ABSTRACT

Oxygen isotope ratios were determined using laser fluorination methods on 15 olivines and seven matrix samples separated from the Pali Aike alkali basalt of southern Patagonia. The δ¹⁸O values were measured in ranges of 4.23 to 5.23‰ and 4.64 to 5.26‰ for olivine and matrix, respectively. The δ¹⁸O values of Pali Aike olivines are lower and have a wider range than those of normal upper mantle (5.0-5.4‰). We considered that the heterogeneity of the oxygen isotope compositions of Pali Aike olivine provides evidence of assimilation at a shallow level and the presence of recycled material. The differences between olivine and matrix δ¹⁸O values indicate that Δ¹⁸O_(melt-ol) (0.14 to 0.50) was within equilibrium fractionation for some and in disequilibrium for most samples at magmatic temperature. Oxygen isotope compositions in disequilibrium indicate that the lower δ¹⁸O for the melt is explained by interaction with low δ¹⁸O hydrothermally altered rocks. The low δ¹⁸O_olivine values for low forsterite (Fo) contents of Pali Aike lavas can be explained by the effect of shallow-level assimilation. The oxygen isotope values for the olivine of high-MgO (or high-Fo) compositions are thought to represent oxygen isotope variation in the mantle source region. The low and variable δ¹⁸O signature of Pali Aike olivine can be explained by crustal recycling, such as a contribution from the hydrothermally altered oceanic crustal melt. These heterogeneous oxygen isotope compositions indicate that Pali Aike olivines require both assimilation at a shallow level and a component of the high-μ end-members (HIMU) mantle derived from recycled mantle lithologies in the form of pyroxenite or eclogite.