Noble gases from oceanic island basalts do not require an undepleted mantle source

Abstract

Several lines of evidence based on relative rare earth abundances and on strontium, lead and neodymium isotopic geochemistry have led to the suggestion that the Earth's mantle may be divided roughly into a region depleted in volatile components, feeding the mid-ocean ridge basalts (MORB's), and a less depleted (‘undepleted’) region which feeds hotspots and thus oceanic island basalts (OIB's) such as Iceland and Hawaii. Models of mantle degassing and atmospheric evolution have been based on differences in argon and xenon isotope ratios measured in samples which supposedly trap rare gases from these two reservoirs^1–7. However, the ultra-mafic Ar and Xe that were first suggested as samples of the undepleted mantle were subject to air contamination and diffusion loss, so that the experimental results are not definitive⁸. New heavy rare gas data on volcanic glasses from Hawaii and Iceland are more directly comparable with the MORB glass data, and supposedly show the isotope pattern expected from the undepleted mantle^2,5. As I report here, however, absolute abundances and elemental ratios corrected for mass fractionation effects indicate that partial loss of the depleted mantle component plus addition of an atmospheric Ar and Xe component provide a better rationale for the isotopic differences in these heavy gases than does the postulated undepleted mantle component.