Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt

Abstract

Zero‐age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6–10 wt. %), strongly depleted in incompatible elements. Fractionation‐corrected Na2O contents (“Nag”) are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid‐ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible‐element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small‐degree partial melting as their cause, rather than long‐lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller‐degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occurring beneath the ridge, nor do they require a heterogeneous source.