Petrogenesis of Braedvikir Tindar, Western Volcanic Zone, Iceland

Braedvikir is a hyaloclastite dominated tindar in the Prestahnukur fissure swarm in the Western Volcanic Zone of Iceland. Major & trace element chemistry shows that Braedvikir is geochemically different from the region’s glaciovolcanic edifices, characterized by its highly depleted levels of FeO* and MgO and enrichment in SiO2. Samples show limited variation in incompatible elements, suggesting little differentiation in melting conditions. Major elements establish a variation in chemistry between lithostratigraphic units on the ridge. Units are genetically related by a 3-kb fractional crystallization model (Rhyolite-MELTS), which generates two pyroxenes and a range of plagioclase compositions. Plagioclase compositions could be indicative of a variety of crustal processes, but the data used in this study cannot conclusively point to any given process. A two-stage eruptive model was formulated using major element distinctions and cross-cutting relationships on the ridge. This model separates the initial explosive event which formed the bulk of the ridge (Unit LT) from a secondary event consisting of an explosive phase (TB1-2) followed by the intrusion of pipes (Lb1-8), a dike set (Ld1-2), and pillows (Lp1). This secondary event is hypothesized to be caused by a recharge event in the magma lens. This supply of more primitive magma could explain the difference in MgO levels between unit LT and all other units