First principles investigations on the elasticity and phase stability of grossular garnet

Abstract

[1] In order to understand the fate of subducted continental materials by means of mechanisms such as tectonic erosion and sediment subduction, it is important to understand the high-pressure stability and elastic properties of grossular garnet. We study these questions using the first principles computation method. Grossular garnet was found to dissociate into an assemblage of CaSiO 3 Ca-perovskite (Pv) and Al 2 O 3 corundum (Cor) at about 23.4 GPa, accompanied by remarkable jumps of compressional wave (8.0%), shear wave (11.6%), bulk sound (5.7%) velocities, and density (12.1%). Although Pv with the grossular garnet composition was suggested experimentally after the decomposition reaction, this phase was found to be less stable than an assemblage of CaPv and Cor at pressures higher than 20 GPa. This indicates that observed Pv with the grossular garnet composition is metastable but can be obtained because of the slow kinetics of the garnet decomposition reaction. Our results imply that the dissociation of grossular garnet (as well as that of jadeite) included in continental materials subducted into the deep mantle increases the complexity of the 660 km discontinuity and can explain seismically observed multiple reflections. We have also investigated the anisotropy of elastic velocities of grossular garnet, CaPv, and Cor and found that CaPv is the most anisotropic mineral. This implies that CaPv could produce seismic anisotropy in the uppermost lower mantle.

Elasticity and phase stability of pyrope garnet from ab initio computation

“…6). (Kawai and Tsuchiya, 2012a). Filled circles indicate experimental results for pyrope (Zou et al, 2012a,b;Zhang et al, 1998;Leger et al, 1990;Gwanmesia et al, 2006).…”

“…The aggregate velocities of the mixture (Fig. 6) are obtained using their elasticity and densities reported in our previous studies (Tsuchiya et al, 2004a;Kawai and Tsuchiya, 2012a), assuming that the elastic properties of the MgPv + Cor solid solutions can be approximated to be those of the MgPv + Cor mixture. The present calculations also clearly show that the breakdown of pyrope at 19.7 GPa produces considerable velocity jumps of the compressional wave (+13.1%), shear wave (+21.7%), and bulk sound (+7.0%) velocities together with a density jump (+10.1%) (Fig.…”

Small shear modulus of cubic CaSiO3 perovskite

“…10.1002/2015GL063446 Tsuchiya, 2015], and grossular (A P =~4%; A S =~10%) [Kawai and Tsuchiya, 2012] at the lower part of the mantle transition zone (MTZ), even a small amount of CaPv could produce large anisotropy with its lattice preferred orientation [Karato, 1998]. This may be related to seismologically reported anisotropy in the MTZ [Montagner and Kennett, 1996].…”

Elasticity of Continental Crust Around the Mantle Transition Zone

“…8.1). We use reported phase relations: jadeite ⇔ CF-type phase + stishovite (Kawai and Tsuchiya 2012a), grossular garnet ⇔ Cor + Ca-pv (Akaogi et al 2009;Kawai and Tsuchiya 2012b), stishovite ⇔ α-PbO 2 phase (Tsuchiya et al 2004), and K-hollandite I ⇔ II (Nishiyama et al 2005;Kawai and Tsuchiya 2012c). Since the high P-T dissociation boundary of grossular garnet and the phase transition boundary of K-hollandite have not been studied theoretically to date, we apply Clapeyron slopes determined experimentally (Akaogi et al 2009;Nishiyama et al 2005) to the static transition pressures obtained by theoretical studies Tsuchiya 2012b, 2013).…”

“…8.1). We employ published density and elastic constants of the NaAlSi 2 O 6 system (jadeite and a CF-type phase) (Kawai and Tsuchiya 2010), SiO 2 stishovite (Karki et al 2001), Ca 3 Al 2 Si 3 O 12 (grossular garnet, corundum (Cor), and Ca-perovskite (pv)) (Kawai and Tsuchiya 2012b), and the KAlSi 3 O 8 systems (K-hollandite) to determine the density variations for each composition with the phase transition pressures along the normal and slab geotherms ( Fig. 8.2).…”