Mantle melting beneath Iceland and Hawaii

Recent samples from Theistareykir, northern Iceland, are relatively close in composition to primary melts from the mantle and are not disturbed by interaction with the pre-existing Icelandic crust. Both source heterogeneity and variations in depth and extent of melting are required to explain the large range in chemical composition. Melting is dominated by melting depleted peridotite similar to the ambient depleted mantle and sampling of the enriched components, which are of minor abundance in the source, is a function of extent and pressure of melting. The dominant enriched component has a HIMU affinity rather than representing the enriched end of the Iceland isotopic arrays and an additional minor enriched component similar to EMI type OIB sources is invoked to explain the isotopic variability. Large and relatively constant 230Th-excesses for the bulk of the samples indicate that melting starts within the garnet stability field and that source heterogeneity does not have an important effect on the U-Th disequilibrium systematics of the extracted melts. 230 Th excesses of about 15% require upwelling velocities of about 1cm/y and residual porosities <0.1%. Lower 230Th excesses in both the most enriched and depleted samples, however, indicate some variations in the processes and rate of melt extraction.; Combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources, and the chemical and isotopic composition of recent Hawaiian lavas argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts.; Creating HIMU sources by recycling oceanic crust requires substantial modification of the trace element budget of the oceanic crust during subduction, and the evidence for sediment in the EM-type OIB sources remains contentious from a quantitative point of view. In contrast to OIB end-members, modeled compositions of the subducted crust are very variable, indicating that intra-mantle processing may lead to the small number of enriched compositions evidenced in OIB.