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Abstract

As a moderately volatile, redox-sensitive chalcophile and siderophile element, Te and its isotopic composition can inform on a multitude of geochemical and cosmochemical processes. However, the interpretation of Te data from natural settings is often hindered by an insufficient understanding of the behavior of Te in high-temperature conditions. Here, we present the results of Te evaporation and isotopic fractionation in silicate melting experiments. The starting material was boron-bearing anorthite-diopside glass with 1 wt% TeO₂. The experiments were conducted over the temperature range of 868-1459 °C for 15 min each, and at oxygen fugacities (logfO₂) relative to the fayalite-magnetite-quartz buffer (FMQ) of FMQ-6 to FMQ+1.5, and in air. Evaporation of Te decreases with decreasing fO₂. For high-temperature experiments performed at >1200 °C Te loss is accompanied by Te isotope fractionation towards heavier compositions in the residual glasses. By contrast, Te loss in experiments performed at temperatures <1200 °C typically resulted in lighter Te isotopic compositions in the residues relative to the starting material. In air, Te evaporates as TeO₂, whereas at lower oxygen fugacities we predict the evaporation of Te₂, using Gibbs free energy minimization calculations. In air, the experimentally determined kinetic isotopic fractionation factor for δ^128/126Te at T > 1200 °C is α_K = 0.99993. At reducing conditions, Te likely substitutes as Te^2- for O^2- in the melt structure and becomes increasingly soluble at highly reducing conditions. Consequently, Te evaporation is not predicted for volcanic processes on reduced planetary bodies such as the Moon or Mercury.