Oxygen isotope variations in primitive tholeiites of Iceland: evidence from a SIMS study of glass inclusions, olivine phenocrysts and pillow rim glasses

Oxygen-isotope compositions are reported for glass inclusions, their host olivines (Fo87–90) and tholeiitic pillow rim glasses from the neovolcanic rift zone of Iceland. The inclusion compositions range from enriched ([La/Sm]n=1.4–4.8) to very depleted ([La/Sm]n=0.08–0.35). The whole rocks and matrix glasses display little variation ([La/Sm]n=0.41–0.51). Glass inclusions vary somewhat stronger in 18O/16O ratios (δ18O=4.0–6.2±0.6‰) than the matrix glasses (δ18O=4.6–5.6±0.6‰). The range of olivine phenocryst composition is even wider (δ18O=2.4–6.9±0.5‰), and in some cases the variations within individual crystals reach 1.0–2.5‰. Most glass inclusions are in O-isotopic equilibrium with the surrounding host olivine but not with the central parts of the host crystals; similarly, most olivines are out of equilibrium with their host matrix glasses. This implies that the glass inclusions and olivine phenocrysts can preserve O-isotopic compositions inherited from contrasting parental mantle-derived magmas, which might have mixed in a common volcanic system. The isotopic heterogeneity of individual olivine crystals allows one to estimate their residence times in the magma reservoir using the rate of 18O diffusion in olivine. The time required to achieve isotopic homogeneity over 90–520 μm ranges from 25 to 1030 yr. The δ18O values found in depleted glass inclusions and their host olivines are unlikely to have resulted from magma contamination by upper crustal rocks. We suggest two possibilities, implying either an O-isotope heterogeneity of the Icelandic mantle or a mixing of two end-members: a refractory component with ‘normal-mantle’ or somewhat elevated δ18O values, high Ca/Na and strong light rare earth element depletion mixed with a component having lower δ18O and Ca/Na but also depleted in incompatible elements. Based on δ18O–La/Sm and δ18O–δ11B relationships, the variations of δ18O between 4.6 and 6.2‰ in enriched and depleted glass inclusions are interpreted to reflect those of primary magmas and, consequently, of their source region.