Oxygen isotope variations in ocean island basalt phenocrysts

Oxygen isotope ratios are reported for olivine phenocrysts from sixty-seven samples of ocean island basalts (OIBs), mid-ocean ridge basalts (MORBs), and related peridotites, including representatives of the various isotopic endmembers defined by radiogenic isotope ratios. OLBs are more homogeneous in delta(18)O by this analysis than suggested by previous studies of whole rocks and glasses. Most OIB samples have oxygen isotope ratios within a restricted range (delta(18)O(olivine) = 5.0-5.4 parts per thousand), comparable to those of olivines in peridotites from ophiolites, in most peridotitic mantle xenoliths, and inferred for the sources of mid-ocean ridge basalts. The exceptions are EM2 lavas, which are enriched in O-18 (delta(18)O(olivine) = 5.4-6.1 parts per thousand), and a small number of samples characterized by low He-3/He-4 and distinctive lead isotope ratios, which are O-18 depleted (delta(18)O = 4.7-5.1 parts per thousand). The observed range in delta(18)O and the correlations with radiogenic isotope ratios are similar to those observed in a detailed study of Hawaiian samples (filer et al., 1996b). These results indicate that recycled crust and/or sediments (or the imprint of extensive metasomatism by fluids derived from such materials) is present as at most a small mass fraction (less than or similar to 1%) in the mantle sources of most OIBs. The results on most EM2 lavas are consistent with the presence of similar to 2-6% recycled sediment in their source regions. Low delta(18)O Values in OIBs can be produced by assimilation of altered lavas from high-levels in the volcanic edifice, assimilation of the oceanic crust underlying the volcano, or incorporation of subducted oceanic crust in mantle sources. The only consistent correlatives with low delta(18)O are low He-3/He-4 and anomalous (207)pb/Pb-204- Pb-206/Pb-204 relationships, and most of the low-delta(18)O samples can be explained by contamination by the underlying oceanic crust or volcanic edifice. High-He-3/He-4 lavas are indistinguishable from MORBs and most other OIBs in terms of delta(18)O, suggesting that to the extent that the lower mantle is sampled by hotspot volcanism, there is no significant vertical stratification in oxygen isotope ratios in the mantle. Copyright (C) 1997 Elsevier Science Ltd