarily recycled ocean crust and refractory lithosphere mantle are well-mixed rather than separated during mantle convection. Large volumes of recycled ocean crust as a separate reservoir would require ten times more separated, depleted mantle, which then should be common rather than rare. The enclosed basin and ultra-slow spreading rate of the Gakkel Ridge may have provided conditions where recycled oceanic mantle lithosphere was able to be preserved.