The potential use of OH-defects in enstatite as geobarometer |
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| Authors: | Felix Prechtel Roland Stalder |
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| Institution: | (1) Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52 f, 6020 Innsbruck, Austria |
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| Abstract: | In this study, single crystals of pure enstatite (Mg2Si2O6) were synthesised under water-saturated conditions at 4 and 8 GPa and 1,150°C with variable silica activity, leading to phase assemblages enstatite + forsterite, enstatite or enstatite + coesite. Run products were investigated using an FTIR spectrometer equipped with a focal plane array detector enabling IR imaging with a lateral pixel resolution of 2.7 μm. IR spectra within the OH-absorption region show two different groups of absorption bands: group 1 (wavenumbers at 3,592 and 3,687 cm?1) shows strongest absorptions for E||n β, whereas group 2 (wavenumbers at 3,067 and 3,362 cm?1) shows strongest absorptions for E||n γ. The groups are related to different defect types, group 1 to tetrahedral defects (T-site vacancies) and group 2 to octahedral defects (M-site vacancies). The intensity ratio of the bands within one group (i.e. A 3067/A 3362 and A 3592/A 3687) and the intensity ratio of E||n γ and E||n α in group 2 bands remain constant within error. In contrast, the intensity ratio of group 2 to group 1 absorption bands e.g. (A 3362)/(A 3687)] is sensitive to the SiO2 activity and pressure. On the basis of the results of this and previous studies, a barometer for pure orthoenstatite coexisting with forsterite can be formulated:\( P\,{\text{GPa}}] = 1.056 + \sqrt {{\frac{{1.025 - A_{{\left( {3362} \right)/\left {(3362) + (3687)} \right]}} }}{0.009}}} , \) where A (3362) and A (3687) are the integral absorbances of the component E||n γ of the absorption bands at 3,362 cm?1 and the component E||n β of the absorption band at 3,687 cm?1, respectively. |
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