The high retentivity of monazite for Pb, its high U and Th concentrations, and its emergence in staurolite-zone metapelites suggests its use as a prograde thermochronometer. However, the monazite-producing reactions in pelites are as yet incompletely understood. We have investigated by experiment the suggestion that rhabdophane (LREEPO(4).nH(2)O) should be considered as an important reactant in monazite-producing reactions in pelitic compositions. Results of our combined hydrothermal and 1-bar heating experiments preclude rhabdophane from being the immediate precursor to neoformed monazite in lower amphibolite-facies metamorphic sequences and restrict rhabdophane occurrence to the uppermost portion of the crust. Dehydration of rhabdophane to form monazite appears to occur in a manner that may be analogous to the dehydration equilibria of zeolite minerals. Dry, 1-bar heating experiments indicate that La-rhabdophane persists (metastably?) to temperatures approaching 500 degrees C. XRD and visible-near-infrared spectroscopy examination of La-rhabdophane heated isothermally in air at temperatures ranging from 200 degrees to 700 degrees C indicate that structural water is largely retained up to 500 degrees C over heating intervals of up to 24 hr. Structural modifications involving H2O appear to take place between 300 degrees and 500 degrees C. Partial decomposition of La-rhabdophane to La-monazite is observed at 600 degrees C while rapid and complete reaction to La-monazite occurs at 700 degrees C. Experiments conducted under elevated water pressure (0.5-2 kbar) at temperatures between 200 degrees and 600 degrees C have not encountered conditions under which rhabdophane is stable relative to La-monazite+H2O, The low rhabdophane dehydration temperatures relative to the temperatures inferred for monazite formation in metapelites indicate that rhabdophane does not convert directly to monazite during pelite metamorphism. Rather, the ''monazite LREE component'' produced by rhabdophane breakdown must be incorporated in intermediate phases such as allanite and, perhaps more likely, LREE-oxides prior to monazite formation.
