Trace Elements Partitioning in the System P₂O₅–CaO–Na₂O ± (SiO₂ + Al₂O₃)–F–H₂O–CO₂ at 500 MPa

The distribution of Ti, Zr, Nb, La, Sm, Yb, and Y was experimentally studied between phosphate-carbonate melts, silicate-bearing melts (at addition of silicate in starting mixtures) and minerals – apatite (Ca₅(PO₄)₃F), fluorite (CaF₂), and nacaphite – (CaNaPO₄F). Four experimental series were carried out in an internally heated gas pressure vessels at a pressure of 500 MPa using four types of starting melts: (1) Ca₅(PO₄)₃F + CaCO₃ + Na₂CO₃ + NaAlSiO₄ at 1100–750 °C;
(2) Ca₅(PO₄)₃F + CaCO₃ + NaF + NaAlSiO₄ at 950 °C; (3) NaPO₃ + CaCO₃ + CaF₂ + NaF + NaAlSiO₄ at 900 °C; and (4) NaPO₃ + CaCO₃ + NaF + NaAlSiO₄ at 900 °C with variable proportions of P₂O₅, CaO, Na₂O, and SiO₂. H₂O, H₂C₂O₄, as well as a mixture of trace element oxides were added in equal mass proportions to all starting compositions. The experimental products were analyzed by electron probe microanalysis. Depending on the CaO and P₂O₅ proportion, the silicate-free starting mixtures in three former series yielded two types of quenched melts: calcite-rich melt with 20 mol % Na₂O at lower P₂O₅ content and sodic-carbonate-phosphate melt with low CaO at higher P₂O₅ content. The solubility of ZrO₂, TiO₂, and Nb₂O₅ in the obtained calcite-rich quenched melts at 650 °C is low and constrained by the crystallization of Zr, Ti, and Nb oxides. At 1000 °C, these oxides are not formed, and the concentrations of ZrO₂, TiO₂, and Nb₂O₅ increase in melts with increasing P₂O₅/(P₂O₅ + CaO) ratio. The REE partition coefficients between apatite and coexisting Ca-rich carbonate melt increase with increasing P₂O₅ content from 0.2 to 0.9 for La₂O₃, from 0.25 to 0.75 for Sm₂O₃, from 0.2 to 0.6 for Yb₂O₃, and from 0.2 to 0.4 for Y₂O₃, reaching in a single case 0.5. In runs of series IV the silicate-bearing starting mixtures yielded two immiscible melts: SiO₂-free phosphate-rich melt with apatite and nepheline, and aluminum-silicate melt in run IV-7. Run IV-8 produced two immiscible melts, sodic-phosphate and silicate, with P₂O₅ content no more than 25 wt %. The concentrations of TiO₂, ZrO₂, Nb₂O₅ are much higher in the phosphate-rich melt than in the silicate melt with the lower phosphorus content. Their partition coefficients in run IV-7 are dTiO₂ = 13.9, dZrO₂ = 2.46, dNb₂O₅ = 3.01, and are less, but still more than one in run IV-8: dTiO₂ = 1.29, dZrO₂ = 2.04, dNb₂O₅ = 1.24.