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Thermodynamic and Elastic Properties of Grossular at High Pressures and High Temperatures: A First‐Principles Study
Author(s) -
Duan Longyu,
Wang Wenzhong,
Wu Zhongqing,
Qian Wangsheng
Publication year - 2019
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb017439
Subject(s) - grossular , pyrope , almandine , eclogite , mantle (geology) , geology , phengite , lawsonite , ringwoodite , coesite , andradite , mineralogy , stishovite , thermodynamics , subduction , geochemistry , quartz , metamorphic rock , fluid inclusions , tectonics , skarn , paleontology , physics
Thermodynamic and elastic properties of grossular (Ca 3 Al 2 (SiO 4 ) 3 , the calcium‐aluminium end‐member of garnet) at high pressures and temperatures are obtained using the first‐principles calculations based on the density functional theory. Our calculated results agree well with available experimental data. The elastic moduli and wave velocities of grossular exhibit the nonlinear pressure and temperature dependences. Although the bulk moduli of grossular are similar to those of pyrope (Mg 3 Al 2 (SiO 4 ) 3 ) and almandine (Fe 3 Al 2 (SiO 4 ) 3 ), its shear moduli are significantly larger than those of pyrope and almandine. Combining our calculated results with previously calculated elasticity of other upper‐mantle minerals, we estimated density and wave velocity profiles for the pyrolitic and eclogitic compositions along different mantle geotherms. We find that a pyrolitic composition along the normal geotherm can well reproduce the reference velocities and densities for the upper mantle, supporting a pyrolitic upper mantle. Although an eclogitic composition also has similar velocities to reference seismic models above 300 km, it is significantly denser than the pyrolitic composition and the ambient mantle. However, the velocities of eclogite along the cold geotherm are relatively larger than those of the pyrolitic composition, indicating that the presence of eclogite in subduction zones could produce high‐velocity anomalies. In addition, the decomposition of grossular to calcium perovskite and corundum at the conditions of the mantle transition zone can produce considerable velocities and density jumps, which probably provides an interpretation for the seismically observed multiple reflections at ~660 km.