Major element zonation following rapid heating of homogeneous glass in superliquidus experiments

High-temperature melting experiments in gas-mixing furnaces are used to simulate magmatic processes and provide insight into melt properties at controlled conditions. These experiments assume that rapid melting results in formation of homogeneous molten samples, especially when glassed starting materials are used. As part of an investigation of evaporative loss of moderately volatile elements (MVE) from lunar magma, we performed short-duration, superliquidus experiments using homogeneous glass starting materials. Powdered glass pellets and glass fragments were suspended on Pt wire loops, and rapidly inserted into the hotspot of a 1-atm vertical gas-mixing furnace at 1350 °C and log fO2 = IW to IW +2 (oxygen fugacity at, or 2 log units above, the Fe-FeO buffer). Samples were held within the furnace for 5–60 min before being drop-quenched into water. Scanning electron microscopy and electron probe microanalysis of sectioned run products evidence progressive MVE loss from samples, but with additional, unexpected, redistribution of refractory elements (SiO2, TiO2, MgO, FeO, Al2O3, CaO). Refractory element zonation reflects early heating processes, and is progressively eradicated during prolonged heating via diffusion. Zonation in shorter duration experiments also reveals variable, and sometimes chaotic, disruption of samples during quenching. We propose that refractive element zonation arises due to phase separation during heating, driven by partial devitrification of the interiors of glassy samples. Our results demonstrate that element redistribution during melting, and physical disruption of samples induced by quenching, can significantly affect chemical homogeneity. These effects should be considered when designing and interpreting data from short-duration high-temperature experiments.