A Highly Depleted and Subduction-Modified Mantle Beneath the Slow-Spreading Mohns Ridge

Abstract

The Mohns Ridge is a very slow-spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian-Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro-peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid-ocean ridge basalts with Na-content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr-number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole-rock composition (14%–20%). If all the melting took place beneath the paleo-Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid-ocean ridges but rather resemble that seen in supra-subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water-rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid-Ocean Ridges and represents the source for the spreading related magmatism.