Triple oxygen isotopic composition of the high-He/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/He OIB associated with the Icelandic and Samoan plumes. If so, the high-He/He 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/He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-He/He OIB locations. To high precision, the DO value of high-He/He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (DO Mantle DO 3He/4He olivine = 0.002 ± 0.004 (2 SEM)‰). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-He/He source. Modelling of mixing processes indicates that if an earlyforming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection. The dO values of olivine with the highest He/He 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, dO values of olivine from high-He/He 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 dO value is a feature of the high-He/He mantle source. The processes responsible for the dO signature of high-He/He mantle are not certain, but dO–Sr/Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate dO components recycled into the high-He/He source. Crown copyright 2016 Published by Elsevier Ltd. This is an open access article under the CCBY license (http://creativecommons.org/ licenses/by/4.0/). http://dx.doi.org/10.1016/j.gca.2015.12.027 0016-7037/Crown copyright 2016 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecomm ⇑ Corresponding author. E-mail address: [email protected] (N.A. Starkey).