The 1875 eruption of the Askja volcano, Iceland: Combined fractional crystallization and selective contamination in the generation of rhyolitic magma

Major 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 major element calculations indicating that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb, depletion in Cs and low values of delta ¹⁸O/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 marked by lower delta ¹⁸O and Cs/Rb ratios in the rhyolites than in associated basalts. -R.A.H.