Chemical diffusion of Zr and Hf under anhydrous conditions was measured in synthetic and natural rutile. The sources of diffusant used were hafnia or zircon powders or a hafnia-rutile mixture. Experiments were run in crimped Pt capsules in air, or in sealed silica glass capsules with solid buffers (to buffer at NNO or QFM). Rutherford back-scattering spectrometry was used to measure Zr and Hf diffusion profiles. From these measurements, Arrhenius relationships were obtained. For Zr diffusion parallel to c at 750 to 1100C:

DZr||c (m2/s) = 9.8 x 10-15exp[-170(kJ/mol)/RT]

For Hf diffusion parallel to c, at 800 to 1000C:

DHf||c (m2/s) = 9.1 x 10-15exp[-169(kJ/mol)/RT]

For Hf diffusion normal to c, at 750 to 1050C,

DHf⊥c (m2/s) = 2.5 x 10-12exp[-227(kJ/mol)/RT]

Diffusivities in synthetic and natural rutile were similar; indicating that these findings could be applied directly in determining Zr diffusivities in rutile in natural systems. These data indicated that rutile should be moderately retentive of Zr chemical signatures, with Zr diffusivities within an order of magnitude of those for Pb in rutile under most geological conditions. When applied to evaluation of the relative robustness of a Zr-in-rutile geothermometer, these findings suggested that Zr concentrations in rutile were somewhat more likely to be affected by later thermal disturbance than a geothermometer based upon Zr concentrations in titanite, and much less resistant to diffusional alteration subsequent to crystallization than the Ti-in-zircon geothermometer.

Zr and Hf Diffusion in Rutile. Cherniak, D.J., Manchester, J., Watson, E.B.: Earth and Planetary Science Letters, 2007, 261[1-2], 267-79