Aluminum Effects on Elastic and Vibrational Properties across the Post-stishovite Transition

Abstract:

Seismic reflectors/scatterers have been widely detected from top to mid lower mantle along the circum-pacific region (Kaneshima, 2016). Stishovite, counting for 25 vol% of the mid-ocean ridge basalt (MORB), will undergo a ferroelastic transition at ~1800 km depth, which is accompanied by significant V_S softening (Carpenter et al., 2000; Ishii et al., 2019). Therefore, mid-lower-mantle seismic reflectors/scatterers are very likely to signal a presence of subducted MORB (Wang et al., 2020). On the other hand, stishovite is able to contain certain amount of aluminum, and aluminum incorporation can substantially reduce its transition pressure (Lakshtanov et al., 2007). Although previous studies have suggested that Al-bearing stishovite may contribute to seismic anomalies in the shallow part of lower mantle, its elasticity behavior across the post-stishovite transition remains unsolved. Here we have conducted synchrotron X-ray diffraction (XRD) and Raman measurements up to 65 GPa on three stishovite crystals with different aluminum contents. Aluminum effects on transition pressure, equation of state (EoS), atomic vibrations, and optical modes of stishovite and post-stishovite have been investigated. Within the framework of Landau theory, these results have been further used to derive elastic constants (C_ij’s), bulk and shear moduli (K_S and G), and compressive and shear wave velocities (V_P and V_S) across the transition for three compositions. Using a typical MORB mineralogy, we have modelled V_P and V_S of MORB along typical slab geotherms and compared them with a standard seismic wave velocity profile (Dziewonski and Anderson, 1981). Our results show that aluminum incorporation could enhance V_S anomaly across the post-stishovite transition in subducted MORB. This finding suggests that Al-bearing stishovite could contribute to seismic reflectors/scatterers associated with negative V_S signal within the upper part of lower mantle.