Noble gas isotopes of mafic phenocrysts and ultramafic xenoliths in alkali basalts of the Patagonian plateau lavas of southermoust South America

SUMINO H; ORIHASHI Y; MOTOKI A; HIRATA D; HALLER MJ; NAGAO K

Pucón

Congreso; International Association of Volcanology and Chemistry of the Earth's Interior, General Assembly; 2004

IAVCEI

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 from Pali Aike and Meseta de Buenos Aires. 3He/4He ratios obtained by in vacuo crushing ranged from 6.3 to 7.2 RA and clearly indicate the absence of deep mantle plume accompanying higher 3He/4He ratio than the MORB-value (8±1 RA). Since trace element composition and Pb isotopes are quite different from those of HIMU basalts (Stern et al., 1990), an involvement of HIMU type plume with low 3He/4He ratio is implausible. The upwelling asthenosphere would therefore be derived from relatively shallow mantle, when the slab-window model is adopted as of present. The slight enrichment in 4He of the Patagonian plateau lavas compared to the MORB mantle suggests that their sources were metasomatized lithospheric mantle, or mixture of asthenospheric mantle with MORB-like helium and lithospheric mantle with lower 3He/4He ratio than 6 RA. In order to clarify the 3He/4He ratio of the lithospheric mantle, we are planning to analyze mantle xenoliths contained in the lavas.