1999/12/01 by John C. Ayers, Calvin Miller, Calvin F. Miller +2 · 3 citations
Earth and Planetary Sciences · #Geological and Geochemical Analysis #High-pressure geophysics and materials #earthquake and tectonic studies
paper · pdf · doi:10.2138/am-1999-11-1206
Monazite has become an important tool for geochronology, but it commonly exhibits complex internal zoning of composition and age. Experiments were conducted to characterize the textural development and the rate and mechanism of growth of finely powdered (<3 μm) natural monazite in quartzite ± H2O at 1.0 GPa and 1000 °C. Coarsely crushed quartz crystals <1 to >500 μm in diameter grew rapidly and progressively engulfed monazite crystals to form arrays of monazite inclusions. The mean diameter of all monazite crystals decreased in the first 24 h, then increased at a constant rate consistent with growth by grain boundary diffusion-controlled Ostwald ripening with a minimum rate constant K1/4 = 4.41 × 10-2 μm/s4. Using small quartz crystals of uniform diameter (~0.5 μm) in the starting material reduced quartz grain boundary mobility and limited the development of inclusions. Monazite grew by matrix volume diffusion-controlled Ostwald ripening with K1/3 = 1.02 × 10-2 μm/s3. In all run products, matrix coarsening produced linear crystal-size distributions that reflect continuous recrystallization and nucleation. Textural evidence suggests that matrix coarsening- induced coalescence was also an important growth mechanism.