Triple oxygen isotopic composition of the high- 3 He/ 4 He mantle Academic Article uri icon

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 ?17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (?17OBulk Mantle ? ?17OHigh 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 ?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, ?18O 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 ?18O value is a feature of the high-3He/4He mantle source. The processes responsible for the ?18O signature of high-3He/4He mantle are not certain, but ?18O–87Sr/86Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate ?18O components recycled into the high-3He/4He source.

publication date

  • March 2016