The unique speciation of iron in calc-alkaline obsidians

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