Geochemical Variations in Off-axis Alkaline Glasses - Snaefellsnes Peninsula, Icelan

Magmatism on Iceland is dominated by the eruption of tholeiitic lavas along two NE-SW trending rift zones that represent the loci of new crustal formation. Small volumes of transitional to alkaline magmas erupted along the ~120 km long Snaefellsnes Peninsula in western Iceland. The Snaefellsnes Peninsula is broken into three distinct volcanic systems; Ljosufjoll, Lysuskard, and Snaefellsjokull. This study examines the geochemical compositions and depths of crystallization of alkaline basaltic glass to see if these features vary spatially along the peninsula. Glass MgO values range from 3.4 to 9.0 wt %, TiO2 ranges from 0.9 -- 4.8 wt. % increasing westward along the peninsula. Estimations of crystallization pressures, based on the pressure dependence of ol+/-plag+/-cpx, average 4.6 +/- 2.8 kbar, translating to depth a depth of 16.1 +/- 10 km generally increasing with distance from the main rift zones. In some cases, cox-melt barometry was also used as a check on the glass cotectic pressures. Crustal thickness beneath Snaefellsnes has been estimated to be ~25 km, suggesting mid to lower crustal magma storage depths. Calculated depths in Snaefellsjokull and Lysuskard are consistent and deep (22 +/- 2.4 km) whereas the Ljosufjoll system is more varied. The Ljosufjoll system displays two groupings of depths, split lengthwise. The northern group exhibits depth between 15-26 km, like the rest of the peninsula, while the southern group has no discernable pattern. Previous data on post-glacial lavas reveals broad spatial trends in REEs across the peninsula. Basaltic glasses exhibit a wide range of La/Sm (1 -- 5.6) suggesting a variation in melting percentage over a small area. Samples west of 22.33°W have higher Dy/Yb (>2.1) indicating a deeper melting depth due to residual garnet as well as a higher La/Sm (>10) indicative of a smaller degree of melting and/or an enriched source. These data have shown that melt storage occurs at near Moho depths along the peninsula but is highly variable in the eastern Ljosufjoll.