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

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 delta(18)O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20 parts per thousand and from 5.07 to 5.34 parts per thousand, respectively, with cpx-ol fractionations Delta(18)O(cpx-ol) ranging from +0.04 to +0.29 parts per thousand. These delta(18)O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the O-18/O-16 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. Lf 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. (C) 1998 Elsevier Science B.V.