Oxygen isotope evidence for the origin of chemical variations in lavas from Theistareykir volcano in Iceland’s northern volcanic zone

Abstract

Oxygen isotope ratios in phenocrysts from recent Theistareykir lavas (Iceland) are consistently 18O-depleted relative to common terrestrial basalts (e.g. δ18O olivine=4.7–4.1‰) and correlate with geochemical indices of ‘enrichment’ (e.g. K2O/TiO2; La/Sm) and major element indices of differentiation (e.g. Mg#; CaO/Na2O). The sense of these correlations is that decreasing δ18O is accompanied by increasing ‘enriched’ geochemical signatures and an increasing extent of differentiation. These trends are similar to (although more subtle than) those defined by highly differentiated and contaminated Icelandic andesites, dacites and rhyolites. The trends we observe are consistent with models in which primary recent Theistareykir magmas are highly ‘depleted’ in their incompatible element geochemistry and similar in δ18O to common terrestrial basalts; differentiation of these magmas is accompanied by contamination by the low δ18O, and on average more ‘enriched’ rocks of the Icelandic crust to produce the observed spectrum in δ18O and other geochemical indices. Our results suggest that geochemical variations among recent Theistareykir lavas are only indirect constraints on the composition and dynamics of the Iceland plume. Extrapolation of the geochemical trends we observe to oxygen isotope compositions within the range of common oceanic basalts suggests that primary recent Theistareykir magmas are exceptionally depleted (e.g. La/Sm=0.2–0.5), indicating unusually high degrees and/or multiple stages of melting of their sources.