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

Abstract

A striking feature of Icelandic volcanism is the effect that the last ice age had on volcanic activity. After the final retreat of ice ∼11 kyr BP, the average eruption rate is estimated to have been 20–30 times greater than it is today. This increase has been attributed to the release of pooled magma through differential tectonic movements during the unloading of ice. However recent work has shown that deglaciation can account for the increase in mantle melting by decreasing the pressure in the upper mantle. We present geochemical data and volume estimates of erupted magmas from Iceland'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 interglacial lavas. The difference in light rare earth element concentrations cannot be accounted for by liquid–crystal fractionation. Averaging the compositions of glacial and postglacial magmas also eliminates the likelihood that the compositional change is due to variations in source composition. An increase in mantle melting from deglaciation can account for both the magma eruption rate and observed changes in trace element concentrations. Finite transport times for magma to travel from the source region to the surface can be estimated from the delay in timing of the increased eruption rates and the end of the last glacial period. This gives transport times of about 1–3 kyr and is consistent with estimates from ( 226Ra/ 230Th) activity ratios measured in ocean island and mid-ocean ridge basalts.