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

Abstract

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.