The dynamic nature of aTiO2: Implications for Ti-based thermometers in magmatic systems

In recent decades, new Ti-based thermometers have found widespread use in geosciences, providing a convenient and powerful tool for investigating the crystallization temperatures of quartz and zircons in magmatic systems. However, a commonly overlooked aspect is the constraint of TiO2 activity (aTiO2liquid–rutile). Many studies assume aTiO2 to be constant or equate the presence of Ti-rich phases, such as ilmenite, with fixed activity levels. Using solubility models and data from natural systems, we demonstrate that aTiO2 is a dynamic parameter, influenced by temperature, mineral assemblage, and TiO2 content in the melt. Focusing on examples from several volcanic fields (Bishop Tuff, Fish Canyon Tuff, Yellowstone, and Shiveluch), we discuss the impact of these factors on aTiO2 and highlight how inadequate constraint of aTiO2 can lead to erroneous interpretations of magma storage conditions.