The solubility of monazite in carbonate melts at upper mantle and crustal conditions

Abstract

Rare earth elements (REEs) are essential components in numerous modern electronic devices and industrial applications, making their abundant and cost-effective supply crucial for societal and economic development. Australia boasts significant REE deposits, in which the REE are predominantly phosphate-hosted, yet extraction from these ores remains complex, hazardous, and economically unviable. REE mineralization is commonly associated with carbonate melts, yet the mechanisms driving their enrichment remain poorly understood. Through high pressure-temperature experimental investigations, this study examines the conditions favouring the crystallization of REE-hosting minerals, focusing on monazite, within carbonate magmas. Piston-cylinder experiments conducted at high pressures and temperatures replicated upper mantle and crustal conditions, and carbonate melts with monazite saturation. Advanced microscopy techniques (Electron Probe Micro-Analyser, Scanning Electron Microscope) were deployed to analyse melt and monazite compositions. Systematic variation of melt compositions enabled the development of a new multivariant linear regression model to predict monazite solubility based on temperature, pressure, and composition. This study reveals substantial monazite solubility in carbonate melt across diverse pressure-temperature-composition ranges, peaking at 61.08 wt% CePO4 equivalent at 2.0 GPa and 1450C. Experimental findings highlight the substantial impact of SiO2 and fluorine concentrations on monazite solubility, with minimal influence from melt Ca# and pressure variations ranging from 1.0 GPa to 2.0 GPa. These results advance our understanding of monazite behavior, particularly in carbonatite magma crystallization. The study concludes with investigations into monazite crystallization within natural carbonatite melts, focusing on fractionation through equilibrium crystallization, and providing further support for the hypothesis of monazite formation from brine melt. Show more