Elasticity of Tremolite at High Pressures
Abstract
Amphiboles are important rock-forming minerals exhibiting diverse compositions. They are thermodynamically stable up to pressures of 3-4 GPa and temperatures of 1200 oC, i.e., conditions relevant for the crust and the upper mantle. Despite being a common rock-forming mineral, little is known about their elasticity. In particular, the full elastic constant tensor and its pressure dependence remain unknown for most of the amphibole varieties. Such information is useful to evaluate potential role that amphibole might play in explaining low velocity zones (LVZ) or newly discovered mid lithospheric discontinuity (MLD).
In this study, we explored the elasticity of tremolite- a calcic amphibole with Ca2Mg5Si8O22(OH)2 stoichiometry. We used first principles simulations based on density functional theory (DFT). We found that the pressure-volume results can be adequately described by Birch-Murnaghan equation of state. The hill averaged bulk modulus at ambient pressure, KH, determined from the full elastic moduli tensor is 83 GPa. The hill averaged shear modulus at ambient pressure, GH, determined from the full elastic moduli tensor is 57 GPa. The bulk and shear moduli are softer than the experimental results by 1 % and 2 % respectively. The experimental results are available only at ambient pressure, whereas we have explored high-pressure conditions relevant for the upper mantle. The full elastic moduli tensor for tremolite shows significant anisotropy with AVP and AVS being 36 % and 26 %. At higher pressures, the elastic moduli stiffen with pressure derivatives for the bulk and shear moduli being 5.4 and 1.3 respectively. While the P-wave anisotropy, AVP reduces upon compression, the S-wave anisotropy, AVS is enhanced at high pressures. Hence, it is important to consider this pressure dependence of elasticity and anisotropy while relating geophysical observations of low shear wave velocities with the elasticity of bulk metasomatized mantle containing amphiboles. Acknowledgement: The work is supported by NSF award EAR 1763215.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR33B0107P
- Keywords:
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- 3909 Elasticity and anelasticity;
- MINERAL PHYSICSDE: 3919 Equations of state;
- MINERAL PHYSICSDE: 7299 General or miscellaneous;
- SEISMOLOGYDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS