Calorimetry and equilibrium phase relations of high-pressure phases in the system CaO-SiO₂-TiO₂

Abstract

With increasing pressure, CaSiO₃ wollastonite transforms to walstromite which dissociates into Ca₂SiO₄ larnite + CaSi₂O₅ titanite, and finally transforms to CaSiO₃ perovskite which is accepted as one of major constituents of the lower mantle. Among the phases, CaSi₂O₅ titanite and CaSiO₃ perovskite convert to triclinic phase and amorphous phase, respectively, on release of pressure. CaSi₂O₅ component is dissolved into CaTiSiO₅ titanite with increasing pressure, and a complete solid solution is formed in the system at about 9 GPa. The titanite solid solutions of all the compositions except for CaSi₂O₅ endmember are quenched at ambient conditions. Thermodynamic properties and stability relations of these Ca-silicates have not yet been fully clarified, and particularly enthalpy measurements of Ti-bearing silicates have been scarcely reported. In this study, we have successfully applied drop-solution calorimetric method combined with gas-bubbling technique to measure enthalpies of titanite solid solutions in the system CaTiSiO₅-CaSi₂O₅ that are dissolved very slowly in lead borate solvent. Using the measured enthalpies, we have estimated enthalpy of CaSi₂O₅ titanite, and have calculated high pressure phase relations in the system CaO-SiO₂-TiO₂. xCaTiSiO₅-(1-x)CaSi₂O₅ titanite solid solutions (x=0.1, 0.2, 0.5, 0.75, 1.0) were synthesized using a multianvil apparatus, confirming single phase titanite solid solutions by powder X-ray diffraction. Pellets of the titanite solid solutions were dropped into 2PbO.B₂O₃ solvent in a Calvet microcalorimeter kept at 705C. In the drop-solution run, Ar gas was passed though the solvent to produce bubbles that enhanced the sample dissolution. The measured enthalpies of the titanite solid solutions decrease linearly with increasing x, which indicates that the solid solution is ideal. The extrapolated enthalpy of CaSi₂O₅ titanite was used to obtain enthalpies of transitions for CaSiO₃ walstromite + SiO₂ coesite -> CaSi₂O₅ titanite (44.9+-2.5 kJ/mol) and 1/3Ca₂SiO₄ larnite + 1/3CaSi₂O₅ titanite -> CaSiO₃ perovskite (66.0+-4.5 kJ/mol). These enthalpy data were used to calculate stability field of titanite solid solution in the CaTiSiO₅-CaSi₂O₅ system, and the calculated results are generally consistent with experimental data by Knoche et al. (1998). The present calculation combined with some quench experiments also indicates that transition boundary for larnite + titanite (R) Ca-perovskite has a slope of 1.8MPa/K at 14.6GPa and 1600C. Our calculated phase relations suggest that coexisting larnite and titanite in Kankan diamonds (Joswig et al., 1999) occurred at depth of about 300-420 km in the mantle, and that Si-rich titanite found in Kokchetav ultrahigh pressure metamorphic belt (Ogasawara et al., 2002) occurred at depth deeper than about 200 km.