An improved understanding of Zr-in-rutile thermometry in application to ultrahigh temperature metamorphic rocks

Abstract

Zirconium in rutile (ZIR) concentrations were analysed in samples from two localities in ultrahigh temperature (UHT: >900°C) metamorphic terranes: Anakapalle, Eastern Ghats Province, India (~1030 °C), and Ayatollah Island, Napier Complex, Antarctica (~1050 °C). ZIR temperatures from analyses of these rocks were categorised according to their microstructural setting—inclusion versus grain boundary—and distance from xenocrystic zircon. ZIR temperatures from three of four samples were all or mostly well below UHT conditions—up to 400 °C in some cases. However, upon reintegrating exsolved zirconium back into rutile grains the distribution of ZIR temperatures in all samples becomes bimodal, more typical of the global UHT ZIR thermometry dataset. There is no obvious trend in the ZIR concentration or temperature data relating to the proximity of rutile to xenocrystic zircon, or to the microstructural setting of rutile, suggesting that rutile and zircon (and quartz) communicate over sufficiently long length scales during the prograde and retrograde history. Stunted Si diffusion does not appear to have been a dominant factor in determining ZIR concentrations and temperatures (contrast with Taylor-Jones & Powell, 2015), as there are no known examples of rutile grains preserving peak (i.e. >1000 °C) temperatures that have not exsolved zircon. The major consequence is that ZIR thermometry may almost never preserve the peak UHT conditions in regional terranes where cooling is prolonged. The bimodal ZIR concentration and temperature distribution may relate to a subtle, undetected microstructural control on access to Si and Zr reservoirs combined with different timing of growth of different rutile grains.