Abstract

Two of the potentially most useful barometers for the granulite facies are based on the following reactions:

anorthite + enstatite = ⅔pyrope + ⅓grossular + quartz (GAPES) (A)

anorthite + diopside = ⅔grossular + ⅓pyrope + quartz (GADS). (B)

Enthalpy of reaction (ΔHR) is one of the major sources of uncertainty in the calibration of these geobarometers. Solution calorimetry of stoichiometric mixes of synthetic phases from each side of each reaction provides an improved method for determination of the enthalpies of reaction. Our results at 973 K are ΔHA = 8.23 (±2.52) kJ/mol and ΔHB = 6.02 (±3.47)kJ/mol. These values discriminate among various ΔHR values compiled from previous solution calorimetry and from internally consistent datasets. For the GAPES reaction there is agreement (though marginal) with all of the derived ΔHR values, but for the GADS reaction, only the ΔHR value compiled from the data of Holland and Powell (1985) is within the calorimetric uncertainty.

The resultant geobarometric equations, retaining entropy and volume data from Newton and Perkins (1982), are (P in kbar, T in K)

GAPES reaction: P = 3.47 + 0.01307T + 0.003504T ln KA (±1.55 kbar) (8)

GADS reaction: P = 2.60 + 0.01718T + 0.003596T ln KB (±1.90 kbar). (9)

A practical field test of these equations, for a well-characteized, granulite facies area in southwestern North Carolina, indicates internal consistency for two-pyroxene-garnet granulites and consistency with the experimental Al2SiO5 diagram compared to a regional kyanite-sillimanite isograd. The empirical adjustments of Newton and Perkins (1982), which subtracted 600 bars from the Reaction A scale and added 1600 bars to the Reaction B scale, are consistent with our calculations with no adjustment. Thus, previous barometric calculations using their adjusted formulae, and tectonic interpretations based on those pressures, should require little revision.

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