Effect of silica activity on OH^- IR spectra of Olivine: Implications for low-aSiO₂ mantle metasomatism

Hydrogen solubility and hydroxyl substitution mechanism in olivine at upper-mantle conditions are not only a function of pressure, temperature, water fugacity and hydrogen fugacity, but are also influenced by silica activity. Olivine synthesized in equilibrium with magnesiowüstite displays hydroxyl stretching bands in the wavenumber range from 3640 to 3430 cm^-1. In contrast, olivine in equilibrium with orthopyroxene shows absorption bands in a narrower wavenumber range from 3380 to 3285 cm^-1. The two fundamentally different spectra are assigned to hydroxyl in tetrahedral and octahedral sublattices, respectively. Olivine in equilibrium with orthopyroxene is also less capable of incorporating hydroxyl, relative to olivines in equilibrium with magnesiowüstite, by about a factor of ten. A comparison of spectra obtained as part of this study with hydroxyl spectra of natural mantle olivines shows that the latter display hydroxyl stretching patterns reminiscent of equilibrium with magnesiowüstite, although undoubtedly olivine in the Earth's mantle coexists with orthopyroxene. This may be attributed to a metasomatic overprint by a low-silica fluid and/or melt that was in reaction relationship with orthopyroxene. A likely metasomatic agent is a carbonatitic melt. When carbonatitic melts decompose to oxides and CO₂, they may temporarily impose a low-aSiO₂ environment inherited by the olivine structure. If this suggestion proves true, Fourier transform IR spectroscopy may be used to fingerprint metasomatic episodes in the lithospheric mantle.