OH-defects in multiple-doped orthoenstatite at 4–8 GPa: filling the gap between pure and natural systems

OH-defects in orthoenstatite were studied experimentally between 4 and 8 GPa at 1150 °C in the system CaO–MgO–Al(2)O(3)–SiO(2)–Cr(2)O(3)–Na(2)O, leading to phase assemblages enstatite ± forsterite ± diopside ± garnet. In enstatite coexisting with garnet, total OH is negatively correlated with pressure. Conversely, in Al-poor systems without garnet, total OH is positively correlated with pressure, and both trends intersect around 8 GPa and ~1000 wt ppm H(2)O. IR-spectra of enstatite reveal several pressure sensitive features, such as (1) the absorbance of the absorption band at 3687 cm(−1), (2) the band position near 3400 cm(−1) and (3) the ratio (A(3240–3570)/A(3240–3730)) and their application as geobarometer in natural samples are evaluated. For garnet-bearing phase assemblages, the band ratio (A(3240–3570)/A(3240–3730)) in orthoenstatite defines a pressure trend in between that observed in the pure system MgO–SiO(2)–H(2)O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.