Pressures of Crystallization of Icelandic Magmas

Abstract

Iceland lies astride the Mid-Atlantic Ridge and was created by seafloor spreading that began about 55 Ma. The crust is anomalously thick (� 20^40 km), indicating higher melt productivity in the underlying mantle compared with normal ridge segments as a result of the presence of a mantle plume or upwelling centered beneath the northwestern edge of the Vatnajo« kull ice sheet. Seismic and volcanic activity is concentrated in � 50 km wide neovolcanic or rift zones, which mark the subaerial Mid-Atlantic Ridge, and in three flank zones. Geodetic and geophysical studies provide evidence for magma chambers located over a range of depths (1� 5^21km) in the crust, with shallow magma chambers beneath some volcanic centers (Katla, Grimsvo« tn, Eyjafjallajo« kull), and both shallow and deep chambers beneath others (e.g. Krafla and Askja). We have compiled analyses of basalt glass with geochemical characteristics indicating crystallization of ol^plag^cpx from 28 volcanic centers in the Western, Northern and Eastern rift zones as well as from the Southern Flank Zone. Pressures of crystallization were calculated for these glasses, and confirm that Icelandic magmas crystallize over a wide range of pressures (0� 001 to � 1 GPa), equivalent to depths of 0^35 km. This range partly reflects crystallization of melts en route to the surface, probably in dikes and conduits, after they leave intracrustal chambers. We find no evidence for a shallow chamber beneath Katla, which probably indicates that the shallow chamber identified in other studies contains silica-rich magma rather than basalt. There is reasonably good correlation between the depths of deep chambers (417 km) and geophysical estimates of Moho depth, indicating that magma ponds at the crust^mantle boundary. Shallow chambers (57� 1km) are located in the upper crust, and probably form at a level of neutral buoyancy. There are also discrete chambers at intermediate depths (� 11km beneath the rift zones), and there is strong evidence for cooling and crystallizing magma bodies or pockets throughout the middle and lower crust that might resemble a crystal mush. The results suggest that the middle and lower crust is relatively hot and porous. It is suggested that crustal accretion occurs over a range of depths similar to those in recent models for accretionary processes at mid-ocean ridges. The presence of multiple stacked chambers and hot, porous crust suggests that magma evolution is complex and involves polybaric crystallization, magma mixing, and assimilation.