Zr-in-rutile thermometry in eclogite at Jinheqiao in the Dabie orogen and its geochemical implications

Abstract

Trace element analysis was made by an electron microprobe on rutiles from ultrahigh-pressure eclogite at Jinheqiao in the Dabie orogen. The results show Zr contents of 37 to 118 μg·g−1, with significant variations between grains within the same thin section. Zr-in-rutile temperatures were calculated by the pressure-dependent thermometer, yielding 595 to 678°C at 4.1 GPa for the upper limit of metamorphic pressures. The temperatures are about 200 to 250°C lower than temperatures of 846±70°C calculated from garnet-omphacite Fe-Mg partition geothermometry, but 150 to 200°C higher than ∼460°C from quartz-rutile O isotope temperatures and rutile Pb diffusion closure temperature for the same specimen. This indicates that the diffusion rate of Zr in rutile is remarkably slower than that of Pb and O in rutile, and that the temperature of Zr-in-rutile is higher than the closure temperature corresponding to cooling age as dated by rutile U-Pb radiometric system. The latter is consistent with experimental result of Zr diffusion in rutile. Diffusion and re-crystallization with intensive retrograde fluid during exhumation of UHP metamorphic rocks may be the major reason why the Zr contents in rutile decreased and thus did not provide the record of the peak metamorphic temperatures. The remarkable difference in Zr content between rutile grains suggests that the distribution of Zr in rutile was in the state of disequilibrium due to differential diffusion and retrograde resetting.