Moderately Volatile Elemental Depletion and Potassium Isotope Fractionation during Evaporation in Laser-Heating Aerodynamic-Levitation Experiments

Moderately volatile element (MVE) depletion and isotopic fractionation are commonly observed in planetary materials. The mechanisms driving these phenomena are subject to intense debate, with some proposing evaporation at various stages of planetary formation as a potential explanation for both elemental depletion and isotopic fractionation. Studying the isotopic fractionation of MVEs can also be useful for investigating the conditions of high-temperature processes in the solar system and on Earth. Evaporation experiments to understand the behavior of isotopic fractionation under variable conditions provide a grounded context for interpreting the geochemical signatures of natural materials. Here we present new experimental data obtained from a novel approach that uses flowing gas to levitate samples and a laser to heat them. This approach offers the advantage of preventing undesired sample-container reactions at elevated temperatures and enabling ultrafast quenching. We analyzed the MVE depletion (e.g., Na, K, Cu, Zn, Ga and Rb) and potassium isotope fractionation associated with heating basalt and loess materials at temperatures up to 2046 °C under multiple oxygen fugacities. Our results show that the oxygen fugacity has a considerable effect on the observed MVE depletion and K isotope fractionation. This effect is likely driven by variations in the ambient gas composition and the specific evaporating species involved. We also found that the starting composition exerts a strong control on the MVE depletion and K isotope fractionation, which we attribute to differences in the relative timescales of K diffusion and evaporation among melt compositions. We additionally computed the evaporation coefficients of K and Zn across various temperature, oxygen fugacities and melt compositions, and systematically explored the relationships between evaporation coefficients and these factors.