Solubility of andradite, Ca3Fe2Si3O12, in a 10 mol% NaCl solution at 800 °C and 10 kbar: Implications for the metasomatic origin of grandite garnet in calc-silicate granulites

The solubility of andradite garnet was determined at 800 °C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% H2O. Experiments were syntheses carried out with high-purity natural wollastonite, reagent hematite or natural specular hematite containing ~1 wt% TiO2, and fluid. All experiments were performed with Mn2O3-Mn3O4 or Fe2O3-Fe3O4 oxygen buffers in a piston-cylinder apparatus with NaCl pressure media for 1–3 days. Andradite saturation was determined by the presence or absence of garnet in quenched charges. Andradite dissolves incongruently to hematite and fluid with CaSiO3 molality (mCS) of 0.0838 ± 0.0015 for the reagent hematite and both buffers. Slightly higher mCS of 0.0895 ± 0.0005 for the natural hematite and Mn-oxide fO2 buffer is interpreted as due to incomplete equilibration and/or ~9 mol% Ti in the run-product andradite. Dissolved Fe molality could be determined only approximately, but must be at least ten times lower than mCS. Quenched fluids were very basic (pH 11–12). The solubility of Fe2O3 in andradite-saturated H2O-NaCl fluids is lower than that of Al2O3 at grossular saturation at the same pressure (P), temperature (T), and fluid composition. The results permit a test of a model of CaSiO3 dissolution in NaCl solutions to three dominant aqueous species: CaCl+, OH−, and H3NaSiO4. Combination of the CaSiO3 molality at andradite saturation with wollastonite solubility at the same conditions (0.1253 ± 0.0047 molal; Newton and Manning 2006) leads to a Gibbs free energy change of the reaction 3 wollastonite + hematite = andradite at 800 °C and 10 kbar of –32.21 ± 2.45 kJ. The good agreement between this value and that derived from previous studies supports the dissolution model of CaSiO3 in NaCl solutions. The low solubility of the Fe2O3 component of andradite contrasts with the high solubility of magnetite and of Fe in pelitic and granitic mineral assemblages measured in acidic chloride solutions by previous workers at lower P and T. The results imply that Fe2O3 is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades.