A helium stratified and ingassed lower mantle: resolving the helium paradoxes

Abstract

It is generally believed a variation of 3He/4He isotopic ratios in the mantle is due to only the decay of U and Th, which produces 4He as well as heat. Here we show that not only 3He/4He isotopic ratios but also helium contents can be fractionated by thermal diffusion in the lower mantle. The driving force for that fractionation is the adiabatic or convective temperature gradient, which always produces elemental and isotopic fractionation along temperature gradient by thermal diffusion with higher light/heavy isotopic ratio in the hot end. Our theoretical model and calculations indicate that the lower mantle is helium stratified, caused by thermal diffusion due to ~ 400 °C temperature contrast across the lower mantle. The highest 3He/4He isotopic ratios and lowest He contents are in the lowermost mantle, which is a consequence of thermal-diffusion fractionation rather than the lower mantle is a primordial and undegassed reservoir. Therefore, oceanic-island basalts derived from the deepest lower mantle with high 3He/4He isotopic ratios and less He contents—the long-standing helium paradox, is solved by our model. Because vigorous convection in the upper mantle had resulted in disordered or disorganized thermal-diffusion effects in He, Mid-ocean ridge basalts unaffected by mantle plume have a relatively homogenous and lower 3He/4He isotopic compositions. Our model also predicts that 3He/4He isotopic ratios in the deepest lower mantle of early Earth could be even higher than that of Jupiter, the initial He isotopic ratio in our solar system, because the temperature contrast across the lower mantle in the early Earth is the largest and less 4He had been produced by the decay of U and Th. Moreover, the early helium-stratified lower mantle owned the lowest He contents due to over-degassing caused by the largest temperature contrast. Consequently, succeeding evolution of the lower mantle is a He ingassed process due to secular cooling of the deepest mantle. This explains why significant amount of He produced by the decay of U and Th in the lower mantle were not released, another long-standing heat–helium paradox.