Triple oxygen isotopic composition of the high-³He/⁴He 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-³He/⁴He OIB associated with the Icelandic and Samoan plumes. If so, the high-³He/⁴He 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-³He/⁴He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high- ³He/ ⁴He OIB locations. To high precision, the δ¹⁷O value of high-³He/⁴He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (δ¹⁷O ^{Bulk Mantle}-δ¹⁷O^{High 3He/4He olivine}=-0.002±0.004 (2×SEM)‰). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-³He/⁴He 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.The δ ¹⁸O values of olivine with the highest ³He/⁴He ratios from a variety of OIB locations have a relatively uniform composition (~5‰). This composition is intermediate to values associated with the depleted MORB mantle and the average mantle. Similarly, δ¹⁸O values of olivine from high- ³He/⁴He OIB correlate with radiogenic isotope ratios of He, Sr, and Nd. Combined, this suggests that magmatic oxygen is sourced from the same mantle as other, more incompatible elements and that the intermediate δ¹⁸O value is a feature of the high- ³He/⁴He mantle source. The processes responsible for the δ¹⁸O signature of high-³He/⁴He mantle are not certain, but δ ¹⁸O-⁸⁷Sr/⁸⁶Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate δ¹⁸O components recycled into the high-³He/⁴He source.