Trace-Elements Partitioning between Phases 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 (with the addition of silicate to the starting mixtures) and minerals: apatite Ca5(PO4)3F, fluorite CaF2, and nacaphite (Na2Ca(PO4)F). Four series of experimental runs were carried out in an internally heated gas pressure vessels (IHPV) at a pressure of 500 MPa, using four types of starting mixtures: (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 (~5 mL), H2C2O4 (~1.5 mg), and a 3-mg 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: (i) calcite-rich melt with 20 mol % Na2O at a lower P2O5 content and (ii) sodic carbonate−phosphate melt with a low CaO concentration at higher P2O5 content and with up to 20 wt % CaO. The solubility of ZrO2, TiO2, and Nb2O5 in the calcite-rich quenched melts at 750°C is low and limited by the crystallization of Zr, Ti, and Nb oxides. At 950°C, these oxides did not form, and the concentrations of ZrO2, TiO2, and Nb2O5 increased in the melts with increasing P2O5/(P2O5 + CaO) ratio. REE concentrations (in wt %) in the apatite and coexisting Ca-rich carbonate melt increased 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 0.5 in a single run. In the runs of series IV, the nepheline-bearing starting mixtures yielded two immiscible melts: (1) SiO2-free sodic phosphate-rich melt with apatite and nepheline in run IV-7 and (2) aluminosilicate melt. Run IV-8 produced two immiscible melts, sodic−phosphate and silicate, with a P2O5 content in the silicate melt no higher than 25 wt %. Concentrations of TiO2, ZrO2, and Nb2O5 are much higher in the phosphate-rich melt than in the silicate melt with a lower phosphorus content. Their partition coefficients in run IV-7 are dTiO2 = 13.9, dZrO2 = 2.46, and dNb2O5 = 3.01, and lower, but these coefficients are still higher than in run IV-8: dTiO2 = 1.29, dZrO2 = 2.04, dNb2O5 = 1.24.