Numerical models, geochemistry and the zero-paradox noble-gas mantle

Numerical models of whole-mantle convection demonstrate that degassing of the mantle is an inefficient process, resulting in ca. 50% of the Ar-40 being degassed from the mantle system. In this sense the numerical simulations are consistent with the Ar-40 mass balance between the atmosphere and mantle reservoir. These models, however, are unable to preserve the large-scale heterogeneity predicted by models invoking geochemical layering of the mantle system. We show that the three most important noble-gas constraints on the geochemically layered mantle are entirely dependent on the He-3 concentration of the convecting mantle derived from the He-3 flux into the oceans and the average ocean-crust generation rate. A factor of 3.5 increase in the convecting-mantle noble-gas concentration removes all requirements for: a He-3 flux into the upper mantle from a deeper high He-3 source; a boundary in the mantle capable of separating heat from helium; and a substantial deep-mantle reservoir to contain a hidden Ar-40 rich reservoir. We call this model concentration for the convecting mantle the 'zero-paradox' concentration. The time-integrated flux of He-3 into the oceans is a robust observation, but only representative of the ocean-floor activity over the last 1000 years. In contrast, ocean-floor generation occurs over tens of millions of years. We argue that combining these two observations to obtain the He-3 concentration of the mantle beneath mid-ocean ridges is unsound. Other indicators of mantle He-3 concentration suggest that the real value may be at least a factor of two higher. As the zero-paradox concentration is approached, the noble-gas requirement for mantle layering is removed. We further consider the role that recycled material plays in ocean-island-basalt generation and show that a source with high He-3 and He-3/(4) He must exist within the mantle. Nevertheless, only a small amount of this material is required to generate both the observed ocean-island He-3/He-4 ratios and the concentrations inferred from basalt samples for this mantle source.