Zirconium Systematics in HT Metamorphic Rutile: Implications for Geothermometry

The Zr-in rutile geothermometer is well-calibrated and widely used for temperature estimates of metamorphic rocks. Although the geothermometer has been successfully used to constrain peak temperatures for granulite facies rocks, experimental data suggest that Zr re-equilibration may occur following post-peak metamorphism. Some field-based studies reported a large range of Zr concentrations in rutile from granulites, possibly indicating such (partial) resetting. Further investigation into the Zr systematics of rutile is needed to enable better interpretation of Zr-in-rutile temperatures and their geologic significance. For this purpose, we investigated trace element contents of rutile grains from slowly cooled granulites. Rutile grains (120-280 μm) in three zircon-bearing granulite-facies metapelites from the Pikwitonei Granulite Domain, Canada, were analyzed by electron probe micro-analyzer. The Zr contents were investigated by measuring compositional profiles and X-ray maps. To investigate diffusive effects, Nb, Cr, and V, which should exhibit higher diffusivities than Zr, were measured simultaneously with Zr. Concentrations of all elements show minor variation within grains, but differ up to a factor 4 between grains. Elemental zoning does not indicate diffusive re-equilibration, demonstrating that the Zr-in-rutile thermometer is robust to post-peak metamorphic resetting and the diffusivity is slower than suggested in experimental studies. Zirconium zoning in Zr-rich grains (3000-4600 ppm) suggests activity of ZrO2 <1 at peak conditions in spite of zircon presence. This indicates that for dry UHT rocks the highest Zr contents best approximate peak temperatures. For the studied Pikwitonei granulites, the highest Zr-in-rutile concentrations correspond to ca. 900 °C, which exceeds previous estimates using solvus equilibria (ca. 820 °C). This demonstrates the advantage of the robust Zr-in-rutile thermometer in constraining peak temperatures of high and ultrahigh-temperature rocks.