Short magma chamber residence time at an Icelandic volcano inferred from U-series disequilibria

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 crystallization, and from variations in crystal sizes and magma compositions. Short-lived disequilibria between daughter nuclides of the ²³⁸U decay series yield information about the processes and timescales of magma differentiation. 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.