Profiles of the Bullen parameter from mantle convection modelling

Ctirad Matyska & David A. Yuen

Abstract

The state of adiabaticity in the mantle can be assessed by using the Bullen parameter profile derived from seismology. We have employed the extended-Boussinesq convection models in a cartesian 2-D domain to extract the Bullen parameter as a function of depth in both base heated and internally heated configurations and for both constant and variable thermal conductivity. We also studied models with depth-dependent thermal expansivity. Our results show that the values of the Bullen parameter for Rayleigh number of 10**6 lie close to one, with sometimes excursions out to 0.9 and 1.1. They are sensitive to the mode of heating and the depth-dependences of the thermal expansivity and changes of thermal conductivity. We obtained a subadiabatic geotherm above the lower boundary layer in all the models with constant thermal expansivity but a decrease of thermal expansivity with depth can result in a subadiabatic geotherm below the upper boundary layer. Thus the profiles of the Bullen parameter have a definite potential of being useful in constraining the physical parameters and flow structures associated with mantle convection.

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Earth Planet. Sci. Lett., 178 (2000), 39-46.