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

Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their U-Pb isotope systematics meas ured before and after the runs. Runs were carried out with monazite gr ain size fractions ranging from 40 to 125 mu m in diameter, and on < 1 to 15 mu m fine powdered material at 400-750 degrees 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 similar to 5500 and similar to 800 ppm, respectively. All hydrothermally treated mona zite grain size fractions yielded concordant U-Pb ages of similar to 3 80 Ma and no loss of Pb and U. In contrast, monazite powder revealed t emperature-dependent Pb loss. Pb concentrations decreased to similar t o 590 ppm at 750 degrees C, similar to 620 ppm at 650 degrees C, and s imilar to 710 ppm at 400 degrees C, indicating Pb loss of similar to 2 6%, similar to 22% and similar to 11%, respectively, of the initial Pb content. No fractionation of Pb isotopes occurred; all analyses yield ed Pb-207/Pb-206 ages close to 380 Ma. U concentrations remained alway s unchanged. All recrystallized monazites from the powdered starting m aterial define a discordia with intercept ages of 379.3 +/- 2.3 and -7 +/- 18 Ma, the latter indicating the hydrothermally induced Pb loss a t the time of the experiment. SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor diss olution features confined to the grain surfaces. Run products from mon azite powders showed recrystallization and grain growth induced by a d issolution-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650 degrees and 750 degrees C. A dissolution-precipitation process in presence of a fluid phase ra ther than Pb volume diffusion acts as a very efficient mechanism for t he resetting of the U-Pb isotope system of monazites, even at low temp eratures. This process is most important in shear zones where channeli zed fluid flow may occur.