Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.
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