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

Oxygen isotope ratios in phenocrysts from recent Theistareykir lavas (Icela nd) are consistently O-18-depleted relative to common terrestrial basalts ( e.g. delta O-18 olivine = 4.7-4.1 parts per thousand) and correlate with ge ochemical indices of 'enrichment' (e.g. K2O/TiO2; La/Sm) and major element indices of differentiation (e.g. Mg#; CaO/Na2O). The sense of these correla tions is that decreasing delta O-18 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 d ifferentiated and contaminated Icelandic andesites, dacites and rhyolites. The trends we observe are consistent with models in which primary recent Th eistareykir magmas are highly 'depleted' in their incompatible element geoc hemistry and similar in delta O-18 to common terrestrial basalts; different iation of these magmas is accompanied by contamination by the low delta O-1 8, and on average more 'enriched' rocks of the Icelandic crust to produce t he observed spectrum in delta O-18 and other geochemical indices. Our resul ts suggest that geochemical variations among recent Theistareykir lavas are only indirect constraints on the composition and dynamics of the Iceland p lume. Extrapolation of the geochemical trends we observe to oxygen isotope compositions within the range of common oceanic basalts suggests that prima ry 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. (C) 2000 Elsevier Science B.V. All rights reserved.