THERMAL AND RARE GAS EVOLUTION OF THE MANTLE

Abstract

Present-day seismic evidence implies that the 660 km-deep seismic velocity jump is associated with neither an internal thermal boundary layer nor a strong internal barrier to flow between the upper and lower mantle. However, the generally preferred geochemical paradigm for mantle rare-gas evolution concludes that the lower mantle has remained an isolated and undegassed reservoir throughout at least the past 4.35 Ga of Earth history, as ~50% of the 40Ar produced by 40K-decay appears to still reside within the mantle. Here we reexplore this problem assuming that present-day differentiation processes have operated throughout Earth history. Present mantle overturn rates are slow (~5-10 Ga to pass a mantle volume through the mid-ocean ridge crucible). If slab subduction has been the mantle's primary heat-loss mechanism, then a simple boundary layer argument suggests that paleo-subduction and mantle overturn rates were proportional to the heat loss-squared, and thus more than twenty times faster in the Archean than at the present day. Nevertheless, simple models of Ar evolution within a convecting mantle demonstrate that whole-mantle convection can retain 25-60% of the 40Ar produced during Earth evolution without the need for postulating an isolated and undegassed lower mantle. These models suggest that the 40Ar constraint should be reinterpreted to be a constraint that ~50% of the mantle (and crust?!) has been undegassed since the 40Ar was produced within it.