INTERPRETING TITANITE U-PB DATES AND ZR TEMPERATURES IN HIGH-GRADE ROCKS: EMPIRICAL CONSTRAINTS ON ELEMENTAL DIFFUSIVITIES

Length scales of compositional heterogeneity in titanite from 750 to 1000°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, as hypothesized by Kohn (2017; Reviews in Mineralogy and Geochemistry); because of this, U–Pb dates from rocks that reached peak temperatures < 850°C should be interpreted as the time of titanite (re)crystallization, not cooling ages. The length scales of Zr diffusion are negligible (<20 μm), even at T > 900°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 due to different diffusion mechanisms operating in natural titanite (a complex solid solution) than those in the near-end-member CaTiSiO₅ used in experiments.