Is the 670 km phase transition able to layer the Earth's convection in a mantle with depth-dependent viscosity?
Résumé
The effect of a viscosity stratification on phase change dynamics have been investigated with axi-spherical convection models. As in previous studies with a constant viscosity mantle an intermittent layering appears for a Clapeyron slope from-2 MPa/K to-3 MPa/K. A viscosity increase in lower mantle requires a more negative Clapeyron slope to produce the layering. This shift is sensitive to the mechanical boundary condition. With a viscosity contrast of 30, a no-slip top condition does not lead to layering in the range of the possible values for the Clapeyron slope. With a tree-slip condition, the threshold is at-4 MPa/K. Just below this threshold, a whole mantle circulation driven by a cylindrical hot plume coexists with layered mantle domains over several billion years. comparison between axisymetrical and 3-D spherical cases with constant viscosity by Machetei et al [1995] shows that the threshold for layering occurs for the same Clapeyron slope in both situations. This result is somewhat surprising in view of cartesian experiments where the mass flux across the phase change interface varies slightly between the 2-D and 3-D cases [Yuen et al, 1994]. This is explained by the fact that the axisymetrical geometry offers the possibility to test the stability of the stratification to 3-D waves triggered along the polar axis and to 2-D ones elsewhere. Accordingly, we conduct this study in axisymetrical geometry which allows to dramatically increase our computing speed to follow the evolution of the convective circulation with sharp phase changes over period of time several times longer than the Earth's age.
Domaines
Géophysique [physics.geo-ph]
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