PRESSURES OF CRYSTALLIZATION AND DEPTHS OF MAGMA CHAMBERS IN ICELAND RIFT ZONES

Iceland straddles the mid-Atlantic Ridge and was created by seafloor spreading that began about 55 Ma. The crust is anomalously thick reflecting location above abnormally productive mantle. The subaerial mid-ocean ridge forms ~ 50 km wide neovolcanic or rift zones characterized by abundant seismic and volcanic activity. Geodetic and geophysical studies provide evidence for magma chambers located over a range of depths (1.5-21 km) in the crust. These studies have located shallow magma chambers beneath some volcanic centers (Katla, Grimsvotn, Eyjafjallajokull), and both shallow and deep chambers beneath other centers (eg. Krafla and Askja). We have calculated the pressures at which Icelandic melt compositions are in equilibrium with ol-plag-cpx. Input data consisted of 507 glass analyses 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. We used the pressures to estimate depths of crystallization of magmas beneath these centers. The results 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, after they leave intracrustal chambers. The results for some centers indicate crystallization over a narrow range of pressure, and allow us to estimate the depths of magma chambers. We find evidence for shallow (3-9 km) chambers (3-9 km) beneath Askja, Blafjall, Hengill, Bardabunga, and Grimsvotn, and evidence for deep chambers beneath Askja, Blafjall, Herdubreid, Hlodufell, Geitafell, Seljahalli, Kalfstindar, Raudafell, Halar, and Efstadalsfjall. These data, along with evidence for lateral transport of magma along fissures and mixing of magmas from different centers suggest extremely complex magma dynamics. The deep chambers beneath volcanic centers in the Western and Northern rift zones are located on average at 19.6 and 20.5 km respectively. These depths correlated well with estimates of crustal thickness from seismic and gravity studies, indicating chambers located at the crust-mantle boundary. The results of this study have important implications for interpreting crustal accretionary processes in Iceland.