Interpreting titanite U-Pb dates and Zr thermobarometry in high-grade rocks: empirical constraints on elemental diffusivities of Pb, Al, Fe, Zr, Nb, and Ce

Length scales of compositional heterogeneity in titanite from 750 to 1000 degrees C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U-Pb dates from rocks that reached peak temperatures<850 degrees C should be interpreted as the time of titanite (re)crystallization, not cooling ages. The length scales of Zr diffusion are negligible (<20 mu m), even at T>900 degrees C; thus, Zr-in-titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.