INCREASED MANTLE MELTING BENEATH SNAEFELLSJOKULL VOLCANO DURING LATE PLEISTOCENE DEGLACIATION

BASALTIC magmatism results when upwelling mantle crosses the peridotite solidus. The greater the overstep of the solidus, the greater the degree of mantle melting and the larger the volume of magma produced. In Iceland most of the active volcanism occurs along the central spreading axis, although some occurs in isolated off-axis volcanoes such as Snaefellsjokull. Because the mantle beneath Snaefellsjokull is not upwelling as vigorously as that beneath the spreading axis, it oversteps its solidus by a much smaller amount and consequently the degree of melting is less. The composition of the resulting magma will be much more sensitive to small perturbations in the amount of upwelling than will the mantle beneath the ridge axis. We show here that the reduction of pressure in the mantle caused by the unloading of ice was sufficient to affect magma composition. Unloading was accompanied by a clear shift towards less undersaturated magma compositions, which reflect a transient increase of about 0.5% in the degree of mantle melting, coupled with a decrease in depth of melting.