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. 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. The presence of unusual symmetric Fe2+ octahedral sites explains the characteristic black to brown color of obsidian glasses, much distinct from that of synthetic Fe-bearing glasses. The Fe3+ sites partly occur in superparamagnetic domains, as evidenced by EPR data. The dramatic intensification of the optical absorption in the visible range indicates the presence of Fe-rich domains with Fe2+-Fe3+ and/or Fe2+-Ti4+ Inter-Valence Charge Transfer (IVCT) processes. These data are consistent with the presence of Fe-rich, mixed valence clusters, located outside the glass structure. The largest of these domains contribute to the black color of obsidians because they cause a background absorption extending throughout the visible spectrum as observed in magnetite. This unusual Fe-speciation reflects the last events of the magmatic history during emplacement and cooling, indicating that obsidians cannot be considered as a quenched witness of the initial magma.