Petrogenesis of subglacial pillow lavas from the flanks of the Bárðarbunga volcano in the Vonarskarð valley

The Bárðarbunga volcano represents one of the largest and most productive volcanic systems in Iceland. It is located in central Iceland and it is thought to be situated directly above the Iceland mantle plume. Several studies have been conducted on early postglacial and Holocene lavas of Bárðarbunga Volcanic System (BVS), however, due to the hardly accessible location of the partly ice-covered Bárðarbunga volcanic center, studies of lavas in the nearest vicinity of the volcano are scarce. This thesis presents a detailed geochemical study of subglacial pillow lavas from ice-free areas in the western and northwestern flanks of the volcano. Major element composition of glassy pillow rims indicates their separation into two different groups of basaltic magma – more primitive and more evolved group, both of which fall close to or on a liquid line of descent defined by units from the BVS. The more evolved group also reveals slight enrichment in the incompatible trace elements when compared to the more primitive group. Results of thermobarometric calculations and indications from the spatial distribution of the pillow lavas, suggest that the more evolved group was stored in a relatively shallow reservoir (4.7 ± 1.8km), located more or less directly beneath the Bárðarbunga volcano. Eruptions of the lavas from the more primitive group most likely were fed from a slightly deeper (8.4 ± 2.4 km) and larger reservoir, which is in agreement with studies on other units from the BVS. Paleo-ice thickness estimates based on dissolved volatile concentrations in pillow rim glasses, suggest that the studied pillow lavas erupted under a relatively thin and uniform ice sheet of about 400-600 m. Thus, different levels of enrichment and degree of melt evolution are most likely not explained by the changes of ice sheet loading, but arise from the heterogeneities of the mantle underlying the Bárðarbunga volcano, as well as from the structure of its plumbing system