Limits...
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.


Range of diopside–enstatite solid solution (grey boxes) in runs, where both pyroxenes coexist. Solvus for 4 GPa (broken line) and 6 GPa (solid line) after Nickel and Brey (1984) for comparison
© Copyright Policy - OpenAccess
Related In: Results  -  Collection


getmorefigures.php?uid=PMC4459430&req=5

Fig4: Range of diopside–enstatite solid solution (grey boxes) in runs, where both pyroxenes coexist. Solvus for 4 GPa (broken line) and 6 GPa (solid line) after Nickel and Brey (1984) for comparison

Mentions: In charges where two pyroxenes coexist, enstatite (En) contains on average 3 mol% diopside (Di) component and diopside contains between 15 and 22 mol% enstatite component (Table 2; Fig. 4). The latter value is slightly higher than expected for a temperature of 1150 °C in the system CaO–MgO–SiO2 (Nickel and Brey 1984). This discrepancy, however, can be explained if the temperature history of the experiments is considered: the observed range in pyroxene Ca-contents is thought to be a result of continuous pyroxene crystallization over a temperature interval of 150 °C (i.e. from 1300 to 1150 °C, cf. Figure 4) and reflects incomplete re-equilibration of Ca in orthopyroxene at the final run temperature of 1150 °C. This assumption is consistent with temperatures of 1275 °C derived from two-pyroxene thermometry based on the calibration by Nimis and Taylor (2000). An excess diopside component in enstatite incorporated at an early stage of the experiments, however, is not likely to affect water incorporation into enstatite to any significant degree as will be discussed in the next paragraph.Fig. 4


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)

Range of diopside–enstatite solid solution (grey boxes) in runs, where both pyroxenes coexist. Solvus for 4 GPa (broken line) and 6 GPa (solid line) after Nickel and Brey (1984) for comparison
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4459430&req=5

Fig4: Range of diopside–enstatite solid solution (grey boxes) in runs, where both pyroxenes coexist. Solvus for 4 GPa (broken line) and 6 GPa (solid line) after Nickel and Brey (1984) for comparison
Mentions: In charges where two pyroxenes coexist, enstatite (En) contains on average 3 mol% diopside (Di) component and diopside contains between 15 and 22 mol% enstatite component (Table 2; Fig. 4). The latter value is slightly higher than expected for a temperature of 1150 °C in the system CaO–MgO–SiO2 (Nickel and Brey 1984). This discrepancy, however, can be explained if the temperature history of the experiments is considered: the observed range in pyroxene Ca-contents is thought to be a result of continuous pyroxene crystallization over a temperature interval of 150 °C (i.e. from 1300 to 1150 °C, cf. Figure 4) and reflects incomplete re-equilibration of Ca in orthopyroxene at the final run temperature of 1150 °C. This assumption is consistent with temperatures of 1275 °C derived from two-pyroxene thermometry based on the calibration by Nimis and Taylor (2000). An excess diopside component in enstatite incorporated at an early stage of the experiments, however, is not likely to affect water incorporation into enstatite to any significant degree as will be discussed in the next paragraph.Fig. 4

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.