| Summary: | Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ 18 O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr–Nd–Pb isotope ratios that terminate at δ 18 O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ 18 O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ 18 O. The hyperbolic 143 Nd/ 144 Nd–δ 18 O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are 4.5× greater and source Nd contents 9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ 18 O and 1 43 Nd/ 144 Nd or 206 Pb/ 204 Pb, extending to δ 18 O of +4.3‰ at 143 Nd/ 144 Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ 18 O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ 18 O in eastern Iceland. It is very difficult to explain the low δ 18 O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ 18 O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ 18 O enriched sources by recycling into the mantle, and there is no evidence for ...
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