Experimental determination of the equilibria: rutile + magnesite = geikielite + C[O.sub.2] and zircon + 2 magnesite = baddeleyite + forsterite + 2 C[O.sub.2]

The P-T conditions of both equilibria were determined precisely by reversal experiments in a piston-cylinder apparatus. On the basis of 8 experiments, brackets for the rutile-magnesite-geikielite equilibrium are 7.0-7.1 kbar at 800 [degrees]C, 8.6-8.7 kbar at 850[degrees]C, and 10.5-10.7 kbar at 900[degrees]C. On the basis of 9 experiments, brackets for the zircon-magnesite-baddeleyite-forsterite equilibrium are 7.1-7.7 kbar at 800[degrees]C, 9.2-9.4 kbar at 850[degrees]C, and 10.7-10.9 kbar at 900[degrees]C. Considering experimental uncertainties in P ([+ or -]300 bars) and T ([+ or -]3[degrees]C), equilibrium curves calculated from both the Berman and the Holland and Powell databases pass through all brackets. Molar Gibbs free energy of formation from the elements at 1 bar and 298 K for geikielite and zircon, derived from the experiments and consistent with the Berman database, are -1481.94 [+ or -] 0.67 kJ and -1917.54 [+ or -] 1.25 kJ, respectively. Corresponding values consistent with the Holland and Powell database are -1479.30 [+ or -] 0.74 kJ and -1918.47 [+ or -] 1.49 kJ. Application of the two equilibria indicate that: (1) the mole fraction of C[O.sub.2] in fluid was 0.54-1.00 when geikielite and baddeleyite formed during contact metamorphism of siliceous dolomites in the Ballachulish aureole, Scotland; (2) the activity of C[O.sub.2] could have been as low as 2 [10.sup.-5] during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of C[O.sub.2] was