An Evaluation of Uncertainties in the Ti-in-Quartz and Ti-in-Zircon Paleothermometers in a Well Characterised Rhyolitic Eruption Deposit

Two widely accepted and utilised paleothermometers have been developed for silicic volcanic and intrusive rocks that use the temperature dependence of Ti solubility in quartz and zircon. Many published studies cite temperatures thus obtained to the nearest degree centigrade and take such data to construct elaborate histories of the grains concerned and their host rocks. Here we present data from the 27 ka Oruanui rhyolite (Taupo, New Zealand) in order to evaluate the uncertainties in such temperature estimates through propagation of the uncertainties in the individual contributing measurements. We have evaluated the Ti concentrations in Oruanui quartz and zircon (the latter with other trace elements and age determinations by U/Th disequilibrium) and in host glasses, using the SHRIMP-RG instrument at the Stanford-USGS facility. Glass compositions (for calculating αTiO2 in the melt) were determined by electron microprobe, and host magma temperatures estimated from Fe-Ti-oxide equilibria. Fe-Ti oxide temperatures have restricted ranges of 750 to 770 °C for typical Oruanui rhyolites as analysed here. Because many of the parameters concerned have independent uncertainties, we quantified propagation of the uncertainties using a Monte Carlo simulation (5000 iterations) to arrive at a ± 26° C 2-sigma uncertainty for temperature estimates from Ti concentrations in either zircon or quartz. Overall, the geological uncertainty (i.e., that associated with the variability in αTiO2) is subordinate to the actual uncertainty on the calibration line fitted through the Ferry and Watson (2007, Contrib. Mineral. Petrol. 154, 429-437) experimental data. The αTiO2 value for the Oruanui matrix glasses is very reproducible (0.74 ± 0.03, total range), implying that the uncertainty we derive represents a reasonable minimum value for systems with less homogeneous matrix glass compositions. Compositions of quartz-hosted melt inclusion glasses, however, show much more diversity in Ti concentrations which would increase uncertainties in αTiO2 and resulting temperature estimates. There is a good correlation between CL brightness and Ti content in the quartz, with temperature estimates having range of 705 to 845°C. Temperature ranges in zircons correlate with Eu* and Hf values, but do not correlate with age prior to eruption, and have a range of 670 to 820 °C. For zircons, the extremes are below the solidus and above the zircon saturation temperature, respectively. There are no traces of bright rims under CL, however, in the quartz crystals, nor any signs of late-stage overgrowths in the zircons indicating any late-stage changes in conditions. Temperatures from early versus late erupted dominant composition Oruanui rhyolites are similar, with diversities in the crystal compositions implying mixing of grains now present within a single sample occurred very shortly prior to eruption.