Whole-mantle versus layered mantle convection and the role of a high-viscosity lower mantle in terrestrial volatile evolution

Terrestrial noble gas isotope geochemistry provides one of the most powerful geochemical tools presently available for understanding the volatile evolution of the Earth. The data require the existence of isolated reservoirs with very different time integrated U/He ratios within discrete portions of the mantle. One mechanism proposed to account for the apparent isolation is a high-viscosity lower mantle. We have conducted numerical experiments to study the influence of a higher viscosity lower mantle on the mixing efficiency of mantle convection and the rate of mantle degassing. We consider cylindrical mantle convection models in which the lower mantle viscosity is increased to up to two orders of magnitude higher than that of the upper mantle, while maintaining the overall vigor of convection as measured by the total surface heat flow. The models incorporate radiogenic ingrowth and degassing of He. We show with models that match the present day heat flow that the high viscosity of the lower mantle alone does not prevent large-scale mixing of the mantle. Both He-3 and He-3/He-4 remain relatively homogeneous through the entire mantle, showing only small-scale heterogeneity Degassing is more rapid in the case of the high-viscosity lower mantle. Present day plate velocities, heat flow and estimates for the amount of degassed radiogenic noble gases over the age of the Earth are best matched by the model with a 30-times increase of viscosity in the lower mantle. (C) 1998 Elsevier Science B.V. All rights reserved.