TRIPHYLITE-SARCOPSIDE MISCIBILITY GAP IN THE FeO-MnO-Li2O-P2O5-H2O SYSTEM: EXPERIMENTAL INVESTIGATION AND THERMOMETRIC APPLICATION TO GRANITIC PEGMATITES

In order to assess the stability of the primary triphyliteþ sarcopside assemblage, we performed hydrothermal experiments between 400 and 700 8C (Ni/NiO oxygen fugacity buffer, P¼1 kbar), starting from the Li(Fe2þ2.5-xMn2þx)(PO4)2 (x¼0.0, 0.5, 1.0) compositions, which represent the ideal compositions of triphyliteþsarcopside assemblages in which both minerals occur in a 1:1 molar ratio. The triphyliteþsarcopside assemblage is observed in all experiments, associated with other phosphates like (Fe2þ,Mn2þ)2P2O7, (Fe2þ,Mn2þ)Fe3þ2(PO4)2(OH)2nH2O, or Fe3þ4(Fe2þ,Mn2þ)3(PO4)6. Electron-microprobe and SIMS analyses show a progressive decrease of the Li contents in the triphylites, balanced by an increase of their Fe2þ-contents, when the temperature increases. These compositional changes are due to the increase of the triphylite-sarcopside miscibility along the Li2(Fe2þ,Mn2þ)2(PO4)2-Fe2þ(Fe2þ,Mn2þ)2(PO4)2 solid solution; the experimental phase diagrams can consequently be used as a geothermometer to calculate the exsolution temperatures of the assemblages. A linear fit of the experimental data leads to the general equation: T (8C)¼(-142 * XFe) - (773 * Li pfu)þ1131, where XFe¼Fe/(FeþMn). The uncertainty is around 615 8C, and the influence of pressure is assumed to be negligible. By using this equation, exsolution temperatures were calculated for nine triphylite-sarcopside assemblages from pegmatites; these temperatures do not represent the crystallization temperatures of the phosphate nodules, but correspond to the closing temperature of the triphylite-sarcopside element exchange. Nevertheless, these temperatures, between 276 and 397 °C, are in fairly good agreement with those generally accepted for the crystallization of primary phosphate assemblages in granitic pegmatites.