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Rheology of the Upper Mantle beneath Southernmost South America Inferred from Peridotite Xenoliths

Recrystallized-grain-size piezometry has been used to estimate the differential stress levels for the mantle source area of a suite of xenoliths collected from the Pali-Aike volcanic field in southernmost South America. The present geothermal gradient below this region, as calculated from the mineral chemistry of the xenoliths, is similar to that expected in suboceanic mantle. Grain boundary migration is the dominant recrystallization mechanism in the deeper parts of the mantle profile represented by the xenoliths. In a xenolith derived from the shallowest level of the mantle (54 km depth), the evidence is ambiguous whether grain boundary migration or subgrain rotation was the dominant recrystallization mechanism. The latter would imply a mantle rheological profile similar to continental rift zones, which is appropriate for the neotectonic setting of the Pali-Aike volcanic field. The former is also consistent with continental extension given the high geothermal gradient in the region. The strain-rate and viscosity profiles show increasing strain-rate and decreasing viscosity with depth.