Zr and Hf diffusion in rutile

Abstract

Chemical diffusion of Zr and Hf under anhydrous conditions has been measured in synthetic and natural rutile. The sources of diffusant used were hafnon or zircon powders or a hafnon–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 Backscattering Spectrometry (RBS) was used to measure Zr and Hf diffusion profiles. From these measurements, the following Arrhenius relations were obtained: For Zr diffusion parallel to c, over the temperature range 750–1100 °C D Zr¦c = 9.8 × 10 − 15 exp(− 170 ± 30 kJ mol − 1 /RT) m 2 s − 1 For Hf diffusion parallel to c, over the temperature range 800–1000 °C D Hf¦c = 9.1 × 10 − 15 exp(− 169 ± 36 kJ mol − 1 /RT) m 2 s − 1 For Hf diffusion normal to c, over the temperature range 750–1050 °C D Hf⊥c = 2.5 × 10 − 12 exp(− 227 ± 62 kJ mol − 1 /RT) m 2 s − 1 Diffusivities for experiments buffered at QFM and NNO are similar to those run in air. Diffusivities in synthetic and natural rutile are likewise similar, indicating that these findings can be applied directly in determining Zr diffusivities in rutile in natural systems. These data indicate that rutile should be moderately retentive of Zr chemical signatures, with Zr diffusivities within an order of magnitude of those for Pb in rutile over most geologic conditions. When applied in evaluation of the relative robustness of the recently developed Zr-in-rutile geothermometer [T. Zack, R. Moraes, A. Kronz, Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer, Contributions to Mineralogy and Petrology 148 (2004) 471–488., E.B. Watson, D.A. Wark, J.B. Thomas, Crystallization thermometers for zircon and rutile, Contributions to Mineralogy and Petrology 151 (2006) 413–433.], these findings suggest that Zr concentrations in rutile will be somewhat more likely to be affected by later thermal disturbance than the geothermometer based on Zr concentrations in titanite [L. Hayden, E.B.Watson, D.A. Wark, A thermobarometer for sphene, Abstract, 16th V.M. Goldschmidt Conference (2006).], and much less resistant to diffusional alteration subsequent to crystallization than the Ti-in-zircon geothermometer [E.B. Watson, D.A. Wark, J.B. Thomas, Crystallization thermometers for zircon and rutile, Contributions to Mineralogy and Petrology 151 (2006) 413–433., E.B.Watson, T.M.Harrison, Zircon thermometer reveals minimum melting conditions on earliest Earth, Science 308 (2005) 841–844.].