Titanite dates crystallization: Slow Pb diffusion during super-solidus re-equilibration

Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U-Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650 degrees C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800 degrees C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid-mediated mass transfer processes on length scales of <100 mu m. These textural features are associated with variation in both Pb concentrations and distinct U-Pb isotopic compositions. Zr-in-titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840 degrees C, in the presence of a high-T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high-T conditions. We estimate diffusivities ranging from 2 x 10(-22) to 5 x 10(-25) m(2)/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide-held assumption that U-Pb titanite dates only reflect cooling ages.