Nb and Ta diffusion in titanite

• Diffusion of Nb and Ta have been measured in natural titanite.
• Nb and Ta profiles were measured with Rutherford Backscattering Spectrometry.
• Nb and Ta diffusivities in titanite are similar, suggesting little diffusive fractionation.
• Titanite should be moderately retentive of Nb and Ta chemical signatures.

Chemical diffusion of Nb and Ta under anhydrous, pO2-buffered conditions has been measured in natural titanite. The source of diffusants were mixtures of CaSiO3, TiO2, and Al2O3 powders, with either Nb or Ta oxides added. Experiments were run in evacuated sealed silica glass ampoules with solid buffers (buffered at NNO). Rutherford Backscattering Spectrometry (RBS) was used to measure Nb and Ta concentration profiles. The following Arrhenius parameters were obtained for Nb and Ta diffusion parallel to c over the temperature range 850–1252 °C under NNO-buffered conditions:DNb=3.23×10−9exp−299±12kJmol−1/RTm2s−1DTa=4.34×10−10exp−281±10kJmol−1/RTm2s−1.In contrast to the findings of Marschall et al. (2013) for rutile, there do not appear to be large differences between Ta and Nb diffusivities in titanite, so the likelihood of diffusive fractionation of these elements will not be great. Values for diffusion coefficients measured parallel to the a-axis for both elements are similar to those parallel to c, indicating that Nb and Ta diffusivities in titanite do not exhibit significant anisotropy. Nb and Ta diffusion in rutile is faster than diffusion of these elements in titanite at higher temperatures (by about 2 and 1.5 orders of magnitude at 700 °C for Nb and Ta, respectively), but because of the higher activation energies for diffusion of Nb and Ta in rutile, they will approach values for titanite with decreasing T. These diffusion data indicate that titanite should be moderately retentive of Nb and Ta chemical signatures, with diffusivities slower than those for Zr, O and Pb in titanite, but faster than those for the REE under most geologic conditions. These differences among diffusivities suggest that information derived from Zr thermometry, U–Pb geochronology, and geochemical tracers such as Ta, Nb and the REE may be decoupled in titanite under some conditions.