Solubility of andradite, Ca3Fe2 Si3O12, 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 (m_cs) of 0.0838 ± 0.0015 for the reagent hematite and both buffers. Slightly higher m_cs of 0.0895 ± 0.0005 for the natural hematite and Mn-oxide f_o2 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 m_cs. 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.