Experimental determination of the equilibria : rutile + magnesite = geikielite + CO 2 and zircon + 2 magnesite = baddeleyite + forsterite

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 ∞C, 8.6–8.7 kbar at 850 ∞C, and 10.5–10.7 kbar at 900 ∞C. On the basis of 9 experiments, brackets for the zircon-magnesite-baddeleyite-forsterite equilibrium are 7.1– 7.7 kbar at 800 ∞C, 9.2–9.4 kbar at 850 ∞C, and 10.7–10.9 kbar at 900 ∞C. Considering experimental uncertainties in P (±300 bars) and T (±3 ∞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 ± 0.67 kJ and –1917.54 ± 1.25 kJ, respectively. Corresponding values consistent with the Holland and Powell database are –1479.30 ± 0.74 kJ and –1918.47 ± 1.49 kJ. Application of the two equilibria indicate that: (1) the mole fraction of CO2 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 CO2 could have been as low as 210 during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of CO2 was <0.18 during one instance of mantle metasomatism. FERRY ET AL.: STABILITY OF RUTILE + MAGNESITE AND ZIRCON + MAGNESITE 1343 ing ultrahigh-pressure (UHP) metamorphic rocks that contain magnesite (Mgs), Rt, and Zrn (Liou et al. 1995; Zhang and Liou 1996; Dobrzhinetskaya et al. 2001) and to diamond-bearing mantle samples that contain Zrn and Bd or Rt and Mg-rich ilmenite (Schärer et al. 1997; Zhao et al. 1999; Sobolev and Yefimova 2000). EXPERIMENTAL METHODS