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 Δ17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Δ17OBulkMantle−Δ17OHigh3He/4Heolivine=−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 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 magmatic oxygen is sourced from the same mantle as other, more incompatible elements and that the intermediate δ18O value is a feature of the high-3He/4He 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.
Highlights
Noble gases provide evidence for the preservation of a reservoir in the Earth’s mantle that has been less melted and degassed than the rest of the mantle (as sampled by mid-ocean ridge basalts, (MORB))
We note that Baffin Island and West Greenland (BI-West Greenland (WG)) samples were screened on the basis of trace element chemistry to be free of crustal contamination (Starkey et al, 2009), further supporting the connection of our measured d18O values to their mantle source value
The mean D17O composition of olivines from noncrustally contaminated, high-3He/4He (>20 Ra, d18O > 4.75‰) ocean island basalts (OIB) is indistinguishable from mean mantle olivine to high precision, despite evidence for the early isolation of high-3He/4He mantle
Summary
Noble gases provide evidence for the preservation of a reservoir in the Earth’s mantle that has been less melted and degassed than the rest of the mantle (as sampled by mid-ocean ridge basalts, (MORB)). N.A. Starkey et al / Geochimica et Cosmochimica Acta 176 (2016) 227–238 evidence for a mantle component with a higher timeintegrated 3He/(U + Th) ratio compared to the MORB mantle source (Kurz et al, 1982). Starkey et al / Geochimica et Cosmochimica Acta 176 (2016) 227–238 evidence for a mantle component with a higher timeintegrated 3He/(U + Th) ratio compared to the MORB mantle source (Kurz et al, 1982) This leads to the assumption that the inferred lower mantle source of high-3He/4He
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