Mobility of heavy rare earth elements in magmatic-hydrothermal systems: Experimental determination of DyPO4 solubility in supercritical fluids

Banerjee, Debarati; Waters, Laura E.; Hurtig, Nicole C.; Gysi, Alexander P.; Harlov, Daniel; Zhu, Chen; Migdisov, Artaches

doi:10.1016/j.chemgeo.2026.123248

Mobility of heavy rare earth elements in magmatic-hydrothermal systems: Experimental determination of DyPO₄ solubility in supercritical fluids

Journal Article · Fri Jan 09 00:00:00 EST 2026 · Chemical Geology

DOI:https://doi.org/10.1016/j.chemgeo.2026.123248· OSTI ID:3012019

Banerjee, Debarati ^[1]; Waters, Laura E. ^[1]; Hurtig, Nicole C. ^[1]; Gysi, Alexander P. ^[1]; Harlov, Daniel ^[2]; Zhu, Chen ^[3]; Migdisov, Artaches ^[4]

New Mexico Institute of Mining and Technology, Socorro, NM (United States)
GFZ Helmholtz Centre for Georesearch (Germany); China Univ. of Geosciences, Wuhan (China); University of Johannesburg (South Africa)
Indiana Univ., Bloomington, IN (United States)
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Dysprosium (Dy) is part of the heavy rare earth elements (REE) and a critical component in the production of high-performance super magnets used in wind turbines, electric vehicles, and other green technologies. The HREE phosphate, xenotime, is commonly associated with hydrothermal systems, where the mobility, fractionation, and deposition of REE are controlled by the stability of aqueous complexes and solubility of REE minerals which depend on temperature, pressure, and fluid composition (salinity, pH, and ligand availability). Here, we conducted a series of synthetic DyPO₄ solubility experiments in NaCl-HCl-bearing aqueous solutions (pH of 2 at 25 °C) from 500 to 700 °C and 1.5 kbar using externally heated pressure vessels. DyPO₄ displays a strong prograde solubility with temperature across all studied fluid salinities (0–0.5 m NaCl). The DyPO₄ solubility isotherms at 500 °C, 600 °C, and 700 °C show a complex relationship with salinity where DyPO₄ solubility is highest (39.2–1552 ppm at 500–700 °C) in 0 m NaCl solutions, decreases strongly (21–331 ppm at 500–700 °C) in the 0.06 m NaCl solutions and remains relatively constant in the 0.25 m (42.8–248 ppm) and 0.5 m NaCl (40.2–124 ppm) solutions. Thermodynamic modeling suggests that pH increases as a function of temperature and salinity, and controls solubility due to the stability of chloride complexes (i.e., DyCl⁺² and DyCl₂⁺) at 500–600 °C, transitioning to hydroxyl complexes (i.e., Dy(OH)₃⁰) dominating at 700 °C and relatively alkaline pH. Comparison with NdPO₄ solubility data indicates that the light REE in monazite is significantly more soluble than the heavy REE in xenotime in saline fluids at high temperature (500–700 °C), implying efficient light/heavy REE fractionation in supercritical fluids. The lower solubility of DyPO₄ relative to NdPO₄ suggests that at high temperatures, acidic (pH 2–4), low-salinity (∼5 wt% NaCl) fluids will favor the precipitation of xenotime over monazite as it will cool from 700 °C to below 500 °C. These observations align with paragenetic mineral assemblages in NYF pegmatites (e.g., Baveno, Italy), where xenotime commonly immobilizes heavy REE during the high-temperature hydrothermal stages.

View Accepted Manuscript (DOE)

Research Organization:: New Mexico Institute of Mining and Technology, Socorro, NM (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB)

Grant/Contract Number:: SC0022269

OSTI ID:: 3012019

Report Number(s):: DOE-NMT--0022269

Journal Information:: Chemical Geology, Journal Name: Chemical Geology Vol. 703; ISSN 0009-2541

Publisher:: ElsevierCopyright Statement

Country of Publication:: United States

Language:: English

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