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

Abstract

OH-defects in orthoenstatite were studied experimentally between 4 and 8 GPa at 1150 °C in the system CaO–MgO–Al ₂O ₃–SiO ₂–Cr ₂O ₃–Na ₂O, 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 ₂O. 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 ₂O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.