Effect of iron and trivalent cations on OH-defects in olivine

Hydrogen incorporation in olivine involves many OH-defects, which will control the hydrogen solubility at mantle conditions. Several of these OH-defects are identified from the investigation of forsterite (the olivine Mg end-member). We study here the effect of Fe2+, Fe3+, Al3+ and Cr3+ on OH-defects in order to improve our understanding of the hydrogen speciation in natural olivine. Low-temperature infrared spectra (-194°C) are collected on synthetic and natural olivines. These spectra are then interpreted in the light of the theoretical determination of the structural, vibrational and infrared spectroscopic properties of Fe-related OH-defects, using first-principles calculations based on density functional theory. The presence of Fe2+ changes the cationic environment around the fully protonated vacancies in forsterite, leading to a slight modification of their infrared signatures. In particular, the presence of Fe2+ in an octahedral site adjacent to a hydrogarnet-type defect is likely responsible for the additional bands observed at 3599 cm-1 and around 3520-3550 cm-1 in Fe-doped olivines. Results show that the OH bands between 3310 and 3380 cm-1 are associated with the presence of trivalent cations. Specifically, two bands at 3323 and 3358 cm-1, commonly observed in natural olivine, are associated with the substitution of Mg2+ by Cr3+ while two similar bands at 3328 and 3353 cm-1 are associated with the substitution of Mg2+ by Fe3+. The presence of these defects and the "titanoclinohumite" defect in natural olivine clearly underlines the prominent role of trace elements on the hydrogen incorporation in lithospheric olivine