ELECTRON-MICROPROBE OBSERVATIONS OF PB DIFFUSION IN METAMORPHOSED DETRITAL MONAZITES

Electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite-sillimanite transition (620 +/- 15 degrees C) and sillimanite-orthoclase (680 +/- 15 degrees 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 ThO2 and UO2 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 mu m for 620 degrees C and 48-58 mu m for 680 degrees 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) x 10(-21) and 1.5(+/-0.3) x 10(-20) cm(2)/s at 620 degrees C and 680 degrees C, respectively. These data derive an activation energy of 2.44(+2.85/-1.26)x 10(5) J/mol and a frequency factor of 3.4 x 10(-7) (8.5 x 10(-12)-2.2 x 10(7)) cm(2)/s, taking account of uncertainties of +/-15 degrees 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.