The 1875 Eruption of Askja Volcano, Iceland: Combined Fractional Crystallization and Selective Contamination in the Generation of Rhyolitic Magma

R. Macdonald, R. S. J. Sparks, H. Sigurdsson, D. P. Mattey, D. W. McGarvie and R. L. Smith
Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ, U.K.
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, U.K.
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A.
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, U.K.
U.S. Geological Survey, 2943C Fulton Avenue, Sacramento, CA 95821, U.S.A.

Abstract: Major and trace element and Sr, Nd and O isotopic data are presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja. Trace element modelling and Sr and Nd data largely confirm previous major element calculations that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb (and Th and U?), depletion in Cs, and low values of δ18O/16O, in the rhyolites are not explained by this mechanism alone. The silicic magmas were selectively contaminated by diffusion from partially molten granitic wall rocks, now found as xenoliths in the eruptive products, the process being particularly marked by lower δ18O and Cs/Rb ratios in the rhyolites than in the associated basalts. This is the first record of a combined fractional crystallization-selective contamination process in an Icelandic silicic complex.

Keywords: volcanoes • rhyolitic magma • isotope data • Askja volcano • Iceland

Mineralogical Magazine; June 1987 v. 51; no. 360; p. 183-202; DOI: 10.1180/minmag.1987.051.360.01
© 1987, The Mineralogical Society
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