The effect of silica activity on the incorporation mechanisms of water in synthetic forsterite: a polarised infrared spectroscopic study

Pure forsterite crystals were grown from hydrous melts using controlled cooling experiments at 2.0 GPa and varying the bulk Mg/Si ratio from 2.0 to 1.5. Oriented single crystals were then studied by polarised infrared spectroscopy. The spectra of the samples with the lowest silica activity (aSiO2) contain the main OH bands in the range 3,620–3,450 cm−1 only. In contrast, the spectra of the samples synthesised with the highest aSiO2 contain additional pleochroic bands at 3,160, 3,220 and 3,600 cm−1. The variations are interpreted in terms of protonated silicon vacancies being dominant at low aSiO2 and Mg vacancies dominant at high aSiO2. Xenolithic mantle olivines generally do not have the spectrum expected for orthopyroxene buffered conditions, suggesting that they re-equilibrated with their host melts during ascent, but mantle olivine from the Zabargad peridotite massif probably is in equilibrium with the coexisting orthopyroxene.