Solubility of Monazite-Cheralite and Xenotime in Granitic Melts, and Experimental Evidence of Liquid-Liquid Immiscibility in Concentrating REE

Abstract

We provide new experimental data on monazite, xenotime and U-Th-bearing cheralite solubility in slightly peralkaline to peraluminous granitic melts using dissolution and reverse (i.e. recrystallization after dissolution) experiments in water-saturated and flux-bearing (P + F + Li) granitic melts, at 800 °C and 200 MPa. Although a positive correlation between rare earth element (REE) solubility and melt peralkalinity is confirmed, monazite solubilities reported here are much lower than the values previously published. We suggest that the presence of elevated phosphorus concentrations in our melts depresses monazite solubility, principally because phosphorus complexes with Al and alkali, which normally depolymerize the melt through the formation of non-bridging oxygens. The new solubility data provide an explanation for the very low REE concentrations generally encountered in phosphorus-bearing peraluminous granites and pegmatites. This accounts for the compatibility of REE in peraluminous systems, as the early crystallization of REE-bearing minerals (mainly monazite and zircon) leads to progressive REE depletion during liquid differentiation. In addition, dissolution and reverse experiments on U-Th-bearing cheralite-monazite display liquid-liquid immiscibility processes in our slightly peralkaline glass. The immiscible liquid forms droplets up to 10 μm in diameter and hosts on average 35 wt% P2O5, 25-30 wt% F, 22 wt% Al2O3, 4 wt% CaO, 5 wt% Na2O, 2 wt% La2O3, and 12 wt% ThO2 + UO2. We believe that the droplets formed during the runs and may have coalesced to larger droplets during quenching. We suggest that liquid-liquid immiscibility is a possible mechanism of REE concentration in highly fluxed melts and should be considered in natural systems where REE are extremely concentrated (up to thousands of μg g-1) in magmatic rocks.