Deglaciation effects on mantle melting under Iceland: results from the northern volcanic zone

A striking feature of Icelandic volcanism is the effect that the last ice a ge had on volcanic activity. After the final retreat of ice similar to 11 k yr BP, the average eruption rate is estimated to have been 20-30 times grea ter than it is today. This increase has been attributed to the release of p ooled magma through differential tectonic movements during the unloading of ice. However recent work has shown that deglaciation can account for the i ncrease in mantle melting by decreasing the pressure in the upper mantle. W e present geochemical data and volume estimates of erupted magmas from Icel and's northern neovolcanic zone which show that the average composition of magmas erupted during the last glacial period in Iceland are significantly more enriched in incompatible trace elements than postglacial and interglac ial lavas. The difference in light rare earth element concentrations cannot be accounted for by liquid-crystal fractionation. Averaging the compositio ns of glacial and postglacial magmas also eliminates the likelihood that th e compositional change is due to variations in source composition. An incre ase in mantle melting from deglaciation can account for both the magma erup tion rate and observed changes in trace element concentrations. Finite tran sport times for magma to travel from the source region to the surface can b e estimated from the delay in timing of the increased eruption rates and th e end of the last glacial period. This gives transport times of about 1-3 k yr and is consistent with estimates from (Ra-226/Th-230) activity ratios me asured in ocean island and mid-ocean ridge basalts. (C) 1998 Elsevier Scien ce B.V, All rights reserved.