Applications and limitations of trace elements-accessory phase thermometry

Abstract

The partition of Zr and Ti in the coexisting rutile and zircon is inferred to be a function of temperatures. The robustness of these two accessory phases, their common occurrence in a variety of magmatic and metamorphic rocks and sluggish diffusion of tetravalent elements (Ti4+ and Zr4+) make these thermometers appropriate for constraining the temperatures of magmatic and metamorphic rocks. Ti-in-zircon thermometer is commonly considered as pressure-insensitive but experimental studies showed that Zr-in-rutile depends on pressure. The present study evaluates the applicability of these thermometers with the help of natural examples reported from different metamorphic rocks. The study reveals that for Zr-in-rutile thermometer, the calibration by Watson (2006, pressure insensitive) yield temperatures similar to calibrated by Tomkins (2007) for alpha and beta quartz fields up to the pressure of 12 kbar. However, at high pressure (>25kbar), the coesite field calibration of Tomkins(2007) yield higher temperature than the calibration of Watson (2006). The study also shows that in most of the cases the Zr-in-rutile thermometers yield higher temperatures than the temperatures recorded in Ti-in-zircon thermometry. Both the thermometers yield wide temperature ranges which deviates significantly from the results of the independent thermometer applied to corresponding rocks. The reasons for the variation include post-peak alteration and retrogression, diffusion, effective removal of one of the phases by getting included in porphyroblastic phases and lattice defect. The results are specifically ambiguous for high to ultra-high temperature metamorphic rocks. Therefore, this study emphasizes that the results of these trace element exchange thermometers should be coupled with other results of other independent geothermometers to record meaningful temperature estimates for magmatic and metamorphic rocks.