Oxygen isotope variations in ocean island basalt phenocrysts
- California Institute of Technology, Pasadena, CA (United States); and others
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. OIBs are more homogeneous in {delta}{sup 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}{sup 18}O{sub olivine} = 5.0-5.4{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 {sup 18}O ({delta}{sup 18}O{sub olivine} = 5.4-6.1{per_thousand}), and a small number of samples characterized by low {sup 3}He/{sup 4}He and distinctive lead isotope ratios, which are {sup 18}O depleted ({delta}{sup 18}O = 4.7-5.1{per_thousand}). The observed range in {delta}{sup 18}O and the correlations with radiogenic isotope ratios are similar to those observed in a detailed study of Hawaiian samples. 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 ({approx_lt}%) in the mantle sources of most OIBs. The results on most EM2 lavas are consistent with the presence of {approximately}2-6% recycled sediment in their source regions. Low {delta}{sup 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. 75 refs., 4 figs., 2 tabs.
- DOE Contract Number:
- FG03-85ER13445
- OSTI ID:
- 576342
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 11 Vol. 61; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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