Abstract. Continental crust is thought to be formed as a result of arc magmatism, but many of the lavas produced in these settings are basaltic, while those that are silicic are commonly evolved, with lower Mg •s than the continental crust. The bulk composition of continental crust can be produced by mixing of end-member basaltic and silicic compositions, via magma mixing or in mechanical, tectonic juxtaposition, but some process is required to remove the cumulates and residues formed during generation of the silicic, "granitic" end-member. We consider convective instability of dense mafic and ultramafic lower crust as a means to remove mafic residues of basalt differentiation in order to produce end-member compositions that can mix to form the bulk composition of the continental crust. Using a range of lower crustal and mantle bulk compositions, ranging from mafic and ultramafic cumulates to primary liquid compositions, we calculated the subsolidus phase assemblage and resulting density. The results show that densities of likely lower crustal lithologies can exceed those of the mantle (by -•50-250 kg m -3) , but the density contrast is a strong function of composition, temperature, and pressure. For a "cold" geotherm with a Moho temperature of 300 øC, relevant to cratonic settings, densities of all of the lower crustal compositions that we considered, except granulite, exceed the density of the underlying mantle at pressures as low as 0.8 GPa. For a "hot" geotherm with a Moho temperature in the range of 800-1000øC, the density of the lower crust is much more variable, with gabbroic and granulite compositions having lower densities than the mantle, while "arc gabbronorite" and ultramafic cumulate compositions having higher densities than the mantle at pressures similar to that for the cold geotherm. Instability times calculated for a two-dimensional Rayleigh-Taylor convective instability, where a dense lower crustal layer sinks into a lower-density mantle, show that high temperatures (>700øC, or >500øC with a background strain rate) are required for this process to occur on a timescale of 10 Myr with rheological parameters expected for the crust and mantle. The high temperature required for dense lower crustal mafic-ultramafic cumulates to sink into the mantle suggests that this process is restricted to arcs, volcanic rifted margins, and continental regions that are undergoing extension, are underlain by a mantle plume, or have had part of the conductive upper mantle removed.

No abstract

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-kin-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 km 3. 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.Though the observations suggest an association between deglaciation and melt production, it is not obvious that the removal of about 2 km of ice can generate the volumes of melt involved. It is, however, easy to show that such an association is plausible. Removal of 2 km of ice is equivalent to moving the whole melting column upward by about 0.6 km. Since the melt generation rate during isentropic decompression is about 0.3% km -•, ice removal will increase the melt fraction by about 0.2%, producing a layer of magma 200 m thick from a melting zone that is 100 km thick. The Reykjanes Peninsula has an area of about 40 x 20 km, giving a total melt volume of 160 km 3 on deglaciation. This simple calculation shows that there is no obvious physical problem in accounting for melt production by deglaciation.The total volume of magma caused by deglaciation cannot, however, exceed the normal output of magma from a spreading ridge over the time interval between glacial advance and retreat. This is because the onset of glaciation increases the pressure in the underlying mantle, reducing the extent of melting. When the pressure is released during deglaciation, the production rate of melt is increased. Over the period of glacial advance and retreat there can be no change/ in the mean melt production rate.Deglaciation of Iceland has previously been proposed by several authors as the cause of increased melt pro-21,815

[1] Temporal variation in the eruption rate and lava composition in the rift zones of Iceland is associated with deglaciation. Average eruption rates after the end of the last glacial period, $12 kyr BP, were up to 100 times higher than those from both the glacial period and recent times (<5 kyr BP). This peak in volcanic activity finished less than 2 kyr after the end of deglaciation. New geochemical data from $80 basalt and picrite samples from the Theistareykir and Krafla volcanic systems show that there is a temporal variation in both the major and trace element composition of the eruptions. Early postglacial eruptions show a greater range in MgO contents than eruptions from other times, and at a fixed MgO content, the concentration of incompatible elements in subglacial eruptions is higher than that in early postglacial eruptions. Recent eruptions from the Krafla system have similar compositions to subglacial eruptions. The high eruption rates and low rare earth element (REE) concentrations in the lava from early postglacial times can be accounted for by increased melt generation rates in the shallow mantle caused by unloading of an ice sheet. Magma chamber processes such as crystallization and assimilation can produce the temporal variation in REE contents if garnet is present. However, garnet is not observed as a phenocryst or xenocryst phase and is not required to match the variation in major element contents observed at Krafla and Theistareykir. If the increase in eruption rates reflects increased melt production rates in the mantle, then the relative timing of deglaciation and the burst in eruption rates can be used to estimate the rate of melt transport in the mantle. The observed duration of enhanced eruption rates after deglaciation can be reproduced if the vertical melt extraction velocity is >50 m yr À1 .