A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (T-C) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including T-C and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z approximate to 29%), titanite (similar to 34%) and apatite (similar to 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a T-C-Z scale ''calibrated'' with field-based T-C data for titanite (greater than or equal to 680 +/- 20 degrees C) and apatite (similar to 500 degrees C). Extrapolation of this scale yields T-C estimates for the following minerals: staurolite (T-C greater than or equal to 1060 degrees C, Z approximate to 25%); garnet (greater than or equal to 1010 degrees C, similar to 26.5%); zircon (greater than or equal to 900 degrees C); monazite, xenotime, and epidote (greater than or equal to 750 degrees C, similar to 32%); and Ca-clinopyroxene (greater than or equal to 670 +/- 30 degrees C, similar to 34 +/- 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature. (C) 1997 Elsevier Science B.V.