An experimental study of the effect of Ca upon garnet-ilmenite Fe-Mn exchange equilibria

The influence of Ca₃Al₂Si₃O₁₂ (grossular) on Fe-Mn partitioning between coexisting garnet and ilmenite has been investigated at 650-1000 °C, 10 kbar for ternary calciummanganese-iron garnets of various Fe-Mn ratios. K^gar-ilm_DFe-Mn shows a slight dependence upon X^gar_Ca. The experimental results have been combined with the FeTiO₃-MnTiO₃ activitycomposition data of O'Neill et al. (1989) to constrain activity-composition relations in garnet. Mixing in ternary calcium-manganese- iron garnets (up to ≈ 30 mol% grossular) may be represented by the simple regular solution model with the difference in the binary interaction parameters (W^gar_CaFe - W^gar_CaMn) = 0.97 ± 0.20 kJ/mol of cation. The results are in good agreement with the recent activity data for the individual calcium-iron and calcium-manganese garnet binaries (Koziol, 1990). For the almandine-spessartine binary solution, the new data reported here give W^gar_FeMn = 0.62 ± 0.20 kJ/mol of cation, in good agreement with previous results from the Cafree system (Pownceby et al., 1987). Reformulation of the thermometer based on Fe-Mn exchange between garnet and ilmenite to take into account the new data gives T9°C) = [14918-2200(2X^ilm_Mn - 1) + 620(X ^gar_Mn - X^gar_Fe) - 972X ^gar_Ca]/R In K_d + 4.38-273.15. The thermometer has a resolution of ± 30-50 °C.