Program > Browse abstracts by speaker > Yaxley Greg

The mechanisms causing rare earth element (REE) enrichment in carbonatite systems remain poorly understood. Monazite (REEPO4) is a key REE ore mineral but its solubility in carbonate melts has not been determined. We conducted high pressure-temperature experiments to evaluate controls on monazite solubility in carbonatite melts. We ran synthetic mixtures of a natural monazite composition with sodic dolomitic carbonate melt. The mass ratio of the monazite to carbonate mixes ensured saturation in monazite. Experiments were conducted at 1.0 and 2.0 GPa and upper mantle and deep crustal temperatures. We examined the effects on monazite solubility of the addition of variable amounts of SiO2 and F- and varying Ca#. Experimental runs crystallised primary monazite crystals in equilibrium with carbonate melt.

Monazite solubility was expressed as the solubility product derived from the monazite solution reaction [REEPO4] = [REE3+] + [PO43-] (square brackets denote the mole fraction of anions in the melt). Data was fit to a multivariant linear regression model to predict solubility based on temperature, pressure, and composition. Monazite solubility in carbonate melt is high, peaking at >60 wt% CePO4 equivalent at 2 GPa and 1450°C. It increases with increasing temperature, decreases with increasing melt SiO2 and to a lesser extent increasing F-, and is not affected by melt Ca/[Ca+Mg] or pressure from 1.0 GPa to 2.0 GPa.

Monazite would be unlikely to crystallise directly from natural carbonatite magma until high levels of crystal fractionation have driven the evolved liquid to high LREE+P2O5 abundances. However, crustally emplaced carbonatites evolve by crystallising calcite and apatite, which may lower P2O5 in the residual liquid. The evolved melt could give rise to monazite formation by partial or complete replacement of earlier apatite, as observed in some carbonatites. Alternatively, reaction of carbonatites with siliceous crust may locally lower monazite solubility sufficiently to lead to saturation.