| 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 due to the presence of a mantle plume or upwelling centered beneath the north western edge of the Vatnajokull ice sheet. Seismic and volcanic activity is concentrated in ∼ 50 km wide neovolcanic or rift zones, that 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 (1.5-21 km) in the crust, with shallow magma chambers beneath some volcanic centers (Katla, Grimsvotn, Eyjafjallajokull), and both shallow and deep chambers beneath others (e.g., Krafla and Askja). I have compiled analyses of basalt glass with geochemical characteristics indicating crystallization of ol-plag-cpx from 29 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 using a method based on phase equilibrium. Comparison with experimental data indicates that calculated pressures are accurate to +/-110 MPa (1sigma) and are precise to better than 80 MPa (1sigma). The results confirm that Icelandic magmas crystallize over a wide range of pressures (1 to ∼1000 MPa), 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. There is reasonably good correlation between the depths of deep chambers (>17 km) and geophysical estimates of crustal thickness suggesting that magma ponds at the crust-mantle boundary. Shallow chambers are located in the upper crust (taken here as <7 km), and probably form at a level of neutral buoyancy. There are also discrete chambers at intermediate depths (∼11 km beneath the rift zones), and there is good evidence for cooling and crystallizing magma bodies or pockets throughout the middle (7-15 km) and lower crust (>15 km). It has been shown that glasses in magmas erupted at the Kverkfjoll volcanic system in the Northern Volcanic Zone (NVZ) have compositions that are consistent with partial crystallization at average pressures of 445+/-69 MPa and 794+/-92 MPa, corresponding to depths of 15.6+/-2.4 km and 27.9+/-3.2 km, and I conclude that magma chambers are located at these depths. These results are consistent with interpretation of recent seismic activity beneath Upptyppingar in the Kverkfjoll volcanic system ∼50 km north of the Kverkfjoll central volcano. The earthquake hypocenters are concentrated at depths of 15-18 km with a few occurring at greater depths (∼25km), and the seismic activity appears to reflect inflow of magma into the base of the crust (Roberts et al., 2007). Custal thickness, temperatures at the base of the crust, and composition of the crust were used to construct geothermal gradients and profiles of density and seismic velocity through the crust to predict these properties in the lowermost crust. Models of mineralogy change and compositional change were considered. The density at the base of the crust is 3120-3134 kg/m3 giving a crust-mantle density difference of 166-188 kg/m3 assuming a mantle density of 3300 kg/m3. The predicted seismic velocity at the base of the crust is 6.8 km/s. The middle and lower crust in the rift zones is relatively hot and porous. It is suggested that crustal accretion occurs over a range of depths as proposed in recent models for crustal accretion 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. |
