Abstract:
The reaction of phlogopite plus quartz to enstatite, potassium feldspar, and aqueous fluid in the system KMASH-KCl was reversed at 2-12 kbar and 750-875 degrees C and at low H2O activities by reversal of the H2O content of concentrated KCI solutions equilibrated with product and reactant assemblages. Synthetic 1M phlogopite [KMg3AlSi3O10(OH)(2)] and enstatite (MgSiO3) maintained end-member stoichiometry throughout, and the potassium feldspar (KAlSi3O8) was a high sanidine, based on unit-cell refinements. The broad P-T-X-H2O and narrow reversal ranges of this investigation were possible because of the low and well-defined H2O activity, yet powerful fluxing action, of concentrated KCI solutions. Solubility experiments on quartz and potassium feldspar in our P-T-X-H2O range showed that fluid-phase solution of silicate constituents was too small to have affected the H2O activity in the experiments. The new determinations are more definitive than previous work done at very low pressures with pure H2O or in CO2-H2O mixtures. They establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kJ and indicate that the synthetic phlogopite has maximal (Al-Si) disorder under our experimental conditions. The standard enthalpy of reaction at 298 K is 106.54 +/- 2.0 kJ (2 sigma) based on our reversals, a value 6 kJ less positive than that currently used by many workers in calculations of biotite stability and H2O activity in the petrogenesis of high-grade metamorphic rocks. The lower thermal stability that we find for phlogopite requires revision in estimates of H2O activity of granulite facies metamorphism: typical values for the natural assemblage orthopyroxene-biotite-garnet-potassium feldspar-plagioclase-quartz at deep-crustal metamorphic conditions (750-850 degrees C, 5-10 kbar) are a(H2O) = 0.4-0.6 compared with values of 0.15-0.30 which would have been estimated with previously available data on phlogopite stability; An important consequence of the expanded H2O activity range of granulites is that alkali chloride solutions of only moderate concentration [X-H2O = 0.5-0.7], which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks, are a feasible alternative to the vapor-absent conditions considered necessary by many workers based on previous low estimates of a(H2O). Participation of concentrated brines in deep-crust/upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large-scale melting of the crust.