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

N.a. Starkey, C.r.m. Jackson, R.c. Greenwood, S. Parman, I.a. Franchi, M. Jackson, J.g. Fitton, F.m. Stuart, M. Kurz, L.m. Larsen

Research output: Contribution to journalArticlepeer-review

Abstract

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 D17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (D17OBulk Mantle D17OHigh 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 earlyforming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection. The d18O 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, d18O 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 d18O value is a feature of the high-3He/4He mantle source. The processes responsible for the d18O signature of high-3He/4He mantle are not certain, but d18O–87Sr/86Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate d18O components recycled into the high-3He/4He source.
Original languageEnglish
Pages (from-to)227-238
JournalGeochimica et Cosmochimica Acta
Volume176
Early online date2 Jan 2016
DOIs
Publication statusPublished - 1 Mar 2016

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