The effect of oxygen fugacity on hydroxyl concentrations and speciation in olivine: Implications for water solubility in the upper mantle

Abstract

The effect of oxygen fugacity on hydroxyl speciation and solubility in San Carlos olivine was investigated in a series of piston cylinder experiments at 2.0 GPa and 1100 and 1300 °C. The presence of enstatite was intended to fix silica activity and experimental fO 2 in most runs was regulated by lining the inner walls of the sample-containing Pt capsules with either Fe, Ni or Re foil. Polarised infrared absorbance spectra show that olivine annealed in capsules lined with Fe foil only contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (relatively high frequencies). Spectra of crystals annealed in capsules lined with Ni or Re foil show that these samples contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (high frequency), and at 3355 and 3325 cm − 1 (low-frequency). The concentrations of hydroxyl defects responsible for absorptions at 3355 and 3325 cm − 1 change as a function of oxygen fugacity and are, therefore, interpreted as Fe 3+-related OH centres. Hydroxyl solubility in our experiments is substantially less than current solubility laws might suggest. These differences may be attributed to variations in experimental procedure, but they also demonstrate that the incorporation of hydroxyl in olivine must be better understood before an accurate model to describe water solubility in the deep Earth can be constructed. Our data show that, when experiments conducted at identical P– T conditions are compared, olivine annealed at low fO 2 (below Fe–FeO) contains less than half as much water as crystals from relatively more oxidised experiments. Because natural mantle conditions approximate to those of our low fO 2 experiments, current solubility laws – which are based on data from relatively high fO 2 experiments – overestimate the amount of water that can be hosted in olivine in the upper mantle. From a consideration of defect equilibria we propose a mechanism for the incorporation of OH in olivine in a way that is appropriate for the actual range of conditions expected in the mantle.