Trace Elements Partitioning in the System PO–CaO–NaO ± (SiO + AlO)–F–HO–CO at 500 MPa

The distribution of Ti, Zr, Nb, La, Sm, Yb, and Y was experimentally studied between phosphatecarbonate melts, silicate-bearing melts (at addition of silicate in starting mixtures) and minerals - apatite (Ca(PO)F), fluorite (CaF), and nacaphite - (CaNaPOF). 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) Ca(PO)F) + CaCO + NaCO + NaAlSiO at 1100-750°С; (2) Ca(PO)F + CaCO + NaF + NaAlSiO at 950°С; (3) NaPO + CaCO + CaF + NaF + NaAlSiO at 900°С; and (4) NaPO + CaCO + NaF + NaAlSiO at 900°С with variable proportions of РО, СаО, NaO, and SiO. HO, HCO, 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 СаО and PO proportion, the silicate-free starting mixtures in three former series yielded two types of quenched melts: *calcite-rich melt with 20 mol % NaO at lower PO content and **sodic-carbonate-phosphate melt with low CaO at higher PO content. The solubility of ZrO, TiO, and NbO in the obtained calcite-rich quenched melts at 650С is low and constrained by the crystallization of Zr, Ti, and Nb oxides. At 1000С, these oxides are not formed, and the concentrations of ZrO, TiO, and NbO increase in melts with increasing PO/(PO+CaO) ratio. The REE partition coefficients between apatite and coexisting Ca-rich carbonate melt increases with increasing PO content from 0.2 to 0.9 for LaO, from 0.25 to 0.75 for SmO, from 0.2 to 0.6 for YbO, and from 0.2 to 0.4 for YO, reaching in 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 PO content no more than 25 wt %. The concentrations of TiO, ZrO, NbO 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 dTiO = 13.9, dZrO = 2.46, dNbO = 3.01, and are less, but still more than one in run IV-8: dTiO = 1.29, dZrO = 2.04, dNbO = 1.24.