In-situ determination of sulfate vs. chloride aqueous complexes in high P-T fluids − A new take on the fractionation of light/heavy REE in c

Rare earth elements (REE) are of critical importance to the development of new technologies and transition to carbon-free energy. REE exhibit complex mineralogy with minerals enriched in the more common light (L)REE (La−Gd) and others in the scarcer, and thus more valuable, heavy (H)REE (Tb−Lu plus Y). Yet, economic geologists are currently unable to predict the extent of LREE over HREE enrichment in the mineral deposits, which prevents the development of sound REE exploration strategies. To develop new metallogenic models, it is necessary to constrain the speciation and solubility of LREE and HREE in high P-T fluids. To do so, we propose to conduct in-situ XAS and SRXF measurements in a RH-DAC to describe REE-Cl-SO4 complexation and its effect on the solubility of monazite/xenotime in Cl and S-bearing hydrothermal fluids.