The unique speciation of iron in calc-alkaline obsidians

Abstract Obsidians found in silicic domes and rhyolitic tuffs are an ideal system to understand the formation conditions of calc-alkaline felsic magmas. They have an unusual black to brown color. The optical absorption and Electron Paramagnetic Resonance (EPR) spectroscopic data presented in this study demonstrate that Fe-speciation in obsidians is different from that observed in other geological glasses (e.g., basaltic glasses) or in synthetic Fe-bearing silicate glasses. Iron sites partly occur in mixed valence clusters, located outside the glass structure: EPR data indicate the presence of superparamagnetic Fe-rich clusters. The absence of crystal field absorption bands extending towards the visible part of the optical spectrum, explains the original color of these glasses that do not show the blue, green or yellow hues that would be expected from the presence of Fe in the glass structure. The temperature dependence of the optical absorption is consistent with the presence of magnetite-like domains that give rise to original and efficient optical absorption processes. In particular, the optical spectra show a dramatic intensification at low temperature, with an amplitude which is not observed in synthetic glasses. The smallest magnetite-like clusters are transparent enough to contribute to the original absorption bands of obsidians, intensified through exchange-coupled pairing and [6]Fe2+-[6]Fe3+ and/or [6]Fe2+-[6]Ti4+ Inter-Valence Charge Transfer (IVCT) processes. The largest ones will contribute to the black color of obsidians. The presence of Fe-clusters in calc-alkaline obsidians reflects the last events of the magmatic history during emplacement and cooling. This unusual Fe-speciation suggests that the Fe-redox states in obsidians cannot be considered as a quenched witness of the initial magma.