Monazite is a light rare earth element (LREE)-bearing phosphate mineral that is present in a wide variety of rock types, has an extremely variable composition reflecting host rock conditions, and is a robust geochronometer that can preserve crystallization ages through a long history of geological events. Monazite crystals typically contain distinct compositional domains that represent successive generations of monazite, which in turn, can provide a detailed record of the geologic history of its host rocks. The electron microprobe can be used to characterize the geometry of compositional domains, analyze the composition of each domain, and, when carefully configured, determine the U-Th-total Pb age for domains as small as 5 μm in width. These data allow the monazite to be linked with, and place timing constraints on, silicate processes in the host rocks. Current applications span a broad range of geologic processes in igneous, metamorphic, hydrothermal, and sedimentary rocks.

Recent advances in the dating of monazite using the electron microprobe indicate that ThSiO₄- and/or CaTh(PO₄)₂-enriched intergrowths in monazite can give ages younger than the original monazite. The morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which Th, Si, and Ca are mobile. This hypothesis has been tested in the piston-cylinder apparatus at 1000 MPa and 900 °C utilizing a natural, unzoned, Th- and Pb-bearing monazite-(Ce) and Na₂Si₂O₅+H₂O. During the experiment, a subset of the monazite grains acquired ThSiO₄-enriched areas with sharp compositional boundaries devoid of Pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite. These experiments support the hypothesis that similar Th-enriched or Th-depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid-induced alteration via coupled dissolution-reprecipitation. If so, such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre-existing radiogenic Pb is removed during alteration.

Thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of Hidra, southwestern Norway. Formation of these inclusions is demonstrated to be the result of dissolution-reprecipitation processes that occurred in an effectively closed chemical system. The reaction was initiated by interaction between the early crystallizing orthophosphate minerals and the evolving pegmatite fluid. This fluid was dominated by H₂O, but also contained F and minor amounts of Cl, and significant quantities of dissolved Na⁺ and K⁺. This is an example of auto-metasomatism and highlights a natural case where Th-U-Si-enriched orthophosphate minerals have been partially altered such that the Th, U, and Si have been converted to secondary inclusions within the now Th-U-Si-depleted orthophosphate via reaction with fluids rich in alkali elements.