First‐Principles Determination of the Dissociation Phase Boundary of Phase H MgSiO₄H₂

Abstract

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition.

Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth

“…Recently discovered phase H (MgSiO4H2) stable at high pressures above 48 GPa, may preserve water in the lower mantle [8], however, later it was shown to dissociate into MgSiO3 (bridgmanite) and H2O (ice-Ⅷ) when pressure is further increased to ~52 GPa (first-principles prediction) [9] or to ~60 GPa (experiment) [10]. Furthermore, phase H is unstable at high temperatures, its upper dissociation boundary being ~1500 K [11] predicted using first-principles calculations within the quasiharmonic approximation (QHA). Such temperatures can only be found in the coldest parts of the lower mantle, in particular, in subducted lithospheric slabs, which are typically ~500 K colder than normal mantle, thus restricting possible abundance of phase H in the mantle.…”

Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle

“…In a cold slab, Phase H, which may be transformed from phase D around the depth of 1,000 km (Tsuchiya, 2013), will undergo the dehydration reaction ($$\mathrm{Phase}\,\mathrm{H}=\mathrm{Bridgmanite}+{\mathrm{H}}_{2}\mathrm{O}$$) at the depth of ∼1,300–1,700 km (52–69 GPa) (Nishi et al., 2018; Ohtani et al., 2014; Tsuchiya, 2013; Tsuchiya & Umemoto, 2019; Walter et al., 2015). The form of H 2 O released by the dehydration of phase H is still debatable.…”

“…Schwager et al (2004) and Schwegler et al (2008) reported that ice-VII can be stable up to normal-mantle temperature above 50 GPa. Other studies suggest that phase H dehydrates above the melting temperature of ice-VII at 50 GPa (Goncharov et al, 2005;Kimura et al, 2014;Millot et al, 2018;Redmer et al, 2011;Tsuchiya & Umemoto, 2019), while the form of H 2 O released by the dehydration of phase H may be superionic ice (Millot et al, 2018;Redmer et al, 2011) or liquid water (Goncharov et al, 2005;Kimura et al, 2014) at 60 GPa. Here we assume that the dehydration will produce the liquid water that may migrate away.…”

“…Previous experiments suggested that phase H has an orthorhombic structure with space group Pnnm (Bindi et al., 2014). Phase H will dehydrate into bridgmanite at the depth of 1,300–1,700 km in a cold slab (Nishi et al., 2018; Ohtani et al., 2014; Tsuchiya, 2013; Tsuchiya & Umemoto, 2019; Walter et al., 2015). Coincidentally, seismic studies (e.g., Courtier & Revenaugh, 2008; Frost et al., 2018; Kaneshima, 2019; Schumacher & Thomas, 2016; Waszek et al., 2018) found seismic discontinuities globally with variable depths (1,300–1,700 km), lateral extent, orientation, and seismic polarity.…”

“…It was first predicted by theoretical calculations (Tsuchiya, 2013) and subsequently confirmed by high‐pressure experiments (Nishi et al., 2014). The stability of phase H at high PT has been studied by theoretical predictions (Tsuchiya, 2013; Tsuchiya & Umemoto, 2019) and experiments (Nishi et al., 2014; Ohtani et al., 2014; Walter et al., 2015). Previous experiments suggested that phase H has an orthorhombic structure with space group Pnnm (Bindi et al., 2014).…”