GEOBAROMETRY OF OFF-AXIS BASALTIC GLASSES FROM THE SNAEFELLSNES PENINSULA, ICELAND

On Iceland, small volumes of Quaternary transitional to alkaline magmas have erupted along the ~120 km long Snaefellsnes Peninsula. This magmatic activity is located >150 km west of the main NE-SW trending rift zones where > 95% of Icelandic magmatism occurs, dominated by the eruption of tholeiitic lavas. It is not clear how and why melt generation occurs in this off-axis setting. Furthermore, the limited crustal extension in the Snaefellsnes region might lead to differences in how melts are transported and stored in the crust compared to the rift zones. To address such questions, basaltic glasses (4.5–9.0 wt% MgO) were collected from 75 eruptive centers (sub-glacial hyaloclastites; post-glacial scoria cones), and analyzed by electron microprobe and LA-ICP-MS. Crystallization depths were estimated from glass compositions based on the pressure dependence of the ol-cpx-plag cotectic. The ENE-WSW trending Ljosufjoll volcanic system can be divided into a narrow western zone with more voluminous eruptions located around the Ljosufjoll central volcano and a broader eastern zone with scattered monogenetic centers. The western zone exhibits crystallization depths of 10-20 km, similar those in the Lysuskard and Snaefellsjokull volcanic systems to the west (12-22 km), whereas the more scattered eastern zone has a wider range of depths (7-32 km). Crustal thickness in Snaefellsnes is ~25 km so the depth estimates suggest storage zones in the middle to lower crust along most of the peninsula, perhaps extending into the uppermost mantle in a few places. LA-ICP-MS data on glass in conjunction with published data on post-glacial lavas indicate broad spatial patterns in incompatible element ratios (e.g. La/Sm and K/Ti) indicating systematic variations in melt generation and source composition. West of 22.33˚W, samples exhibit higher Dy/Yb indicative of deeper melting depth caused by residual garnet in the mantle source.