EXPERIMENTAL DETERMINATION OF THE EQUILIBRIA RUTILE + MAGNESITE = GEIKIELITE + CO2 AND ZIRCON + MAGNESITE = BADDELEYITE + FORSTERITE + CO2

The experiments were motivated by the common prograde development of geikielite (Gk) from rutile (Rt) and of baddeleyite (Bd) from zircon (Zrn) in contact metamorphosed siliceous dolomites and by the occurrence of magnesite (Mag), Rt, and diamond inclusions in Zrn from UHP metamorphic rocks. The P-T conditions of both equilibria were precisely determined at 7.2-11.3 kbar and 800°-900°C by 17 reversal experiments in a 2.54 cm diameter piston-cylinder apparatus with NaCl pressure medium. A P correction of 0±200 bars for the apparatus was determined from a reversal of the brucite-periclase-H₂O equilibrium. Synthetic minerals in stoichiometric proportions were sealed in an inner Pt tube with silver oxalate in quantities that produced excess CO₂ over what would fully carbonate the mineral assemblage. The inner tube was then sealed in an outer Pt tube with hematite and H₂O to minimize reduction of CO₂. Direction of reaction was determined by weight loss of the inner capsule, measured with a precision of ±2 mg, when it was punctured at the end of the experiment. Significant reaction (>30 mg signal) occurred in experiments ranging from 7-12 h (900°C) to 24-28 h (800°-850°C) in duration. A small further weight loss (2-21 mg) of the punctured capsule after drying at 300°C was interpreted as a combination of H₂O adsorbed on minerals and reduction of CO₂ to H₂O + CO by infiltration of H₂ from the sample assembly. Experimental pressures were therefore corrected downward to the value for reaction in a pure CO₂ fluid using thermodynamic data and the measured XCO₂ of fluid at the end of the experiment. Reversal brackets, with corrected P, for the Rt-Mag-Gk and Zrn-Mag-Bd-Fo equilibria, respectively, are: 7.0-7.1 and 7.1-7.7 kbar at 800°C, 8.6-8.7 and 9.2-9.4 kbar at 850°C, and 10.5-10.7 and 10.7-10.9 kbar at 900°C. Equilibrium curves computed both from the Berman and from the Holland and Powell data bases pass through all brackets considering experimental uncertainties in P (±300 bars) and T (±3°C). Our new data confirm that Gk and Bd formed in the Ballachulish aureole, Scotland, in the presence of relatively CO₂-rich fluids with XCO₂=0.6-1.0.