THERMAL STABILITY OF CHLORITOID AT HIGH PRESSURE AND RELATIVELY HIGH OXYGEN FUGACITY

The equilibrium conditions for the oxidation reaction of chloritoid to staurolite, magnetite, quartz, and vapor have been determined experimentally in the pressure range 10-25 kb using the hematite-magnetite buffer. At 10 kb total pressure the reaction is in equilibrium at 575° C. At 5 kb a value of 544° C was obtained by extrapolation of the high pressure results with an adjustment for the changing thermodynamic properties of water in the low pressure range.At oxygen fugacities along the nickel-nickel oxide buffer, the thermal stability of chloritoid is promoted by 50-60° C above the hematite-magnetite values. Here the breakdown products staurolite, almandine, and magnetite are probably more stable.The recently produced petrographic evidence for the oxidation breakdown of chloritoid and the relative lack of sensitivity of the reaction to moderate changes in total pressure and oxygen fugacity indicate that it may have potential as an important indicator on the temperature scale of progressive metamorphism. Several independent temperature estimates of the chloritoid breakdown event by workers in the field agree very well with the present value of about 550° C for the oxidation reaction of chloritoid to staurolite, magnetite, quartz, and vapor at the hematite-magnetite buffer in the pressure range 5-10 kb. © 1968 Oxford University Press.