Raman Spectroscopy of Orthopyroxene and Clinopyroxene at Simultaneous High Pressures and Temperatures: Implications for the Upper Mantle
Abstract
The upper mantle has a complicated structure which is controlled by the subduction of crust and the upwelling of the hot mantle. The pyroxene (ortho and clino) minerals comprise 30% of the upper mantle. Previous studies of pyroxenes show them to have atypical elastic properties compared to other silicates and are complex in structure, transforming to several different structures in a narrow range of pressure and temperature. The goal of this study is to use Raman spectroscopy to monitor lattice vibrational properties of pyroxenes at temperatures and pressures corresponding to the Earth's crust and mantle. Specifically, we will determine the phase stability of a variety of different pyroxenes, and pressure- and temperature- dependence of the Raman-active spectral peaks. The former helps us to plot out the phase diagram of these complicated minerals; the latter provides information on the thermodynamic behavior of the pyroxenes as they are subjected to extreme conditions. Raman spectroscopy measurements versus temperatures to 470 K on a natural enstatite (Mg90) and diopside (deKalb) isobarically to 20 kbar have been performed in an externally heated membrane diamond cell. The figure shows typical results on enstatite at 11 kbar (1.1 GPa) to 468 K. The pressure derivatives the Raman modes at room temperature match those from our previous studies. For example, the 687 cm^(-1) line in enstatite has a (∂lnν/∂lnV)_T of 0.6±0.3 versus pressure while the temperature shift at 1.8 GPa (∂lnν/∂lnV)_p is 0.7±0.1 versus temperature. Similar results were found for the other modes in both enstatite and diopside. Thus, intrinsic anharmonicity was substantially reduced from high values at ambient temperatures as shown by the reduced volume dependence of the Raman modes versus temperature, meaning at mantle conditions, the vibrational properties scale with volume and are unaffected by temperatures. Higher pressures and temperatures are anticipated and will be further discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMMR43A2129C
- Keywords:
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- 3924 MINERAL PHYSICS / High-pressure behavior;
- 3934 MINERAL PHYSICS / Optical;
- infrared;
- and Raman spectroscopy;
- 3939 MINERAL PHYSICS / Physical thermodynamics;
- 3994 MINERAL PHYSICS / Instruments and techniques