THERMAL AND RARE-GAS EVOLUTION OF THE MANTLE

Present-day seismic evidence implies that the 660 km-deep seismic velo city jump is associated with neither an internal thermal boundary laye r nor a strong internal barrier to flow between the upper and lower ma ntle. However, the generally preferred geochemical paradigm for mantle rare-gas evolution concludes that the lower mantle has remained an is olated and undegassed reservoir throughout at least the past 4.35 Ga o f Earth history, as similar to 50% of the Ar-40 produced by K-40-decay appears to still reside within the mantle. Here we reexplore this pro blem assuming that present-day differentiation processes have operated throughout Earth history. Present mantle overturn rates are slow (sim ilar to 5-10 Ga to pass a mantle volume through the mid-ocean ridge cr ucible). If slab subduction has been the mantle's primary heat-loss me chanism, then a simple boundary layer argument suggests that paleo-sub duction and mantle overturn rates were proportional to the heat loss-s quared, 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 Ar-40 produced during Earth evolution without the need f or postulating an isolated and undegassed lower mantle. These models s uggest that the Ar-40 constraint should be reinterpreted to be a const raint that similar to 50% of the mantle (and crust?!) has been undegas sed since the Ar-40 was produced within it. (C) 1998 Elsevier Science B.V. All rights reserved.