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.