Numerical Models of Mantle Convection Based on Thermodynamic data for the MgO-SiO2 Olivine-Pyroxene System

Recent experiments of Chudinovskikh and Boehler (pers. com. 2002) on the phase transition of MgSiO₃ ilmenite to perovskite have shown a strong negative Clapeyron slope of -6 MPa/K. For a representative geotherm this phase transition occurs near 660 km. This is close to the transition depth of γ-spinel to perovskite+periclase in the olivine Mg_{2SiO_4} subsystem of the magnesium endmember olivine-pyroxene MgO-SiO₂ system. We have performed numerical modelling experiments on 2-D mantle convection in the the magnesium endmember olivine-pyroxene MgO-SiO₂ system, to investigate the impact of the recently obtained Clapeyron slope in this system characterized by closely spaced multiple phase transitions (Zhao et al.,1992, Phys. Earth Planet. Inter., 72, 185-210). The numerical model applied uses a database of thermodynamic properties obtained by adapting an existing database for the MgO-FeO-SiO₂ system (Saxena, 1996. Geochim. Cosmochim. Acta, 60, 2379-2395). To this end Saxenas database was adapted for the recently obtained more negative Clapeyron slope of Chudinovskikh and Boehler (Jacobs et al., 2002, Fall AGU Meeting). The mantle convection code uses an anelastic liquid approximation and a tabulated representation of the thermodynamic quantities: density, thermal expansivity and specific heat (van den Berg et al., 2001, EOS, Trans. AGU, 82(47), F1133). Convection experiments were done for a 2-D cartesian model of aspect ratio 2.5. We present results comparing models applying the original Saxena (1996) database and the modified database, adapted for the new value of the Claperyron slope for the ilmenite to perovskite transition (Jacobs et al, 2002), showing an increased degree of layering in the style of mantle convection resulting from the introduction of the more negative Clapeyron slope.