The effect of deglaciation on mantle melting beneath Iceland

The spreading ridge on Iceland shows large variations in eruption rate over the last 10,000 years. An increase of about 30 times the steady state value, between 10,000 and 8000 years ago, coincides with the disappearance of ice at the end of the last ice age. We examine the possibility that deglaciation caused this increase by modeling the effect on melt generation of the removal of an axisymmetric ice sheet from a spreading ridge. Our calculations take into account the influence of both a nonhydrostatic stress field and viscous flow. The results show that the average melting rate is increased by about 30 times its steady state value when a 2-km-thick ice sheet melts in 1000 years. The effect of the glacial cycle of loading and unloading is to cause a nonlinear modulation in the melt production. The factor of 30 increase in melt generation during the 1-kyr unloading can occur only because melt generation is reduced for about 60 kyr after the ice load is applied. The total volume of magma that can be produced from the deglaciation of Iceland is about 3100 km³. The concentrations of the light rare earth elements in the melt produced by deglaciation near the center of the ice sheet are about 15% less than those in melts produced by steady state melting. Transport time for the melt to reach the surface is not yet well constrained but is likely to be less than ∼1 kyr.