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

Stalder R, Karimova A, Konzett J - Contrib Mineral Petrol (2015)

Bottom Line: 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 (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-SiO2-H2O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.

View Article: PubMed Central - PubMed

Affiliation: Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52 f, 6020 Innsbruck, Austria.

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 (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-SiO2-H2O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.

No MeSH data available.


Related in: MedlinePlus

IR-spectra of OH-stretching vibrations shown as E//nα + E//nβ + E//nγ from polarised measurements on oriented crystal sections. Comparison of different phase assemblages at constant pressure (a), pressure dependence of the phase assemblage En + Fo + Gt (b) and En + Di (c). When data from two runs with the same conditions were available (e.g. AK36/37, AK38/43 and AK56/59), average spectra are shown. Bands assigned to OH associated with metal vacancies in pure enstatite are shown in b as dotted lines. The subsequently used division between high-wavenumber and low-wavenumber bands at 3570 cm−1 is also displayed as broken line. Spectra are normalised to 1 mm thickness and offset for clarity
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Fig2: IR-spectra of OH-stretching vibrations shown as E//nα + E//nβ + E//nγ from polarised measurements on oriented crystal sections. Comparison of different phase assemblages at constant pressure (a), pressure dependence of the phase assemblage En + Fo + Gt (b) and En + Di (c). When data from two runs with the same conditions were available (e.g. AK36/37, AK38/43 and AK56/59), average spectra are shown. Bands assigned to OH associated with metal vacancies in pure enstatite are shown in b as dotted lines. The subsequently used division between high-wavenumber and low-wavenumber bands at 3570 cm−1 is also displayed as broken line. Spectra are normalised to 1 mm thickness and offset for clarity

Mentions: IR-spectra of orthoenstatite exhibit many absorption bands between 3000 and 3700 cm−1 (Fig. 1), some of them amalgamating to broad absorption features similar to those known from natural specimens (e.g. Bell et al. 1995; Prechtel and Stalder 2012). Band positions for E//nα are more similar to E//nγ than to E//nβ (in accord with Prechtel and Stalder 2011, 2012), but considerable deviations from this behaviour are observed for samples with high Al-content. High-wavenumber bands >3570 cm−1 are most strongly developed for E//nβ, but in some samples also show significant contributions for E//nα. IR-spectra summed over all three polarisation directions E//nα + E//nβ + E//nγ show systematic changes dependent upon phase assemblage (Fig. 2a) and pressure (Fig. 2b, c). Specifically, high-wavenumber absorption bands such as the one at 3687 cm−1 tend to increase with pressure in comparison with low-wavenumber absorption bands, and the band near 3600 cm−1 shows a positive correlation between intensity and bulk Al-content, culminating in maximum intensities in garnet-bearing assemblages (Fig. 2a). Integral absorbances for bands >3570 cm−1 and <3570 cm−1 are listed in Table 3, and water concentrations determined by the calibrations of Bell et al. (1995), Libowitzky and Rossman (1997) and Stalder et al. (2012) are plotted in Fig. 3. With the calibration of Bell et al. (1995), spectra with a larger proportion of low-wavenumber bands yield systematically higher water contents than with the calibration of Libowitzky and Rossman (1997) (Fig. 3b), which is in agreement with Mosenfelder and Rossman (2013). In general, water concentrations of samples synthesised under similar P–T–x conditions are in good agreement. The only exception is the discrepancy between AK38 and AK43 (En + Di at 8 GPa), which was caused by partial fluid loss from AK38.Fig. 1


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

Stalder R, Karimova A, Konzett J - Contrib Mineral Petrol (2015)

IR-spectra of OH-stretching vibrations shown as E//nα + E//nβ + E//nγ from polarised measurements on oriented crystal sections. Comparison of different phase assemblages at constant pressure (a), pressure dependence of the phase assemblage En + Fo + Gt (b) and En + Di (c). When data from two runs with the same conditions were available (e.g. AK36/37, AK38/43 and AK56/59), average spectra are shown. Bands assigned to OH associated with metal vacancies in pure enstatite are shown in b as dotted lines. The subsequently used division between high-wavenumber and low-wavenumber bands at 3570 cm−1 is also displayed as broken line. Spectra are normalised to 1 mm thickness and offset for clarity
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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Fig2: IR-spectra of OH-stretching vibrations shown as E//nα + E//nβ + E//nγ from polarised measurements on oriented crystal sections. Comparison of different phase assemblages at constant pressure (a), pressure dependence of the phase assemblage En + Fo + Gt (b) and En + Di (c). When data from two runs with the same conditions were available (e.g. AK36/37, AK38/43 and AK56/59), average spectra are shown. Bands assigned to OH associated with metal vacancies in pure enstatite are shown in b as dotted lines. The subsequently used division between high-wavenumber and low-wavenumber bands at 3570 cm−1 is also displayed as broken line. Spectra are normalised to 1 mm thickness and offset for clarity
Mentions: IR-spectra of orthoenstatite exhibit many absorption bands between 3000 and 3700 cm−1 (Fig. 1), some of them amalgamating to broad absorption features similar to those known from natural specimens (e.g. Bell et al. 1995; Prechtel and Stalder 2012). Band positions for E//nα are more similar to E//nγ than to E//nβ (in accord with Prechtel and Stalder 2011, 2012), but considerable deviations from this behaviour are observed for samples with high Al-content. High-wavenumber bands >3570 cm−1 are most strongly developed for E//nβ, but in some samples also show significant contributions for E//nα. IR-spectra summed over all three polarisation directions E//nα + E//nβ + E//nγ show systematic changes dependent upon phase assemblage (Fig. 2a) and pressure (Fig. 2b, c). Specifically, high-wavenumber absorption bands such as the one at 3687 cm−1 tend to increase with pressure in comparison with low-wavenumber absorption bands, and the band near 3600 cm−1 shows a positive correlation between intensity and bulk Al-content, culminating in maximum intensities in garnet-bearing assemblages (Fig. 2a). Integral absorbances for bands >3570 cm−1 and <3570 cm−1 are listed in Table 3, and water concentrations determined by the calibrations of Bell et al. (1995), Libowitzky and Rossman (1997) and Stalder et al. (2012) are plotted in Fig. 3. With the calibration of Bell et al. (1995), spectra with a larger proportion of low-wavenumber bands yield systematically higher water contents than with the calibration of Libowitzky and Rossman (1997) (Fig. 3b), which is in agreement with Mosenfelder and Rossman (2013). In general, water concentrations of samples synthesised under similar P–T–x conditions are in good agreement. The only exception is the discrepancy between AK38 and AK43 (En + Di at 8 GPa), which was caused by partial fluid loss from AK38.Fig. 1

Bottom Line: 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 (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-SiO2-H2O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.

View Article: PubMed Central - PubMed

Affiliation: Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52 f, 6020 Innsbruck, Austria.

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 (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-SiO2-H2O and that found in orthopyroxenes from natural mantle peridotites, suggesting that the application of IR-spectra as proxy for pressure is justified.

No MeSH data available.


Related in: MedlinePlus