PARTIAL RESETTING OF THE U-PB ISOTOPE SYSTEM IN MONAZITE THROUGH HYDROTHERMAL EXPERIMENTS: AN SEM AND U-PB ISOTOPE STUDY

Abstract

Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their U-Pb isotope systematics measured before and after the runs. Runs were carried out with monazite grain size fractions ranging from 40 to 125 μm in diameter, and on < 1 to 15 μm fine powdered material at 400-750°C, 3 kbar and 28 days. U-Pb analyses of the starting material resulted in concordant U-Pb ages of 377 +/- 2 Ma. U and Pb concentrations were ~ 5500 and ~ 800 ppm, respectively. All hydrothermally treated monazite grain size fractions yielded concordant U-Pb ages of ~ 380 Ma and no loss of Pb and U. In contrast, monazite powder revealed temperature-dependent Pb loss. Pb concentrations decreased to ~ 590 ppm at 750°C, ~ 620 ppm at 650°C, and ~ 710 ppm at 400°C, indicating Pb loss of ~ 26%, ~ 22% and ~ 11%, respectively, of the initial Pb content. No fractionation of Pb isotopes occurred; all analyses yielded207 Pb/206Pb ages close to 380 Ma. U concentrations remained always unchanged. All recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379.3 +/- 2.3 and - 7 +/- 18 Ma, the latter indicating the hydrothermally induced Pb loss at the time of the experiment.SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces. Run products from monazite powders showed recrystallization and grain growth induced by a dissolution-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650° and 750°C.A dissolution-precipitation process in presence of a fluid phase rather than Pb volume diffusion acts as a very efficient mechanism for the resetting of the U-Pb isotope system of monazites, even at low temperatures. This process is most important in shear zones where channelized fluid flow may occur.