Infrared Spectroscopic Investigation of Hydroxyl in Beta-(Mg,Fe)2sio4 and Coexisting Olivine - Implications for Mantle Evolution and Dynamics

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Physics and Chemistry of Minerals
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Wadsleyite (beta-(Mg,Fe)2SiO4) is a major constituent of the Earth's transition zone and is known to accommodate OH. The portion of the transition zone between 400-550 km could be an important source or sink for hydroxyl in plumes and slabs intersecting this region. Micro-infrared spectroscopy has been carried out on the beta-phase and coexisting metastable olivine synthesized in a multianvil apparatus at 14 GPa and 1550-1650 K under hydrous conditions. Single-crystal and polycrystal specimens of both phases were analyzed in the 1800-8500 cm-1 frequency region to determine the speciation, abundances, and partitioning behavior of the hydrous components in coexisting beta-phase and olivine. beta-phase spectra consistently show three distinct OH bands at 3329, 3580, and 3615 cm-1. OH concentrations range from 10000-65000 H/10(6) Si. A strong positive correlation of grain size and extent of transformation with OH concentration in the beta-phase indicates that grain-growth and transformation rates are enhanced in a hydrous environment. Olivine spectra are variable, but consistently show a prominent broad-band absorbance representing molecular H2O, consistent with the infrared signature of the starting material. OH concentrations in olivine range from < 300-1400 H/10(6) Si. The highest OH concentrations measured for olivine and the beta-phase may represent solubility limits, in which case the OH solubility ratio between these two phases is approximately 1:40. Where both phases coexist and are undersaturated with OH, the partitioning ratio of OH between them is about 1:100. The large solubility contrast between olivine and the beta-phase suggests a mechanism for hydrating the transition zone via olivine carried down in subducting slabs. Plumes impinging on an OH-rich upper transition region could cause H-2 or H2O to be released upon transformation of the beta-phase to olivine, resulting in initiation of secondary upwellings. If dissolution of OH weakens the beta-phase, and if OH is present in the mantle, the region between 400-550 km could be a zone of low viscosity.


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