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 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.