Oxygen isotope variations in ocean island basalt phenocrysts

Abstract

Oxygen isotope ratios are reported for olivine phenocrysts from sixty-seven samples of ocean island basalts (OlBs), mid-ocean ridge basalts (MORBs), and related peridotites, including representatives of the various isotopic endmembers defined by radiogenic isotope ratios. OIBs are more homogeneous in δ^(18)O by this analysis than suggested by previous studies of whole rocks and glasses. Most 0I13 samples have oxygen isotope ratios within a restricted range (δ^(18)O_(olivine) = 5.0–5.4‰), 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 ^(18)O (δ^(18)_(olivine) = 5.4–6.1‰), and a small number of samples characterized by low ^3He/^4He and distinctive lead isotope ratios, which are ^(18)O depleted (δ^(18)O = 4.7-5.1‰). The observed range in δ^(18)O and the correlations with radiogenic isotope ratios are similar to those observed in a detailed study of Hawaiian samples (Eiler 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 (⪅ l%) in the mantle sources of most OIBs. The results on most EM2 lavas are consistent with the presence of ∼2–6% recycled sediment in their source regions. Low δ^(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 δ^(18)O are low ^3He/^4He and anomalous ^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb relationships, and most of the low-δ^(18)O samples can be explained by contamination by the underlying oceanic crust or volcanic edifice. High-^3He/^4He lavas are indistinguishable from MORBs and most other OIBs in terms of δ^(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.

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