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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Starkey, N.A., Jackson, C.R.M., Greenwood, R.C., Parman, S., Franchi, I.A., Jackson, M., Fitton, J.G., Stuart, F.M., Kurz, M., Larsen, L.M.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2016
• Subjects: Baffin Island; Baffin; Ocean Island
• Online Access: https://oro.open.ac.uk/48184/; https://oro.open.ac.uk/48184/1/Starkey%20et%20al.%202016.pdf; https://doi.org/10.1016/j.gca.2015.12.027

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- 3 He/ 4 He OIB associated with the Icelandic and Samoan plumes. If so, the high- 3 He/ 4 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- 3 He/ 4 He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high- 3 He/ 4 He OIB locations. To high precision, the Δ 17 O value of high- 3 He/ 4 He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Δ 17 O Bulk Mantle − Δ 17 O High 3He/4He olivine = −0.002 ± 0.004 (2 × SEM)‰). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high- 3 He/ 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. The δ 18 O values of olivine with the highest 3 He/ 4 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, δ 18 O values of olivine from high- 3 He/ 4 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 δ 18 O value is a feature of the high- 3 He/ 4 He mantle source. The processes responsible for the δ 18 O signature of high- 3 He/ 4 He mantle are not certain, but δ 18 O– 87 Sr/ 86 Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate δ 18 O components recycled into the high- 3 He/ 4 He source.