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

Abstract

The solubility of andradite garnet was determined at 800 °C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% H 2 O. Experiments were syntheses carried out with high-purity natural wollastonite, reagent hematite or natural specular hematite containing ~1 wt% TiO 2 , and fluid. All experiments were performed with Mn 2 O 3 -Mn 3 O 4 or Fe 2 O 3 -Fe 3 O 4 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 CaSiO 3 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 O 2 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 Fe 2 O 3 in andradite-saturated H 2 O-NaCl fluids is lower than that of Al 2 O 3 at grossular saturation at the same pressure ( P ), temperature ( T ), and fluid composition. The results permit a test of a model of CaSiO 3 dissolution in NaCl solutions to three dominant aqueous species: CaCl + , OH , and H 3 NaSiO 4 . Combination of the CaSiO 3 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 CaSiO 3 in NaCl solutions. The low solubility of the Fe 2 O 3 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 Fe 2 O 3 is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades.