Olivine major and trace element compositions from 12 basalts from the southern Payenia volcanic
province in Argentina have been analyzed by electron microprobe and laser ablation inductively
coupled plasma mass spectrometry. The olivines have high Fe/Mn and low Ca/Fe and many fall at
the end of the global olivine array, indicating that they were formed from a pyroxene-rich source
distinct from typical mantle peridotite. The olivines with the highest Fe/Mn have higher Zn/Fe, Zn
and Co and lower Co/Fe than the olivines with lower Fe/Mn, also suggesting contributions from a
pyroxene-rich source. Together with whole-rock radiogenic isotopes and elemental concentrations,
the samples indicate mixing between two mantle sources: (1) a pyroxene-rich source with EM-1
ocean island basalt type trace element and isotope characteristics; (2) a peridotitic source with
more radiogenic Pb that was metasomatized by subduction-zone fluids and/or melts. The increasing
contributions from the pyroxene-rich source in the southern Payenia basalts are correlated
with an increasing Fe-enrichment, which caused the olivines to have lower forsterite contents at a
given Ni content. Al-in-olivine crystallization temperatures measured on olivine–spinel pairs are between
1155 and 1243C and indicate that the magmas formed at normal upper mantle (asthenospheric)
temperatures of 1350C. The pyroxene-rich material is interpreted to have been brought
up from the deeper parts of the upper mantle by vigorous asthenospheric upwelling caused by
break-off of the Nazca slab south of Payenia during the Pliocene and roll-back of the subducting
slab beneath Payenia. The pyroxene-rich mantle mixed with peridotitic metasomatized South
Atlantic mantle in the mantle wedge beneath Payenia.