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

Magmatism on Iceland is dominated by the eruption of tholeiitic magmas along two NE-SW trending rift zones that represent the loci of new crust formation. However, small volumes of Quaternary transitional to alkaline magmas are also found along the ~120 km long Snaefellsnes Peninsula, located >150 km west of the western rift zone. Key questions to answer include how and why melt generation occurs in this off-axis setting and whether melt transport and storage in the crust is influenced by the limited extension compared to the main rift zones. Basaltic glass from post-glacial scoria cones and sub-glacial hyaloclastite deposits have been collected from 70 localities throughout the peninsula and will be analyzed by electron microprobe and LA-ICP-MS. Initial results show that the glasses have 4.7–9.0 wt% MgO and 0.9–4.8 wt% TiO₂. Preliminary estimates of crystallization pressures, based on the pressure dependence of the ol-cpx-plag cotectic, average 4.6 ± 1.9 kbar, equivalent to a depth of 16 ± 6 km. Crustal thickness beneath Snaefellsnes is ~25 km, and so these depth estimates indicate magma storage zones in the lower to middle crust. LA-ICP-MS data on post-glacial basalt glasses together with literature data on post-glacial lavas reveal broad spatial trends in REE compositions across the peninsula. Samples west of 22.33˚W have higher Dy/Yb indicative of deeper melting due to residual garnet in the mantle source. Samples east of 22.33˚W form two ESE-WNW alignments, and the northern group has higher La/Sm than the southern group, suggesting a lower degree of melting or a more enriched source. These data will be used to evaluate whether melting is driven by mantle upwelling related to topography on the base of the lithosphere related to the abandoned Hunafloi-Snaefellsnes rift. Completion of the glass analyses will provide a more complete picture of magma storage and transport and spatial variations within the Snaefellsnes Peninsula.