DI43B-03
The Triple Oxygen Isotopic Composition of High 3He/4He Mantle

Thursday, 17 December 2015: 14:10
303 (Moscone South)
Stephen Wayne Parman1, Colin Jackson2, Natalie Starkey3, Richard Greenwood3, Ian Franchi3, Matthew G Jackson4, J Godfrey Fitton5, Finlay M. Stewart6 and Lotte M Larsen7, (1)Brown University, Providence, RI, United States, (2)Geophysical Laboratory, Washington Dc, DC, United States, (3)Open University, Planetary and Space Sciences, Milton Keynes, United Kingdom, (4)University of California Santa Barbara, Department of Earth Sciences, Santa Barbara, CA, United States, (5)University of Edinburgh, Edinburgh, United Kingdom, (6)Scottish Universities Environmental Research Center at the University of Glasgow, East Kilbride, United Kingdom, (7)Geological Survey of Denmark and Greenland, Copenhagen, Denmark
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 [1]. 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 ∆17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Δ17OBulk Mantle - Δ17OHigh 3He/4He olivine = -0.002 ± 0.004 (2 x 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 δ18O 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 oxygen remains coupled to the more incompatible elements during melt production and migration and that the intermediate δ18O value is a feature of the mantle source. The processes responsible for the δ18O signature of high-3He/4He mantle are not certain, but δ18O-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.

[1] S. Mukhopadhyay (2012) Nature 486: 101-106.

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