The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layer ing in a relatively short time of about 50 Ma. Due to secular cooling mantl e differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two di fferent parameterizations of the melting phase-diagram are used in the mode ls. The results indicate that during the Archaean melting occurred on a sig nificant scale in the deep regions of the upper mantle, at pressures in exc ess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram par ameterization used in the model. The results reveal a strong correlation be tween lateral variations of temperature and the thickness of the continenta l root. This shows that cold regions in cratons are stabilized by a thick d epleted root. (C) 1999 Elsevier Science B.V. All rights reserved.