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Experimental determination of the equilibria: rutile + magnesite = geikielite + CO₂ and zircon + 2 magnesite = baddeleyite + forsterite + 2 CO₂

¹ Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
² Department of Earth and Space Sciences, University of California at Los Angeles, Los Angeles, California 90095, U.S.A.

Correspondence: ^* E-mail: jferry@jhu.edu

The first 20% of the full text of this article appears below.

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 2·10^–5 during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of CO₂ was <0.18 during one instance of mantle metasomatism.

	INTRODUCTION

 
Geikielite (Gk) and baddeleyite (Bd) are common trace minerals in contact metamorphosed siliceous dolomitic limestones that develop during prograde metamorphism at the expense of detrital rutile (Rt) and zircon (Zrn) by the reactions:

(1)

and

(2)

[these and all other abbreviations for minerals follow Kretz (1983)]. Isograds can be mapped in contact aureoles based on reactions 1 and 2, and they lie at grades above that of . . . [Full Text of this Article]

This article has been cited by other articles:

				R. C. Newton and C. E. Manning Hydration state and activity of aqueous silica in H2O-CO2 fluids at high pressure and temperature American Mineralogist, August 1, 2009; 94(8-9): 1287 - 1290. [Abstract] [Full Text] [PDF]