The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/4He ratios

A key requirement for any model of mantle evolution is accounting for the high He-3/He-4 ratios of many ocean island basalts compared to those of mid-ocean ridge basalts. The early, popular paradigm of primitive, undegassed mantle stored in a convectively isolated lower mantle is incompatible with geophysical constraints that imply whole mantle convection. Thus it has been suggested more recently that domains with high He-3/U ratios have been created continuously from the bulk mantle throughout Earth history. Such models require that the He-3/He-4 ratio of the convecting mantle was at least as high as the highest values seen in OIB at the time the OIB source was generated. These domains must also be created with sufficient He to impart distinctive He isotopic signatures to ocean island basalts. However, the He isotope evolution of the mantle has not been consistently quantified to determine if such scenarios are plausible. Here a simple model of the He evolution of the whole mantle is examined. Using a wide range of possible histories of continental extraction and He degassing, the bulk convecting mantle was found to have had He-3/He-4 ratios as high as those seen in the Iceland hotspot only prior to 3 Ga. Such high He-3/He-4 ratios can only be preserved if located in domains that are not modified by convective mixing or diffusive homogenisation since that time. Further, there are difficulties in producing, with commonly invoked magmatic processes, domains with sufficiently high He-3/U ratios and enough 3 He to be able to impart this signature to ocean island basalts. The results are consistent with models that store such He signatures in the core or a deep layer in the mantle, but are hard to reconcile with models that continuously generate high He-3/He-4 domains within the mantle. (C) 2008 Elsevier B.V. All rights reserved.