Compression and Metastability of the Amphibole Tremolite to High Pressures and Temperatures through Raman Spectroscopy
Abstract
Amphiboles, which contain up to 2 wt% water in the form of hydroxyl units, contribute significantly to the transport of water to the deep lithosphere and upper mantle in subduction zones, where dehydration reactions make the water available for metasomatic processes and flux melting in the overlying mantle wedge. In this study, we investigate the high-pressure and temperature behavior of tremolite, a calcic amphibole with the end member composition [ ]Ca2Mg5Si8O22(OH)2. Raman spectroscopy is used to characterize tremolite at pressures to 49 GPa at room temperature using a symmetric type diamond anvil cell with methanol-ethanol-water and neon as pressure transmitting media, and at temperatures up to 540 K at ambient pressure, with an experimental focus on the bonding environment of the hydroxyl unit and the calcium and magnesium cation environments. Trends in the intensity of Raman peaks that involve Ca-O displacement suggest changes in the calcium cation bonding environment above 10 GPa. The behavior of the hydroxyl stretching vibration is concordant with previous infrared spectroscopic results, implying that the role of Davydov splitting in this amphibole is small. Moreover, as with a range of other hydroxyl-bearing minerals, the hydroxyl librations have slightly negative mode shifts. The peak shifts of ~16 modes with increasing pressure and increasing temperature are each fit to second degree polynomials and the results are used to determine the isothermal and isobaric mode-Grüneisen parameters (γiT and γiP), respectively. The intrinsic mode anharmonicity, calculated from the Grüneisen parameters, indicate that the internal mode vibrations are largely quasi-harmonic while the lower frequency vibrations associated with cations tend to display significant anharmonicity. The behavior inferred from the anharmonicity of vibrations is consistent with observations of previous static compression studies. Our work suggests that the general topology of the tremolite structure remains metastable to ~50 GPa, and provides constraints and guidance for future work on the metastability of amphiboles in subduction zones and the upper mantle, as well as the intrinsic crystallographic stability of the monoclinic amphibole structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMMR41B0053O
- Keywords:
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- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3999 General or miscellaneous;
- MINERAL PHYSICS;
- 3620 Mineral and crystal chemistry;
- MINERALOGY AND PETROLOGY;
- 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGY;
- 5430 Interiors;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 7208 Mantle;
- SEISMOLOGY