A New Mantle Convection Model with Depth-Dependent Viscosity and its Implications for the Thermal and Chemical Evolution of the Earth
Abstract
Motivated by studies that infer a significant viscosity contrast of up to 30-100 times between the upper and lower mantle of the Earth, we explore a 2D model of thermal mantle convection that includes a viscosity contrast. Mass is free to flow between the upper and lower mantle but is impeded by the viscosity change. An analytic solution for the plate driven flow reveals recirculation of material within the upper mantle's low viscosity region. The lower mantle acts as a high viscosity convecting layer with an open upper boundary and controls the mass exchange between the upper and lower mantle. The effects of plate deformation, convergent and transform boundary stresses, the 660 km phase transition, and toroidal flow are incorporated into the energy balance of the upper mantle and act to regulate the surface heat flux. The upper mantle properties and plate dynamics are found to govern the temperature drop across the surface boundary layer while the viscosity contrast results in a temperature difference between the upper and lower mantle. The model is used to investigate the effects of the viscosity contrast on the thermal history of the Earth. The model also calculates the mass flux between the upper and lower mantle and can be generalized to track geochemical fluxes through time. While the model is simple in its layered formulation it captures the large-scale laterally averaged dynamics of the system and reveals fundamental relationships between viscosity structure, heat flow, and mass flux between the upper and lower mantle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMDI23A1659C
- Keywords:
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- 0545 COMPUTATIONAL GEOPHYSICS / Modeling;
- 3225 MATHEMATICAL GEOPHYSICS / Numerical approximations and analysis;
- 5418 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Heat flow;
- 8125 TECTONOPHYSICS / Evolution of the Earth