Triphylite–Sarcopside Miscibility Gap In the FeO–MnO–Li2O–P2O5–H2O System: Experimental Investigation and Thermometric Application To Granitic Pegmatites

Abstract In order to assess the stability of the primary triphylite + sarcopside assemblage, we performed hydrothermal experiments between 400 and 700 °C (Ni/NiO oxygen fugacity buffer, P = 1 kbar), starting from the Li(Fe 2+ 2.5– x Mn 2+ x )(PO 4 ) 2 ( x = 0.0, 0.5, 1.0) compositions, which represent the ideal compositions of triphylite + sarcopside assemblages in which both minerals occur in a 1:1 molar ratio. The triphylite + sarcopside assemblage is observed in all experiments, associated with other phosphates like (Fe 2+ ,Mn 2+ ) 2 P 2 O 7 , (Fe 2+ ,Mn 2+ )Fe 3+ 2 (PO 4 ) 2 (OH) 2 · n H 2 O, or Fe 3+ 4 (Fe 2+ ,Mn 2+ ) 3 (PO 4 ) 6 . Electron-microprobe and SIMS analyses show a progressive decrease of the Li contents in the triphylites, balanced by an increase of their Fe 2+ -contents, when the temperature increases. These compositional changes are due to the increase of the triphylite–sarcopside miscibility along the Li 2 (Fe 2+ ,Mn 2+ ) 2 (PO 4 ) 2 –Fe 2+ (Fe 2+ ,Mn 2+ ) 2 (PO 4 ) 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 (°C) = (–142 * XFe) – (773 * Li pfu ) + 1131, where XFe = Fe/(Fe + Mn). The uncertainty is around ±15 °C, 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.