Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland

• Published in: Journal of Petrology
• Main Authors: NICHOLSON, HUGH, CONDOMINES, MICHEL, FITTON, J. GODFREY, FALLICK, ANTHONY E., GRÖNVOLD, KARL, ROGERS, GRAEME
• Format: Text
• Language: English
• Published: Oxford University Press 1991
• Subjects: Articles; Krafla; Iceland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/32/5/1005; https://doi.org/10.1093/petrology/32.5.1005

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, K 2 O 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.