[S-IT19] Deep Earth Sciences

[SIT19-P07] Effect of Al₂O₃ component on the viscosity of bridgmanite.

*Longli Guan¹, Daisuke Yamazaki¹, Noriyoshi Tsujino¹ (1.Institute for Planetary Materials, Okayama University, Misasa, Japan)

Keywords:Al2O3 component, bridgmanite, deformation, viscosity, lower mantle

Seismic tomography has imaged slab stagnation and plume deflection in the Earth’s shallow lower mantle (~1000 km) (French et al., 2015; Fukao and Obayashi, 2013). Because of no phase transition of major minerals for density change, these observations are considered to result from the viscosity jump detected at those depths (Rudolph et al., 2015). However, in terms of mineralogy, causes of viscosity variation in the lower mantle have not been clarified yet. The substitution difference of Al₂O₃ in bridgmanite with pressure is one of the possible origins of the observed viscosity increase at ~1000 km as the substitution mechanisms of Al₂O₃ in bridgmanite with pressure were investigated to be the presence of Tschermak substitution (TS) and the absence of oxygen vacancy (OV) type substitution at those depths (Liu et al., 2017) and the bridgmanite with OV component was more compressible than those without Al₂O₃ component and with Tschermak substitution (Brodholt, 2000). However, no direct experimental data of creep strength difference between Al-free and Al-bearing bridgmanite with OV and TS type was reported. Therefore, in this study, at first, to investigate the effect of Al₂O₃ component on the rheological properties of bridgmanite, we conducted the uniaxial deformation experiments on pure bridgmanite and Al-bridgmanite with OV and TS type by using the D111 apparatus at a pressure of 26 GPa and temperature of 1700-2100 K. Both pure and Al-bridgmanite aggregates were largely deformed with the maximum strain reaching 0.4. Deformation of samples was mainly controlled by dislocation creep indicated by the formation of lattice preferred orientation. In the samples deformed at high temperature and low stress, diffusion creep may also operate. The strain ratios of Al-free bridgmanite to Al-bridgmanite with OV and TS type were ranged from 0.5 to 1 and 1 to 2, respectively, depending on the experimental conditions. Our results suggest that OV component could weaken the bridgmanite to a certain extent and the Al-bridgmanite with TS type could be harder than Al-free and Al-bearing bridgmanite under the present experimental conditions. Thus, the substitution difference of Al₂O₃ in bridgmanite with pressure has the possibility to make some contributions to the observed viscosity increase at ~1000 km.