THE INSTABILITY OF RHABDOPHANE AND ITS UNIMPORTANCE IN MONAZITE PARAGENESIS

The high retentivity of monazite for Pb, its high U and Th concentrati ons, and its emergence in staurolite-zone metapelites suggests its use as a prograde thermochronometer. However, the monazite-producing reac tions in pelites are as yet incompletely understood. We have investiga ted by experiment the suggestion that rhabdophane (LREEPO(4).nH(2)O) s hould be considered as an important reactant in monazite-producing rea ctions in pelitic compositions. Results of our combined hydrothermal a nd 1-bar heating experiments preclude rhabdophane from being the immed iate precursor to neoformed monazite in lower amphibolite-facies metam orphic sequences and restrict rhabdophane occurrence to the uppermost portion of the crust. Dehydration of rhabdophane to form monazite appe ars to occur in a manner that may be analogous to the dehydration equi libria of zeolite minerals. Dry, 1-bar heating experiments indicate th at La-rhabdophane persists (metastably?) to temperatures approaching 5 00 degrees C. XRD and visible-near-infrared spectroscopy examination o f La-rhabdophane heated isothermally in air at temperatures ranging fr om 200 degrees to 700 degrees C indicate that structural water is larg ely 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 20 0 degrees and 600 degrees C have not encountered conditions under whic h rhabdophane is stable relative to La-monazite+H2O, The low rhabdopha ne dehydration temperatures relative to the temperatures inferred for monazite formation in metapelites indicate that rhabdophane does not c onvert directly to monazite during pelite metamorphism. Rather, the '' monazite LREE component'' produced by rhabdophane breakdown must be in corporated in intermediate phases such as allanite and, perhaps more l ikely, LREE-oxides prior to monazite formation.