Monazite-(Ce) formation as a product of hydrothermal chevkinite-(Ce) alteration: Experimental constraints

Chevkinite-group minerals are rare earth element (REE) carriers in alkaline magmatic and metasomatic systems, yet their hydrothermal stability and alteration pathways remain poorly constrained. We experimentally investigated the hydrothermal transformation of chevkinite (Cvk)-(Ce) under conditions relevant to alkaline rock environments, with particular emphasis on the formation and composition of secondary monazite (Mnz)-(Ce). Twenty-four cold-seal autoclave experiments were conducted at 500–600°C and 200–400 MPa for durations of 21–63 days, using natural Cvk-(Ce) from the Diamer district (Pakistan) together with quartz, albite, and variable accessory components, and reacting them with F- and Ca-bearing aqueous fluids. Mnz-(Ce) crystallized in 200 MPa in six experiments, typically as microcrystalline aggregates (<20 µm) surrounding or replacing primary Cvk. The main associated phases were britholite-(Ce) and titanite. Electron backscattered diffraction confirmed full crystallinity, while electron probe microanalysis) showed that all products are compositionally homogeneous Mnz-(Ce) with dominant phosphate components (∼97.7 wt%), minor silicate substitution (∼2.3 wt%), and no significant REE fractionation relative to the precursor Cvk. Experimental results indicate that Mnz-(Ce) formation is promoted by F- and P-rich fluids and inhibited by high Ca, Fe, and Al activities, which instead favour epidote-supergroup minerals and britholite. Systems with low Ca preferentially stabilized Mnz-(Ce) and albite. These experiments demonstrate that Mnz-(Ce) can form readily during hydrothermal alteration of Cvk -(Ce) under closed-system conditions, providing new constraints on REE mobility, mineral stability, and metasomatic processes in alkaline geological environments.