Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland

The Krafla volcanic system consists of a central volcano and associated fissure swarm in the NE axial rift zone of Iceland. Lavas spanning the whole of Krafla's exposed volcanic history (estimated to be 0-> 300 ka) have been analysed and range in composition from olivine tholeiite to rhyolite. Major-element compositions suggest that fractional crystallization exerts the main control over the differentiation process. However, K2O and the very incompatible trace elements, Rb, Th, and U, are all enriched beyond the extent expected by closed-system fractional crystallization. Fractionation coupled with periodic replenishment and tapping of the reservoir is unlikely to be responsible for this enrichment, despite the geophysical evidence suggesting a large number of inflations and deflations of a shallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authors that crustal assimilation is operating on a local scale beneath Krafla. A model is suggested, fitting both the Th- and O-isotopic data, which involves the partial melting and incorporation of a hydrothermally altered wall-rock contaminant during fractional crystallization (i. e., AFC processes). This process of partial melting is likely to enhance the most highly incompatible element concentrations (e. g., increasing Rb/Zr) more than expected by closed-system fractional crystallization.