Abstract
THE questions of how quickly magmas can differentiate and how long they reside in magma chambers are fundamental to our understanding of magma evolution and the behaviour of volcanoes. Timescales of a few years to a few hundred years have been inferred from physical modelling of crystallization1,2, and from variations in crystal sizes and magma compositions3–5. Shortlived disequilibria between daughter nuclides of the 238U decay series yield information about the processes and timescales of magma differentiation4,6–14. Here I report excesses of 226Ra over 230Th and 210Pb that decrease with differentiation, in lavas from the Vestmannaeyjar volcanic system (Iceland). The decreasing disequilibria can be explained by crystal fractionation of alkali basalt to form hawaiite and mugearite and a 10-year magma chamber residence time. Such rapid differentiation and so short a residence time in a deep reservoir probably result from the injection of a small volume of alkali basalt into a relatively cold crust.
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