CONSTRAINTS ON THE UPPER PRESSURE STABILITY OF BLUESCHIST FACIES METAMORPHISM ALONG THE REACTION: GLAUCOPHANE = TALC+2 JADEITE IN THE Na2O-MgO-Al2O3-SiO2-H2O SYSTEM

Blueschist-facies metamorphism, which is associated with high pressure and low temperature subduction zone metamorphism, is usually recognized by the presence of the sodic-amphibole glaucophane. Determining the upper- and lower-pressure stability of end-member glaucophane places important constraints on the conditions of blueschist metamorphism. An experimental investigation into the upper-pressure stability, of glaucophane has been done in the system Na2O-MgO-Al2O3-SiO2-H2O over the range of 600 to 750 degrees C and 2.5 to 4.5 GPa. Mixtures of synthetic glaucophane, jadeite, and talc were used to determine the location of the reaction boundary glaucophane = 2 jadeite + talc by reversing the sense of reaction direction. The upper-pressure stability of glaucophane is located over a pressure interval (0.2-0.3 GPa in width) whose midpoints lie at 2.6 GPa at 600 degrees C and at 3.1 GPa at 700 degrees C, across which the proportion of glaucophane decreases to zero as talc and jadeite increase. This band has a positive dP/dT slope of around 0.005 GPa/degrees C. Glaucophane showed the largest change in composition, consistent with the incorporation of significant amounts of the nyboite (17-34 mol%) and cummingtonite (10-18 mol%) components. Talc showed minor, but definite incorporation of Na (0.08 atoms per formula unit, apfu) and Al (0.15 apfu) consistent with incorporation of the components aspidolite and tschermak-talc. Jadeite displayed no clear change from its ideal composition. The results from this study were combined with the lower-pressure stability of glaucophane + quartz reported earlier by Corona and Jenkins (2007) to refine the thermodynamic values for glaucophane (Delta H-f degrees = -11,956.36 kJ/mol), report an initial set of values for the amphibole nyboite (Delta H-f degrees = -12,165.85 kJ/mol) and phyllosilicate aspidolite (Delta H-f degrees = -6163.86 kJ/mol), and to develop activity-composition relationships for both the amphibole and talc in this chemical system. The results of this study place an upper-pressure limit to a key index mineral of the blueschist-facies at about 3.2 GPa at 700 degrees C, which is just above the quartz-coesite transition, allowing nearly end-member glaucophane to remain stable up to the conditions of ultra-high-pressure metamorphism.