Textural development of monazite during high-grade metamorphism: Hydrothermal growth kinetics, with implications for U,Th-Pb geochronology

Monazite has become an important tool for geochronology, but it commonly ex hibits complex internal zoning of composition and age. Experiments were con ducted to characterize the textural development and the rate and mechanism of growth of finely powdered (<3 mu m) natural monazite in quartzite +/-H2O at 1.0 GPa and 1000 degrees C. Coarsely crushed quartz crystals <1 to >500 mu m in diameter grew rapidly and progressively engulfed monazite crystals to form arrays of monazite inclusions. The mean diameter of all monazite c rystals decreased in the first 24 h, then increased at a constant rate cons istent with growth by grain boundary diffusion-controlled Ostwald ripening with a minimum rate constant K-1/4 = 4.41 x 10(-2) mu m/s(4). Using small q uartz crystals of uniform diameter (similar to 0.5 I-Lm) in the starting ma terial reduced quartz grain boundary mobility and limited the development o f inclusions. Monazite grew by matrix volume diffusion-controlled Ostwald r ipening with K-1/3 = 1.02 x 10(-2) mu m/s(3). In all run products, matrix c oarsening produced linear crystal-size distributions that reflect continuou s recrystallization and nucleation. Textural evidence suggests that matrix coarsening-induced coalescence was also an important growth mechanism. During annealing of fluid-filled rock, growing host crystals may occlude sm all monazite crystals, preserving their isotopic composition. Large monazit e crystals may pin grain boundaries, while smaller crystals may move with g rain boundaries by recrystallizing, a process that resets isotopic systems. Monazite crystals on grain boundaries may grow by Ostwald ripening to form rims and by coalescence. Accurate interpretations of monazite ages therefo re require knowledge of the texture/growth history of the rock and its date d grains.