Effect of silica activity on OH-IR spectra of olivine: Implications for low-aSiO(2) 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 magnesiowustite displays hydroxyl stretching bands in the wavennumber range from 3640 to 34 (3) over bar0 cm( -1). In contrast, olivine in equilibrium with orthopyroxene shows absorptio n bands in a narrower wavenumber range from 3380 to 3285 cm(-1). The two fu ndamentally different spectra are assigned to hydroxyl in tetrahedral and o ctahedral sublattices, respectively. Olivine in equilibrium with orthopyrox ene is also less capable of incorporating hydroxyl, relative to olivines in equilibrium with magnesiowustite, by about a factor of ten. A comparison o f spectra obtained as part of this study with hydroxyl spectra of natural m antle olivines shows that the latter display hydroxyl stretching patterns r eminiscent of equilibrium with magnesiowustite, although undoubtedly olivin e 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 r eaction relationship with orthopyroxene. A likely metasomatic agent is a ca rbonatitic melt. When carbonatitic melts decompose to oxides and CO2, they may temporarily impose a low-aSiO(2) environment inherited by the olivine s tructure. If this suggestion proves true, Fourier transform IR spectroscopy may be used to fingerprint metasomatic episodes in the lithospheric mantle .