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. Fractionationcorrected Na20 contents CNas") 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., K20/TiO• 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 K•O•iO•. 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 Nas) 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. Introduction Based on comparisons with experimental work, the major element compositions of mid-ocean ridge basalts (MORB) imply that they are derived by relatively large degrees of melting of peridotitic mantle at reasonably shallow depths. Recent modeling of the melting processes in ascending mantle by McKenzie and coworkers [McKenzie and Bickle, 1988, McKenzie and O'Nions, 1991] has shown that the mantle will melt progressively during adiabatic ascent, continuously producing melt (the "melting column"), and that pooling of the melts so formed can produce the magmas observed at the surface. Global studies of MORB geochemistry [e.g., Klein and Langmuir, 1987; Brodholdt and Batiza, 1989; Klein and Langrnuir, 1989] have shown that variations in major and minor element contents are related to ridge depth and crustal thickness, suggesting systematic variations in the degree and depth of melting beneath large areas of. the global ridge system. Klein and Langmuir [1987] proposed that the low fractionation-corrected Na contents (denoted Na8, to indicate contents corrected for low-pressure crystal ...
