Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites

Abstract

Electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite-sillimanite transition (620 ± 15°C) and sillimanite-orthoclase (680 ± 15°C) zones of the Cretaceous Ryoke metamorphic belt, southwest Japan. Monazites from pelitic gneisses are of metamorphic origin, euhedral to subhedral and chronologically homogeneous, giving chemical ThU-total Pb isochron (CHIME) ages of 98.8 ± 3.3–98.0 ± 3.2 Ma. Two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca. 1700 Ma. Most detrital monazite grains are heterogeneous in the ThO 2 and UO 2 concentrations and have multiple or single rims as young as ca. 100 Ma. Several detrital monazite grains are well rounded in form, exhibit homogeneous Th and U distributions and show a Pb diffusion profile in the margin. The width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss: 18–22 μm for 620°C and 48–58 μm for 680°C. Assuming the isothermal diffusion of Pb from homogeneous monazite spheres during a 5 Ma duration of peak metamorphism, we obtain diffusion coefficients of 1.9(±0.3) × 10 −21 and 1.5(±0.3) × 10 −20 cm 2/s at 620°C and 680°C, respectively. These data derive an activation energy of 2.44(+2.85/−1.26) × 10 5 J/mol and a frequency factor of 3.4 × 10 −7 (8.5 × 10 −12–2.2 × 10 7 cm 2/s, taking account of uncertainties of ±15°C in the temperatures and of ±20% in the diffusion coefficients. The diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for Pb in monazite that has been poorly understood so far.