Physics
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999pepi..114..129a&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 114, Issue 3-4, p. 129-140.
Physics
25
Scientific paper
Enthalpies of perovskite solid solutions in the system MgSiO3-Al2O3 and pyrope were measured by high temperature lead borate solution calorimetry. The enthalpy of garnet-perovskite transition for Mg3/4Al1/2Si3/4O3 (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 magnesiowustite. 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.
Akaogi Masaki
Ito Emi
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