Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number

M Wolstencroft,J H Davies

doi:10.5194/se-2-315-2011

Abstract

Abstract. We investigate the influence on mantle convection of the negative Clapeyron slope ringwoodite to perovskite and ferro-periclase mantle phase transition, which is correlated with the seismic discontinuity at 660 km depth. In particular, we focus on understanding the influence of the magnitude of the Clapeyron slope (as measured by the Phase Buoyancy parameter, P) and the vigour of convection (as measured by the Rayleigh number, Ra) on mantle convection. We have undertaken 76 simulations of isoviscous mantle convection in spherical geometry, varying Ra and P. Three domains of behaviour were found: layered convection for high Ra and more negative P, whole mantle convection for low Ra and less negative P, and transitional behaviour in an intervening domain. The boundary between the layered and transitional domain was fit by a curve P = α Raβ where α = −1.05, and β = −0.1, and the fit for the boundary between the transitional and whole mantle convection domain was α = −4.8, and β = −0.25. These two curves converge at Ra ≈ 2.5 × 104 (well below Earth mantle vigour) and P &amp;approx; −0.38. Extrapolating to high Ra, which is likely earlier in Earth history, this work suggests a large transitional domain. It is therefore likely that convection in the Archean would have been influenced by this phase change, with Earth being at least in the transitional domain, if not the layered domain.

Highlights

Mantle convection has had a dominant control on Earth’s surface evolution
The results show that at low Ra and high P whole mantle convection is preferred, while at high Ra and more negative P, layered convection is preferred (Table 2, Fig. 3)
Our work shows three domains of behaviour: a layered convection domain at high Ra and more negative P, a whole mantle convection domain at low Ra and less negative P, and a transitional domain at intermediate values of Ra and P

Summary

Introduction

Mantle convection has had a dominant control on Earth’s surface evolution. It has been known for many years that mineral phase changes with negative Clapeyron slope (γ ), where γ = dP/dT, are capable of layering mantle convection (Olson and Yuen, 1982; Christensen, 1995). Layering occurs when the lateral temperature variations of convection produce laterally varying vertical deflections of the boundary away from its equilibrium position. If there is an appropriate restoring density contrast between the phases, the deflection is accompanied by buoyancy forces that act against the convective thermal buoyancy. If the Clapeyron slope and density difference are sufficient, the buoyancy forces resulting from the phase boundary deflections can overcome the local convective thermal buoyancy, resulting in layered convection (Fig. 1)

Methods

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Discussion

Conclusion