Elasticity of Orthoenstatite at High Pressure and Temperature: Implications for the Origin of Low V_P/V_S Zones in the Mantle Wedge

Abstract

Orthopyroxene (opx) is an important mineral in petrologic models for the upper mantle. Its elastic properties are fundamental for understanding the chemical composition and geodynamics of the upper mantle. Here we calculate the elastic properties of orthoenstatite (MgSiO3), the Mg end‐member orthopyroxene under upper mantle pressure and temperature conditions using first principle calculations with local density approximation. Bulk and shear moduli increase nonlinearly with pressure at mantle temperatures, but the shear modulus and VS show very weak pressure dependence in comparison with VP. Compared to other major minerals in the upper mantle, orthoenstatite has the lowest compressional velocities (VP), shear velocities (VS), and VP/VS ratio down to the depth of approximately 300 km. The enrichment of opx in the upper mantle can cause the unusually low VP/VS observed in the mantle wedge.

Thermodynamic and Elastic Properties of Grossular at High Pressures and High Temperatures: A First‐Principles Study

“…An isotropic distribution function of strain Grüneisen azimuth angles is assumed in this method, which is true for isotropic material, and the anisotropy of the crystal is considered through the anisotropic pressure dependence of lattice parameters. The method has been applied successfully to many minerals, including MgO (Wu & Wentzcovitch, 2011), ferropericlase , bridgmanite (Shukla et al, 2015), stishovite and the CaCl 2 -type silica (Yang & Wu, 2014), superhydrous phase B (Yang et al, 2017), ringwoodite (Núñez Valdez et al, 2012), olivine and wadsleyite (Núñez Valdez et al, 2013), pyrope (Hu et al, 2016), Orthoenstatite (Qian et al, 2018), diopside , and Corundum .…”

“…. The elastic data of orthopyroxene, clinopyroxene, and olivine were obtained from previous elastic studies (Núñez Valdez et al, 2013;Qian et al, 2018;Zou et al, 2018), and the nonnegligible effect of 10% Fe in orthopyroxene, 10% in clinopyroxene, and 8% in olivine is adopted (Bass & Weidner, 1984;Duffy & Vaughan, 1988;Kandelin & Weidner, 1988;Zhang et al, 2013). For garnet, we derived the elastic data from the pyrope (Hu et al, 2016), almandine (Arimoto et al, 2015), grossular (this study), and majoritic garnet (Aoki & Takahashi, 2004;Irifune et al, 2008;Kono et al, 2012).…”

East‐West Differential Underthrusting of the Indian Lithospheric Plate Beneath Central Tibet Revealed by Imaging VP/VS

“…Contrary to the two hypotheses above, Hacker and Abers (2012) argue that it is difficult to preserve abundant, stable α quartz under upper mantle conditions and that most minerals have V P /V S > 1.75, so even 100% enrichment of orthoenstatite cannot produce the observed low V P / V S in the subduction zone mantle wedge. However, in a recent study, Qian et al (2018) calculated the elastic properties of orthoenstatite, the Mg end-member orthopyroxene under upper mantle pressure and temperature conditions, and supported that enrichment of orthopyroxene in the mantle can cause the unusually low V P /V S observed in mantle wedge. Here our low V P /V S found in shallow parts of eastern Tibet can be ascribed to enrichment of orthopyroxene in the subduction zone mantle wedge.…”

“…A combination of V P and V S provides more insight into the mantle physical state than either V P and V S alone, which have different sensitivities to partial melting, temperature, and composition (Wagner et al, 2008). The ratio V P /V S has been widely used to infer the thermal and compositional state of the upper mantle (Hacker & Abers, 2012;Lee, 2003;Niu et al, 2004;Qian et al, 2018;Rossi et al, 2006;Scafidi et al, 2009;Schutt & Humphreys, 2004;Wu & Wentzcovitch, 2017). Usually, high V P /V S indicates that partial melting existed (Barron & Priestley, 2009).…”

Once Again on Preliminary Reference Earth Model

“…Detailed reviews of the experimental work on this topic (through 2016) have been presented in Anderson (2007), Litasov and Shatskiy (2016), Liu and Lin (2014), and Sumita and Bergman (2007), and the results of these studies are analyzed from the perspective of geomechanics in Guliyev (2017cGuliyev ( , 2018a. The results of new studies are reflected in numerous publications (Antonangeli et al, 2018;Birch, 1952;Chantel et al, 2016;Chen et al, 2018;Das et al, 2017;Fan, Xu, et al, 2019;Hakim et al, 2018;Huntington et al, 2017;Komabayashi, 2014;Komabayashi et al, 2019;Kusakabe et al, 2019;Mao et al, 2018;Qian et al, 2017;Sakairi et al, 2017;Sakamaki et al, 2016;Singh et al, 2017;Stekiel et al, 2017;Takahashi et al, 2019;Tanto et al, 2018;Tateno, 2018;Thompson et al, 2017;Wakamatsu et al, 2018;Xiong et al, 2018;Xu et al, 2019).…”