Abstract
OH-defects in orthoenstatite were studied experimentally between 4 and 8 GPa at 1150 °C in the system CaO–MgO–Al2O3–SiO2–Cr2O3–Na2O, 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 H2O. 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 (A3240–3570/A3240–3730) and their application as geobarometer in natural samples are evaluated. For garnet-bearing phase assemblages, the band ratio (A3240–3570/A3240–3730) in orthoenstatite defines a pressure trend in between that observed in the pure system MgO–SiO2–H2O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.
Highlights
Hydrous defects make nominally anhydrous minerals (NAMs) important hosts for water in the Earth’s mantle and affect their physical properties such as rheology
Water concentrations show a positive correlation with pressure for the phase assemblage enstatite + diopside (Fig. 5), 1 3
Water incorporation into enstatite depends on the coexisting phase assemblage, but this trend is governed by the Al-content in enstatite rather than the coexistence with an Al-rich phase
Summary
Hydrous defects make nominally anhydrous minerals (NAMs) important hosts for water in the Earth’s mantle and affect their physical properties such as rheology We present results from an experimental study at 4–8 GPa in the system CaO–MgO–Al2O3–SiO2–Cr2O3– Na2O, which is intermediate in its complexity between the strongly simplified systems such as MgO–SiO2–H2O and the natural peridotitic systems. In this system, orthoenstatite ± diopside ± forsterite ± garnet coexist, reproducing the assemblage typically stable in peridotitic upper mantle at depths
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