Noble gas isotope ratios of olivine phenocrysts and mantle-derived xenoliths in Patagonian plateau lavas from southernmost South America

The Pliocene to Quaternary Patagonian plateau lavas of southernmost South America are considered to be generated by slab-window processes associated with the collision of a Chile Ridge segment with the Chile Trench (e.g., Gorring et al., 1997). In the slab-window model, OIB-like asthenospheric upwelling plays an important role for the magma genesis; however, the origin of the upwelling asthenosphere remains still unknown. Since noble gas is a powerful tracer of a mantle plume rising from deep within the Earth, we measured noble gas isotopic composition of olivine phenocrysts separated from Pliocene to Recent alkali basalts and mantle-derived xenoliths from Patagonia, South America. 3He/4He ratios obtained by in vacuo crushing ranged from 6 to 7 RA and clearly indicate the absence of deep mantle plume accompanying higher 3He/4He ratio than the MORB-value (8±1 RA). The upwelling asthenosphere would therefore be derived from relatively shallow mantle, when the slab-window model is adopted as of present. The low 3He/4He and 4He/40Ar* ratios of the mantle-derived xenoliths suggest the subcontinental lithospheric mantle beneath this region had been affected by metasomatism causing selective removal of He relative to other noble gases and incompatible elements. Systematically low 40Ar/36Ar ratios of the alkali basalts and mantle xenoliths compared with MORB-source mantle imply that slab-derived atmospheric Ar was incorporated into the mantle wedge beneath South America.