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Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area |
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| Authors: | G bor Dobosi Hilary Downes David Mattey and Antal Embey-Isztin |
| Institution: | a Hungarian Academy of Sciences, Laboratory for Geochemical Research, Budaorsi út 45, H-1112, Budapest, Hungary b Department of Geology, Birkbeck College, University of London, Malet St., London WC1E 7HX, UK c Department of Geology, Royal Holloway University of London, Egham Hill, Egham, Surrey TW20 OEX, UK d Department of Mineralogy and Petrology, Hungarian Natural History Museum, Ludovika tér 2, H-1083, Budapest, Hungary |
| Abstract: | Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ18O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ18O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ18O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the 18O/16O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered. |
| Keywords: | Oxygen isotopes Alkali basalts Phenocrysts Olivine Upper mantle Laser methods Pannonian basin |
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