Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer

Rutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature ( T) and pressure ( P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile - quartz - zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100 degreesC. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene: T (in degreesC) = 127.8 x ln (Zr in ppm) - 10 No pressure dependence was observed. An uncertainty in absolute T of +/- 50 degreesC is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly +/- 10 degreesC, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients.