PHASE EQUILIBRIA MODELLING OF MONAZITE IN A TH-BEARING SYSTEM

Integrating monazite into existing pressure–temperature frameworks is an essential step towards fully realising the significant potential of this mineral in petrochronology. To further this endeavour, we present a predictive and readily adaptable equilibrium thermodynamic calculation framework involving solid solution for monazite, apatite, allanite, xenotime and Y+LREE+P+F+Th-bearing silicate melt. This framework comprises over 90 wt% oxide of the elemental components of these phases (Y, La, Ce, Nd, Th, P, Si, Ca) and includes all of the major end-member substitutions. We investigate the response of monazite and other accessory phases to closed and open system melting processes and changes to the major and trace element composition of the whole rock. We find that the incorporation of additional elements into monazite (La, Nd, Th, Ca, P and Si) displaces both the lower and upper bounds of monazite stability to higher temperatures relative to previous estimates. Exploration of bulk composition changes reveals that both Al and Ca affect the size and shape of the accessory mineral stability fields, in line with previous studies. We also show that increases to bulk LREE increase the mode and stability of monazite and decrease the proportion of the Th endmembers, cheralite and huttonite, in monazite. Changes to bulk Th have limited effect on the mode or stability field of monazite due to the generally low fraction of Th-endmembers in monazite, but do significantly change the total amount of Th-in-monazite.

Our modelling shows that monazite can be stable to much higher temperatures than previously modelled, to >1100°C in both open and closed systems, consistent with the natural rock record. Our models replicate the compositions and compositional trends from a natural dataset of over 5000 pressure–temperature-linked monazite analyses and present the first predictions of monazite growth above the solidus. We also present models for specific natural rock and monazite compositions which show considerable promise for the application of this framework to natural examples. The provision of this readily adaptable phase equilibria calculation framework adds an important new tool to the petrochronology toolbox.