An Experimental Determination of the High-Pressure Stability Limits of Magnesian Cordierite under Wet and Dry Conditions

The high-pressure breakdown of Mg-cordierite to talc + sillimanite + quartz in the hydrous system and to enstatite, sillimanite, and quartz in the wet and dry systems was determined experimentally. The latter reaction was determined by indirect means because of melting in the system for P_H₂O = Pₜₒₜₐₗ and because of impossibly sluggish reaction rates in the dry system. Experimental data for the hydrothermal reaction of enstatite and sillimanite to cordierite and corundum was used to locate the upper pressure limit of cordierite for P_H₂O = Pₜₒₜₐₗ and reversal data on the anhydrous reaction of cordierite to sapphirine (+ enstatite ?) + quartz and on the reaction of enstatite + sillimanite to sapphirine + quartz at high temperatures were extrapolated to lower temperatures to locate the equilibrium breakdown of cordierite to enstatite + sillimanite + quartz for PH₂O = 0. The upper pressure experimental stability boundary of anhydrous cordierite agrees with published thermodynamic data. The maximum pressure of stability of anhydrous cordierite is 8.2 kbar and of cordierite under conditions of P_H₂O = Pₜₒₜₐₗ is 11.2 kbar. This discrepancy is attributed to the stabilization of cordierite by molecular water in the porous structure. This concept is supported by calculations based on the dP/dT slope of the breakdown to talc, sillimanite, and quartz by experiments on cordierite breakdown under conditions of reduced water activity using anhydrous oxalic acid and MgI₂ solutions, and by analogy with the crystal chemistry and thermochemistry of zeolites. The observed granulite assemblage hypersthene + sillimanite + quartz, the high-pressure assemblage equivalent to cordierite, is plausibly explained by current estimates of the pressure-temperature conditions of granulite-grade metamorphism if P_H₂O was very low during the metamorphism.