Oxygen isotopic signature of mantle xenoliths from the Patagonia region, Argentina

In order to provide petrological information about the continental lithospheric mantle in the Patagonian region, a series of oxygen isotopic analyses were performed in mantle xenoliths. Hosted by Miocene-Pleistocene alkaline basalts erupted in the Andean back-arc setting, the studied mantle xenoliths are represented by anhydrous spinel-bearing lherzolites to harzburgites, and subordinately websterites and olivine websterites. Using a conventional vacuum extraction line employing BrF₅, oxygen isotopic analyses were conducted in whole-rock, olivine, clinopyroxene and orthopyroxene at the Department of Geological Sciences, Indiana University, Bloomington - USA. Isotopic ratios were measured on a Finnigan MAT 252 stable isotope mass spectrometer. Values of NBS-28 quartz are δ ¹⁸O = +9.6 +/- 0.2 ‰ . Errors obtained were better than +/- 0.1 ‰ (2SD). Whole-rock data suggests a wide range of δ ¹⁸O from +5.5 to +10.1 ‰ , whereas mantle rocks, in general, presented values ranging around 5.5 +/- 0.2 ‰ . Olivines showed two distinct isotopic ranges, with values of +5.0 to +5.9 ‰ , similar to values for upper mantle olivines, but also low δ ¹⁸O values, around 3.2 ‰ . Pyroxenes demonstrated a wide range of values, between +4.5 and +6.3 ‰ similar to upper mantle pyroxenes. The discrepancy of some whole-rock and olivine values in relation to mantle compositions could be interpreted as an inter-mineral disequilibrium, mineral zonation reflecting or not, metasomatic interaction, or even crust participation during subduction