Oxygen isotopic heterogeneity of Pali Aike basaltic magmas from southern Patagonia as evidenced by oxygen isotope compositions of olivines

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.