Origin and implications of the amagmatic segment of the Gakkel Ridge

Global ocean crust has an average thickness of 6–7 kilometers, suggesting a globally pervasive, rather uniform mantle composition. At some ultra-slow spreading ridges, crustal thickness is highly variable and mantle peridotite can be found at the surface. The peridotites, however, are mostly recovered in fracture zones that expose the deeper crust, or at the edges of ridge segments where there is a central volcano. The Gakkel Ridge is unusual in this regard because it contains a 400-kilometer-long sparsely magmatic zone (SMZ) with extensive mantle peridotite exposure, negligible crustal thickness and limited basaltic lava outcrops. This segment is also bracketed by two other sections of ridge that have active volcanism, including the adjacent Western Volcanic Zone (WVZ) where no peridotites were recovered. What is the origin of this enigmatic expanse of ridge, and is it simply a curiosity or does it have global implications for ocean ridges and mantle recycling? We have undertaken systematic geochemical analysis of 267 basaltic glass samples from the WVZ and the few recovered basalts from the SMZ. The WVZ has normal-thickness oceanic crust and predominantly produces depleted normal mid-ocean ridge basalt (N-MORB). Gradients in chemical composition can be accounted for by a combination of more depleted mantle and lower extents of melting as the SMZ is approached. Across an abrupt boundary, the SMZ has negligible crustal thickness and is dominated by exposed mantle peridotite and a few samples of enriched mid-ocean ridge basalt (E-MORB). Quantitative models suggest the SMZ is the result of cold, ancient ocean mantle lithosphere that has been metasomatized by enriched, low degree melts. While the SMZ is a rare occurrence, simple mass balance considerations suggest such occurrences should instead be very common. While recycled ocean crust is commonly called upon, sometimes as an isolated lower mantle reservoir, the mass of depleted ocean mantle lithosphere would be more than ten times greater. Indeed, using current ridge production rates, over the last 2.5 billion years the total volume of recycled mantle lithosphere would be equivalent to the volume of the entire lower mantle. While vestiges of such lithosphere are frequently invoked from Os isotopes or melt inclusions, almost all of these occurrences are coincident with predominant basalts, and occur in regions with normal crustal thickness. Why are there not vast regions dominated by depleted lithosphere, negligible crust, or common occurrences of basalts that come from highly depleted reservoirs? An obvious solution is that mantle convection is highly efficient at mixing crustal and mantle components on a scale finer than is sampled by melting, permitting relatively uniform crustal thickness and composition on a global basis.