Phase transformation of hydrous ringwoodite to the lower-mantle phases and the formation of dense hydrous silica

Abstract

8 In order to understand the effects of H 2 O on the mineral phases forming under the 9 pressure-temperature conditions of the lower mantle, we have conducted laser-heated 10 diamond-anvil cell experiments on hydrous ringwoodite (Mg 2 SiO 4 with 1.1wt% H 2 O) at Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our 12 experimental results show that hydrous ringwoodite (hRw) converts to crystalline dense 13 hydrous silica, stishovite (Stv) or CaCl 2 -type SiO 2 (mStv), containing 1 wt% H 2 O 14 together with Brd and MgO at the pressure-temperature conditions expected for shallow 15 lower-mantle depths between approximately 660 to 1600 km. Considering lack of sign 16 for melting in our experiments, our preferred interpretation of the observation is that Brd 17 partially breaks down to dense hydrous silica and periclase (Pc), forming Brd + Pc + Stv 18 mineralogy. Our experiments may provide an explanation for the enigmatic coexistence 19 of Stv and Fp inclusions in lower-mantle diamonds.

Elasticity of Hydrous SiO2 Across the Post‐Stishovite Transition in the Lower Mantle

“…Or an alternative reaction path has been recently proposed by Chen et al. (2020) who observed hydrous ringwoodite decomposition to form hydrous SiO 2 at conditions corresponding to a cold “660” boundary. If the melt is generated, it can be ∼20% less dense and would migrate upwards to the mantle transition zone (Fei, 2021; R. Huang et al., 2023).…”

“…Hydrous ringwoodite can dehydrate to generate melt (Schmandt et al, 2014). Or an alternative reaction path has been recently proposed by Chen et al (2020) who observed hydrous ringwoodite decomposition to form hydrous SiO 2 at conditions corresponding to a cold "660" boundary.…”

“…This gravitational instability would lead to the upward migration of melt into the mantle transition zone. Previous laser‐heated diamond‐anvil cell (LHDAC) experiments have shown that hydrous stishovite can be generated when hydrous ringwoodite is transported down into the lower mantle, although whether it is formed during solid‐state decomposition or partial melting remains controversial (Chen et al., 2020). Hydrous stishovite is expected to be chemically distinct from the ultramafic lower mantle (Ringwood, 1975), and thus, its elastic properties are important for understanding those low V S layers.…”

Stability and Thermoelasticity of Diaspore by Synchrotron X-ray Diffraction and Raman Spectroscopy

“…Subducted slabs bring water into the deep Earth, which has a profound influence on the geodynamics of the Earth's interior, such as earthquakes and the formation of island arc magma (Zheng et al, 2016;Zheng and Zhao, 2017;Zheng, 2019;Lempart et al, 2020). Since hydrous minerals are believed to exist in subduction plates and subduct into the mantle together with slab components, the P-T stability field of hydrous minerals under high P-T conditions is vital for the proposed transportation of water into the deep Earth (Chen et al, 2020;Thompson et al, 2020). Moreover, slab geotherms strongly affect the depth at which hydrous minerals dehydrated in subducting slabs, and thus affect water transport in the deep Earth.…”