Variable viscosity convection and partial melting in the continental asthenosphere

Abstract

Convection beneath continents may be associated with the generation of partially molten zones at depth. Experimentally determined non‐Newtonian, temperature‐ and pressure‐dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase. In our models, convection is chaotic and exhibits a strong time dependence. Using model “dry” and “water‐undersaturated” solidus relations, partially molten regions develop at asthenospheric depths forming low‐viscosity regions. The existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m.y., in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics. For an average continental heat flux of about 53.3 mW/m2, maximum melt fractions range between 0 and 2% (dry solidus) and 2 and 4% (water‐undersaturated solidus). On the basis of experimental deformation studies, we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle. The grain sizes obtained (millimeters to centimeters) are in good agreement with grain sizes observed in natural examples. The smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere. The largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions. As our models scale well to natural observations, these results appear to preclude diffusion creep (n = 1) as a viable upper mantle steady state deformation mechanism.