Calorimetric study on majorite-perovskite transition in the system Mg4Si4O12-Mg3Al2Si3O12:: transition boundaries with positive pressure-temperature slopes

Enthalpies of perovskite solid solutions in the system MgSiO3-Al2O3 and pyrope were measured by high temperature lead berate solution calorimetry. The enthalpy of garnet-perovskite transition for Mg(3/4)A(1/2)Si(3/4)O(3) (pyrope) composition was estimated to be 62.3 +/- 7.9 kJ/mol at 298 K, assuming the ideal solution model for perovskite. Using the data together with previously measured enthalpy of garnet-perovskite transition in MgSiO3, and available phase equilibrium data, the majorite-perovskite transition boundaries in the system MgSiO3-Mg3/4Al1/2Si3/4O3 were calculated. The calculated boundaries reveal that the majorite-perovskite transition occurs in a wide pressure interval of about 2.5-3 GPa which corresponds to about 60-70 km in the mantle. The steep gradients of seismic velocities of PREM at the depth range just below the 660 km discontinuity may be attributed to the garnet-perovskite transition of mantle majorite. The calculated boundaries show a positive pressure-temperature slope, 2 +/- 1 MPa/K, for majorite composition appropriate in the mantle, in contrast to a negative Clapeyron slope of postspinel transition in which spinel dissociates to perovskite and magneslowustite. The positive slope boundary and a large density increase of the majorite-perovskite transition play a role to enhance mantle convection. In a pyrolite mantle, combined effect of the majorite-perovskite transition and the postspinel transition would reduce a resistance to material exchange between the upper and lower mantle to about one fourth of that associated with the single postspinel transition in a pure olivine mantle. (C) 1999 Elsevier Science B.V. All rights reserved.