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Fayalite oxidation processes in Obsidian Cliffs rhyolite flow, Oregon
-
Audrey M. Martin
,
Etienne Médard
Veröffentlicht/Copyright:
12. Mai 2015
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+1.6□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 Fe2+0.52Fe3+2.32□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
This would imply that Fe3+ can also be incorporated in the M1 site of olivine.
Received: 2014-5-8
Accepted: 2014-10-29
Published Online: 2015-5-12
Published in Print: 2015-5-1
© 2015 by Walter de Gruyter Berlin/Boston
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Schlagwörter für diesen Artikel
Olivine;
fayalite;
laihunite;
oxyfayalite;
rhyolite;
lithophysae;
oxidation
Artikel in diesem Heft
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- Highlights and Breakthroughs. Zircon dissolution and growth during metamorphism
- Review Paper. Linear partitioning in binary solutions: A review with a novel partitioning array
- Presidential Address. The many facets of apatite
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- Pieczkaite, ideally Mn5(PO4)3Cl, a new apatite-supergroup mineral from Cross Lake, Manitoba, Canada: Description and crystal structure
- First-principles prediction of pressure-enhanced defect segregation and migration at MgO grain boundaries
- Optical properties of siderite (FeCO3) across the spin transition: Crossover to iron-rich carbonates in the lower mantle
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