Garnet-ilmenite-perovskite transitions in the system Mg4Si4O12-Mg3Al2Si3O12 at high pressures and high temperatures:: phase equilibria, calorimetry and implications for mantle structure

Phase relations in the System Mg4Si4O12-Mg3Al2Si3O12 were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000degreesC using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle, Both of glass and crystalline starting materials were used for the experiments. At 1000degreesC, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg4Si4O12-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg3Al2Si3O12-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000degreesC supersede preliminary phase relations determined at about 900degreesC in the previous study. The phase relations at 1000degreesC are quite different from those reported previously at 1600degreesC where gal-net s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg4Si4O12, The stability field of Mg3Al2Si3O12 ilmenite was determined at 800-1000degreesC and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al2O3 content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2+/-10.4 and 168.6+/-8.2 kJ/mol, respectively, for Mg4Si4O12, and 150.2+/-15.9 and 98.7+/-27.3 kJ/mol, respectively, for Mg3Al2Si3O12. Thermodynamic calculations using these data give rise to phase relations in the system Mg4Si4O12-Mg3Al2Si3O12 at 1000 and 1600degreesC that are generally consistent with those detern-dried experimentally, and confirm that the single-phase field of ilmenite expands from Mg4Si4O12 to Mg3Al2Si3O12 with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg2SiO4-Fe2SiO4 System are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600degreesC lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for gamet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs. (C) 2002 Elsevier Science B.V. All rights reserved.