Stability and physical properties of brucite at high pressures and temperatures: Implication for Earth’s deep water cycle

• Brucite can stably exist in cold subducted slabs to the top of the lower mantle. • Velocity-density model for subducted hydrous harzburgite, utilizing thermodynamic equation of state of brucite. • The low density of hydrous harzburgite may affect the sinking rate of subducted slabs. Brucite is a common hydrous mineral on Earth and may contribute to the deep water cycle of the Earth, but its stability and structure under mantle conditions remain uncertain. In this study, we investigated the stability of brucite up to 60 GPa at 800 K and 45 GPa at 1850 K. Within the experiment P-T conditions, no theoretically predicted new phase was observed, and brucite remained in the P 3 ¯ m 1 structure. With the determined thermal EoS of brucite and the elastic parameters of mantle minerals, we modeled the velocity and density profile of subducted hydrous harzburgite in the top lower mantle, assuming that the water was stored in brucite and phase D. Based on the modelling, 1 wt.% water will reduce the velocity and density of harzburgite by ∼ 5% and ∼ 2%, respectively, yet whether the water is stored in brucite or phase D has weak influence on both density and velocity. With a water content up to 2.4 wt.%, the density of hydrous harzburgite could be reduced to 2.2(2)%–2.8(2)% lower than the surrounding mantle, while the V P and V S of hydrous harzburgite are still 0.3(1)%–0.7(1)% and 0.7(2)%–1.8(2)% higher than that of the normal mantle. Thus, the low-density hydrous harzburgite may slow down the subducting of slab, despite being a high-velocity body in seismic observations.