FLUORINE-AIDED RE-EQUILIBRATION OF U-PB AND ZR IN TITANITE

The mechanism(s) by which U-Pb dates and 4+ cation concentrations re-equilibrate can be enigmatic. Titanite petrochronology can be used to determine both the timing (U-Pb) and temperature (Zr thermometry) of geologic events. Such data are often interpreted in the context of thermally mediated volume diffusion. Some studies of natural titanite have estimated granulite-facies closure temperatures for both Pb and Zr in titanite—higher than what experiments predict—yet, some titanite does equilibrate or recrystallize at amphibolite-facies conditions. We investigated the re-equilibration process of titanite grains in quartzofeldspathic orthogneiss from the Western Gneiss Region (WGR) of Norway using LASS (laser ablation split stream ICP-MS), EPMA, and EBSD. The titanite grains have BSE-bright cores with twins, BSE-dark rims that truncate core zoning, and adjacent neoblasts. The cores have partially reset U-Pb dates as old as ~650 Ma, whereas the rims and neoblasts are uniformly young, ≤ 400 Ma. The cores have high 4+ element concentrations (e.g., 500 ppm Zr) and low F (0.3 wt%), whereas the rims and neoblasts are depleted in Zr (125 ppm) and enriched in F (0.5 wt. %). The Zr concentrations in the cores and rims + neoblasts are compatible with temperatures of ~850°C and ~750°C, respectively. EBSD shows that the rims are crystallographically continuous with the core and the neoblasts have rotated away from the core by as much as 20°. The F-rich rims with reset U-Pb dates and different Zr concentrations are consistent with fluorine-aided recrystallization. These data suggest that i) high temperatures or dynamic recrystallization alone may not be enough to reset titanite, and ii) a F-rich metamorphic fluid was locally responsible for resetting titanite in the WGR after UHP metamorphism.