Oxidation state of Paleozoic subcontinental lithospheric mantle below the Pali Aike volcanic field in southernmost Patagonia

Mantle xenoliths in the Quaternary Pali Aike alkaline basalts of southernmost Patagonia include lherzolites and harzburgites with and without garnet. The values of fO2 for all xenoliths range from 0.33 logarithmic unit below the fayalite–magnetite–quartz buffer (FMQ − 0.33) to FMQ + 0.75, which overlap those for abyssal peridotites. The fO2 data, together with the bulk rock and mineral compositions, suggest that the subcontinental lithospheric mantle (SCLM) below Pali Aike formed through the accretion of oceanic lithosphere. The oceanic accretion likely occurred in mid to late Paleozoic time when the southern Patagonian terrane formed along the southwest margin of Gondwana. Relict spinel inclusions in garnet suggest that garnet-facies peridotites formed from spinel-facies peridotites most likely in response to pressure increase during the accretion. Comparable fO2 for garnet- and spinel-facies peridotites suggests that this spinel-garnet transformation was not accompanied by changes in fO2. Metasomatism by asthenosphere-derived melt through a slab window resulted in the formation of Ti-bearing minerals, and lowering of Mg and enrichment of Ti in bulk rocks and minerals. The replacement of olivine by orthopyroxene formed orthopyroxenites in extreme cases. No significant change in fO2 is associated with this metasomatism since similar oxidation state is observed between un-metasomatized (FMQ − 0.23 to + 0.48) and metasomatized (FMQ − 0.33 to + 0.75) samples. This is explained by the fO2 of the asthenospheric melt (~ FMQ − 0.50) similar to the SCLM prior to the metasomatism.