Theoretical modeling of monazite growth in a low-Ca metapelite

Monazite growth and composition has been modeled in the system K2O–Na2O–CaO–MnO–MgO–FeO–Al2O3–SiO2–H2O–Y2O3–Ce2O3–P2O5–F and a pseudosection is presented that is contoured for monazite modal amount and YPO4 composition, garnet amount and YAG composition, and xenotime amount. Monazite amount and composition is strongly controlled by the presence of xenotime and when xenotime is absent, by the growth or consumption of garnet. Monazite grows continuously in the subsolidus region over most reasonable metamorphic P–T paths with YPO4 content generally increasing with increasing temperature, especially where xenotime is present. When melt is present, monazite is consumed with increasing melting and, in the absence of xenotime, decreases in YPO4 content. Two prograde forward models are presented to illustrate the effect of garnet fractional crystallization on the results. When xenotime is present and the modal amount of garnet is low (e.g., at low pressure), fractional crystallization has little effect on phase amount or composition. Where xenotime is absent and garnet is more abundant (e.g., at intermediate to high pressure), garnet fractionation dominates both monazite growth history and composition. In particular, YPO4 content of monazite drops to nearly zero at intermediate grades in a rock in which garnet has sequestered the Y as YAG component. This Y may be released by diffusion at higher temperature and promote monazite growth with a higher YPO4. Therefore, the growth history of monazite as inferred from YPO4 zoning may be quite complex and reflect different reactions and processes, even in a simple bulk composition.