Effects of Glaciation and Deglaciation on Mantle Melting beneath Iceland.

Previous observations and numerical models show that large temporal variations in lava compositions and eruption rates in Iceland over the last 12,000 years are associated with the last deglaciation. Deglaciation decreases the surface load and the pressure of the mantle underneath, which in turn increases the decompression melting rate in the mantle. Conversely, the increase in surface load due to glaciation decreases the decompression melting rate. While previous numerical works only consider glacial unloading, our numerical model investigates the effects of both loading and unloading by glaciers on compression and decompression in the mantle. This has important implications on the mantle melting rates and the melt compositions. In our study with a simplified axisymmetric ice sheet, we find that loading of 2-km thick ice sheet with radius of 180 km in 1,000 years inhibits melting completely for 5,000 years until the solid mantle begins to return to its solidus pressure by upwelling from the spreading ridge. The melting rate then increases gradually and recovers back fully to the steady state value at around 35,000 years after the glacial loading. The result shows that melting recommences first at the deepest section of the melting region, which is the least depleted in incompatible elements compared to other sections of the melting region. This could explain why geological observations in Iceland found some subglacial eruptions more enriched in incompatible elements than late post-glacial steady state eruptions. Furthermore, our numerical model shows the same behavior as the geological observations that the effect of deglaciation on the compositional variations in the melt is longer lived than that on the eruption rates. In our calculations, the temporal variation of incompatible element concentrations lasts for approximately twice as long as the temporal variation of the eruption rates during and after deglaciation.