Magmatic stability of eudialyte-group minerals (EGM) and element distribution between EGM and peralkaline silica-undersaturated melts

Eudialyte-group minerals (EGM) are unique tracers of peralkaline silica-undersaturated melts. They receive global interest as potential resources for high-field-strength elements (HFSE, e.g., Zr, Nb, Ta) and rare-earth elements (REE), which are critical materials for modern technologies. The main condition for magmatic crystallization of EGM in general is that the concentration of Zr in the parental melt should reach saturation. The solubility of EGM in peralkaline melts from the system Na₂O–CaO–Al₂O₃–SiO₂ ± H₂O at temperatures between 750 and 1000 °C and pressures of 100 and 200 MPa was investigated. Newly formed crystalline phases in the run products are EGM grains, parakeldyshite and albite. EGM are stable between 750 and 900 °C, and melt incongruently to parakeldyshite between 900 and 1000 °C. EGM crystallization from peralkaline silica-undersaturated melts at nominally dry conditions and 750–850 °C requires a minimum of 0.2–0.22 wt% ZrO₂ in the melt. In experiments with the addition of H₂O, saturation is attained at much higher ZrO₂ concentrations (1.1–2.85 wt%) in the same temperature interval. REE and HFSE are strongly compatible with eudialyte-group minerals as the EGM-melt distribution coefficients (D) vary from 2 to 90 with falling temperature. The lowest D values are observed in experiments with the highest Zr solubility, i.e., at high temperature and in hydrated compositions. Light REE and especially La tend to have lower D values than the heavy REE.