Lead diffusion in monazite

We have measured the tracer diffusion rate of Pb in natural, annealed, gem-quality monazite from Alexander Co., NC, USA, and Riverside Co., CA, USA. Observations from the literature concerning low T annealing in monazite suggest the possibility that Pb diffusion at geologic conditions occurs, in effect, within a similarly annealed lattice. Our experiments were performed by evaporating an aqueous solution containing Pb-204 Onto a crystal face and then heating the charge to 1000-1200 degrees C for 4 to 36 days. To the extent that the compositions of the two samples differed, we observed no effect of composition upon the rate of Pb diffusion. We do find that transport parallel to the c-axis is similar to 2-5 times slower than that measured perpendicular to c. For Pb diffusion perpendicular to c, the Arrhenius parameters are Q = 180 +/- 48 kJ/mol and log D-o = -14.18 +/- 1.54 (for Do in m(2)/s). While these parameters now allow for a direct calculation of closure temperatures for monazite, we believe a more useful form of our data for geochronologists is the fraction of Pb lost from crystals, expressed as a function of T, time, and crystal size. We have applied this formalism to discordant monazite U-Pb ages from the recent literature and find that Pb diffusion at rates extrapolated from our experiments can account for the observed degree of discordancy. We suggest that, rather than employing the concept of closure temperature to evaluate possible Pb loss in monazite, the diffusion data presented here be used in Pb loss models based upon T and duration conditions appropriate for a given geological setting. Copyright (C) 1997 Elsevier Science Ltd.