Fayalite oxidation processes in Obsidian Cliffs rhyolite flow, Oregon
Résumé
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: ( I) 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 degrees 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+square(1)(SiO4)(2). This zone is made of nano-lamellae of amorphous silica SiO2 and laihunite-3MFe(1.6)(2+)Fe(1.6)(3+)square(0.8)(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 Fe0.522+Fe2.323+square(1.16)(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 0.24Fe(M1)(2+laihunite-1M) + 0.06O(2) = 0.16 Fe-M1(3+''oxyfayalite'') + 0.08 square(''oxyfayalite'') + 0.04 (Fe23+O3hematite) This would imply that Fe3+ can also be incorporated in the M1 site of olivine.