Geochemical and Isotopic Evidence for Crustal Assimilation Beneath Krafla, Iceland |
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Authors: | NICHOLSON HUGH; CONDOMINES MICHEL; FITTON J GODFREY; FALLICK ANTHONY E; GR?NVOLD KARL; ROGERS GRAEME |
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Institution: | 1Department of Geology and Geophysics, University of Edinburgh West Mains Road, Edinburgh EH9 3JW, UK
2Scottish Universities Research and Reactor Centre East Kilbride, Glasgow G75 OQU, UK
3Universit? Blaise Pascal and C.N.R.S., U.R.A. 10, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France
4Nordic Volcanological Institute 101 Reykjavik, Iceland |
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Abstract: | The Krafla volcanic system consists of a central volcano andassociated 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 rangein composition from olivine tholeiite to rhyolite. Major-elementcompositions suggest that fractional crystallization exertsthe 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-systemfractional crystallization. Fractionation coupled with periodicreplenishment and tapping of the reservoir is unlikely to beresponsible for this enrichment, despite the geophysical evidencesuggesting a large number of inflations and deflations of ashallow magma reservoir (Tryggvason, 1986). Th- and O-isotope results confirm the work of previous authorsthat crustal assimilation is operating on a local scale beneathKrafla. A model is suggested, fitting both the Th- and O-isotopicdata, which involves the partial melting and incorporation ofa hydrothermally altered wall-rock contaminant during fractionalcrystallization (i. e., AFC processes). This process of partialmelting is likely to enhance the most highly incompatible elementconcentrations (e. g., increasing Rb/Zr) more than expectedby closed-system fractional crystallization. |
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