Abstract

This study investigates the oxidation of fayalite Fe22+SiO4 that is present in lithophysae from a rhyolite flow (Obsidian Cliffs, Oregon). Textural, chemical, and structural analyses of the successive oxidation zones are used to constrain: (1) the oxidation processes of olivine, and (2) the role of temperature, chemical diffusion, and meteoric infiltration. Petrologic analyses and thermodynamic modeling show that the rhyolite flow emplaced at 800–950 °C. Fayalite-bearing lithophysae formed only in the core of the lava flow. Variations in the gas composition inside the lithophysae induced the oxidation of fayalite to a laihunite-1M zone Fe12+Fe23+1(SiO4)2. This zone is made of nano-lamellae of amorphous silica SiO2 and laihunite-3M Fe2+1.6Fe3+1.60.8(SiO4)2 + hematite Fe2O3. It probably formed by a nucleation and growth process in the fayalite fractures and defects and at fayalite crystal edges. The laihunite-1M zone then oxidized into an “oxyfayalite” zone with the composition Fe2+0.52Fe3+2.321.16(SiO4)2. This second oxidation zone is made of lamellae of amorphous silica SiO2 and hematite Fe2O3, with a possible small amount of ferrosilite Fe2+SiO3. A third and outer zone, composed exclusively of hematite, is also present. The successive oxidation zones suggest that there may be a mineral in the olivine group with higher Fe3+ content than laihunite-1M. The transformation of laihunite-1M to this “oxyfayalite” phase could occur by a reaction such as  
0.24FeM12+laihunite-1M+0.06O2=0.16FeM13+oxyfayalite+0.08oxyfayalite+0.04Fe23+O3hematite

This would imply that Fe3+ can also be incorporated in the M1 site of olivine.

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