Exploring the role of mantle eclogite at mid-ocean ridges and hotspots: U-series constraints on Jan Mayen Island and the Kolbeinsey Ridge

Abstract

Sensitivity to rates of melting and to the presence of garnet in the melting residue makes U-series isotope disequilibrium a helpful tool for assessing the presence and role of mantle eclogite rocks in generating magmatic crust at both mid-ocean ridges and hotspots. Here we present 238U-234U-230Th-226Ra-210Pb isotope disequilibria data for a suite of fresh lavas from the tectonically and geochemically complex Jan Mayen region, including Jan Mayen Island; the Northern Kolbeinsey Ridge, which hosts the anomalous Eggvin Bank bathymetric high; and the Southern Mohns Ridge. Age-constrained, unaltered samples from the Northern Kolbeinsey Ridge and Eggvin Bank, including fresh popping rocks from Eggvin Bank, are characterized by relatively high (230Th/238U)=1.23 to 1.36 and low (226Ra/230Th)=1.24 to 1.28. These data are best explained by contributions to magma mixtures from an eclogite-dominated, upwelling mantle source with only modest aging or crystal fractionation effects during transport or storage in the crust. Young lavas from Jan Mayen Island have lower (230Th/238U)=1.15 and (226Ra/230Th)=1.05 to 1.16, and forward modeling suggests a detectible, but smaller mafic source contribution to a peridotitic magma mixture in faster upwelling mantle. Globally, most ocean island basalts also show low (226Ra/230Th) disequilibria, suggesting that in addition to high upwelling rates, mafic lithologies make significant melt contributions at most hotspots and that upwelling mantle plumes likely entrain highly fusible, recycled mafic rocks. Eggvin Bank apparently overlies a highly unusual mantle source that likely contains a remnant of delaminated, formerly underplated, ancient subcratonic mafic rocks, entrained by upwelling beneath the northernmost Kolbeinsey Ridge.