Triple oxygen isotopic composition of the high-<SUP>3</SUP>He/<SUP>4</SUP>He mantle

Measurements of Xe isotope ratios in ocean island basalts (OIB) suggest that Earth's mantle accreted heterogeneously, and that compositional remnants of accretion are sampled by modern, high-He-3/He-4 OIB associated with the Icelandic and Samoan plumes. If so, the high-He-3/4(H)e source may also have a distinct oxygen isotopic composition from the rest of the mantle. Here, we test if the major elements of the high-He-3/He-4 source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-He-3/(4) He OIB locations. To high precision, the Delta O-17 value of high-He-3/He-4 olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Delta O-17(Bulk Mantle) - Delta O-17(High3He/4He) olivine = -0.002 +/- 0.004 (2 x SEM)parts per thousand). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-He-3/(4) He source. Modelling of mixing processes indicates that if an early-forming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection.