Abstract
The heats of solution of synthetic anhydrous Mg-cordierite and of its high-pressure breakdown assemblage sapphirine + quartz (+ enstatite?) have been measured in a lead borate melt at 694°C. The ΔH of this reaction at this temperature and one atmosphere is 6.1±1 kilocalorie per mole of cordierite.
A P-T stability diagram of cordierite relative to other synthetic phases in the system MgO-Al2O3-SiO2 was constructed which satisfies the heat of reaction data and all other reliable observations pertaining to the stability of anhydrous cordierite. The stability field of cordierite is limited by boundaries of very small dP/dT slopes. The maximum pressure of cordierite stability is about 8 kilobars. Above an invariant point near 950°C the sapphirine-bearing assemblage is the stable breakdown product of cordierite. Below 950°C the stable breakdown assemblage is enstatite + sillimanite + quartz. New heat of solution data for orthorhombic enstatite are presented which allow a calculation of the lower-temperature breakdown boundary. This calculation is in good agreement with the boundary deduced above. The calculated breakdown pressure of cordierite at 700°C is 5.6±1.5 kilobars. This is much lower than estimates of earlier workers and shows that cordierite stability is greatly restricted under very dry conditions.
Heat of solution data of natural low-iron cordierite and sapphirine samples are presented. These indicate that synthetic cordierite is energetically close to natural cordierite and is therefore an adequate stability model to apply to natural occurrences but that the synthetic sapphirine prepared by the breakdown of cordierite is quite different from natural sapphirine. An estimate of the breakdown relations of cordierite relative to natural sapphirine is presented, which looks quite like the diagram of the synthetic system except that the invariant point is shifted to considerably lower temperatures.
A consequence of the present work is that if conditions of metamorphism were very dry, pressures of only six to eight kilobars would have been necessary to produce the dense anhydrous assemblages equivalent to natural cordierite which are found in some ancient granulites. The subcrustal pressures considered necessary by some workers should not be regarded as established by presently available evidence.
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Newton, R.C., Charlu, T.V. & Kleppa, O.J. A calorimetric investigation of the stability of anhydrous magnesium cordierite with application to granulite facies metamorphism. Contr. Mineral. and Petrol. 44, 295–311 (1974). https://doi.org/10.1007/BF00371558
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DOI: https://doi.org/10.1007/BF00371558