| Summary: | Krafla is an active central volcano in the, NE axial rift zone of Iceland associated with a fissure swarm which trends NNE-SSW. Lava/hyaloclastitc samples were collected from the volcanic system, covering the last 4 interglacial and 3 glacial periods. A stratigraphic framework for the volcanic system has been established by use of the Hekla tephra layers and the distinctive volcanic products of glacial and interglacial periods. Recent volcanic activity has shown that there is a shallow magma reservoir beneath the central volcano, which supplies magma laterally to the fissure swarms forming dykes and to the surface directly for eruption. The Krafla rocks contain the following phenocryst assemblages ol + plag, ol + plag + cpx, plag + cpx ± ol, plag + cpx + opx + FeTi oxides, plag + FeTi oxides cpx ± fayalitic ol. These distinct assemblage suggest that the Krafla suite evolves predominantly by fractional crystanation. They are also similar to the assemblages found in other mid-ocean ridge basalt (MORB) suites.Most samples, excluding rhyolites,contain plagioclase xenocrysts,implying that there has been at least 2 stages of magma mixing prior to eruption The absence of plagioclase xenocrysts in the rhyolites suggests that they formed in isolation from the most primitive magmas with or without crustal assimilation. Fractional crystallisation using the observed phenocryst assemblage explains many aspects of the 4 major-element compositions of the suite. Least-squares modelling confirm this observation, although it also suggests that the rhyolites contain higher concentrations of K₂O than would. be predicted by closed-system fractional crystallisation. This discrepancy, may be, explained by open system fractionation and/or by crustal assimilation. Numerical modelling of phase equilibria suggests that fractional crystallisation occurs over a range of pressures from 1-3.5 kbar. As was suspected, magma mixing also'appears to be a significant process in the Krafla f system and may explain sorne of the scatter on ...
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