Large H ₂ O solubility in dense silica and its implications for the interiors of water-rich planets

Abstract

Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in H2O-rich setting at high pressures and temperatures (P−T) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between SiO2 and H2O as archetypal materials for rock and ice, respectively, at high P−T. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the CaCl2-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (Si1−xH4x)O2 (x=0.12). At pressures above 60 GPa, H2O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

Water Content of the Dehydration Melting Layer in the Topmost Lower Mantle

“…Even if using a relatively dry transition zone model (∼0.4 wt.% water based on seismic velocity [Houser, 2016]), the melt fraction will be ∼0.5 vol.% (calculation details are given in the supporting information). Note that the secondary minerals in the slabs and ambient mantle including CaSiO 3 ‐perovskite, garnet, and stishovite may also contain large amounts of water (Chen et al., 2020; Katayama et al., 2003; Lin et al., 2020; Nisr et al., 2020). However, they have no phase transition when crossing 660‐km depth.…”

Waterworlds Probably Do Not Experience Magmatic Outgassing

“…For the purposes of this paper, we define waterworlds as <2 M Earth planets with water inventories that are easily sufficient to submerge all land (typically >0.1 wt% H 2 O). While silicates and water become miscible under high pressures and temperatures such as those found within the interiors of sub-Neptunes (Nisr et al 2020;Vazan et al 2020), because we limit ourselves to Earth-sized waterworlds, we assume silicates and water separate into distinct layers during formation. Specifically, we consider planets where the majority of the planet's water inventory is degassed during magma ocean solidification and resides at the surface as steam, liquid water, or ice (e.g., Elkins-Tanton 2008; Hamano et al 2013;Ikoma et al 2018;Kite & Ford 2018).…”

Water transport to the core–mantle boundary

“…But phase δ- H solid solution and the other potential hydrated oxide phases, intriguing as they are as potential hosts for water, may not be the likely primary host for water in slab crust. Recent studies suggest a new potential host for water—stishovite and post-stishovite dense SiO 2 phases [ 13 , 14 ].…”

“…SiO 2 minerals make up about a fifth of the slab crust by weight in the transition zone and lower mantle [ 15 ] and recent experiments indicate that the dense SiO 2 phases, stishovite (rutile structure—very similar to CaCl 2 structure) and CaCl 2 -type SiO 2 , structures that are akin to phase H and other hydrated oxides, can host at least 3 wt% water, which is much more than previously considered. More importantly, these dense SiO 2 phases apparently remain stable and hydrated even at temperatures as high as the lower mantle geotherm, unlike other hydrous phases [ 13 , 14 ]. And as a major mineral in the slab crust, SiO 2 phases would have to saturate with water first before other hydrous phases, like δ- H solid solution, would stabilize.…”