Inferring upper-mantle temperatures from seismic and geochemical constraints: Implications for Kaapvaal craton

Abstract

Based on self-consistent thermodynamic approach, we infer the temperature distribution models at 100 to 300 km depth for the upper mantle beneath the Kaapvaal craton from absolute P- and S-wave velocities and geochemical constraints (garnet-bearing lherzolite xenoliths, average composition of garnet peridotite and primitive mantle composition). For the computation of phase equilibrium relations, we have used a method of minimization of the total Gibbs free energy combined with a Mie-Grüneisen equation of state. Our forward calculation of phase equilibria, seismic velocities and density and inverse calculation of temperature include anharmonic and anelastic parameters as well as mineral reaction effects, including phase proportions and chemical compositions of coexisting phases. Sensitivity of density and velocities to temperature, pressure and composition was studied. Calculated velocities are between the fastest and slowest seismic models reported for southern Africa. The estimated temperatures depend rather strongly on bulk composition and proportion of phases stable at various depths of the upper mantle. The relatively small differences between the xenolith compositions translate into substantial variations in inferred temperature. Temperatures inferred from the IASP91 model and from some of regional models beneath the Kaapvaal craton, irrespective of the composition model, display an inflection with a negative temperature gradient at depths below ∼200–220 km, leading to unrealistic temperature behavior. We find that the cratonic upper mantle cannot be treated as uniform in terms of bulk composition because a fixed composition leads to a non-physical behavior of geotherms. A sharp change in composition from depleted garnet peridotite to fertile pyrolitic material seems unable to explain inflexions of geotherms as well as an anticorrelated behavior for T P and T S. To avoid temperature inflexions, a continuous change in composition and a substantial increase in fertility (gradual increase in FeO, Al 2O 3 and CaO content) at depths between 200 and 275 km are required to get monotonous temperature profiles. The mantle beneath the Kaapvaal craton is chemically stratified: an upper layer at depths between 100 and ∼175 km consisting of depleted garnet peridotite and a lower layer (200–275 km) made of a more fertile material. At depths of about 275 km, Kaapvaal cratonic mantle does not differ from normal mantle. S-velocity and density model for southern Africa is constructed. The results indicate the possibility of the existence of a solid-state low-velocity zone, which may be associated with temperature gradients alone without hydrous phases and partial melting.