THE ATiO2 PROBLEM: CURRENT LIMITATIONS TO Ti-BASED THERMOMETERS IN MAGMATIC SYSTEMS

Ti-thermometers are currently among of the most widely used methods to estimate crystallisation temperatures in silicic magmatic systems and beyond. Their success stems from their convenience: it is easy, fast, and cheap to measure in-situ Ti in mineral phases by LA-ICP-MS or EPMA, which are available in many institutions. Quartz and zircons, the most commonly used minerals for Ti-thermometers, are frequently present in most high-Si magmatic rocks. Geoscientists can obtain crystallisation temperatures for these mineral phases by applying a relatively easy to use equation derived from experimental calibrations (e.g. Wark and Watson, 2006 for quartz). However, utilisation of Ti thermometers requires a good constraint on aTiO₂, an aspect that is largely overlooked when these thermometers are employed. Thus, accurate estimations of aTiO₂ are key to obtain reliable temperature through such thermometers.

This presentation will review current methods for estimating aTiO₂ and their associated limitations, as well as common errors when utilising and interpreting Ti-based thermometers. Despite being often constrained to a fixed value, aTiO₂ must be understood as a dynamic parameter throughout crystallisation, heavily dependent on the TiO₂ content of the melt and on the crystallisation temperature, as demonstrated by its basis on thermodynamics and TiO₂-solubility models. The goal of this presentation is to provide guidelines for Earth scientists to successfully apply Ti-thermometers, in order to avoid inaccurate calculations and erroneous interpretations. When properly applied, Ti thermometers can be powerful tools for researchers, offering numerous possibilities to address persistent questions in magmatic systems.

Wark, D. A., & Watson, E. B. (2006). TitaniQ: a titanium-in-quartz geothermometer. Contributions to Mineralogy and Petrology, 152(6), 743-754.