Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle. Issue 11 (28th October 2022)

• Record Type: Journal Article
• Authors: Song, Zijun; Wu, Zhongqing; Wang, Wenzhong; Hao, Shangqin; Sun, Daoyuan
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-28
• Subjects: elasticity -- first‐principles calculations -- mid‐mantle discontinuities -- phase H -- water transport; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024893
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Phase H (MgSiO4 H2 ), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1, 300–1, 700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1, 500 km on V P, V S, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic V S impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs. Plain Language Summary: The deep water cycle (DWC) strongly influences the evolution of the planet. But whether hydrous minerals could be carried into the lower mantle or not, which fundamentally impacts our understanding of the DWC, remains controversial. Dense hydrous magnesium silicates (DHMSs), which have been widely conducted by theoretical and experimental investigations in recent years, are considered to be the major carriers for transporting water to the deep mantle in a cold slab. However, the elastic data of phase H, the deepest DHMSs, are not available under the mantle temperatures and pressures. Here we conducted first‐principles calculations to investigate the thermodynamic properties and elastic properties of phase H under the lower mantle conditions. We find that the dehydration of phase H into bridgmanite, which may occur at the depth of ∼1, 300–1, 700 km in a cold slab, will produce a seismic V S impedance contrast of ∼17%. Thus, the dehydration of the moderate amount of phase H at these depths could generate the seismic discontinuities detected in some subduction zones. Our results provide evidence of water transport in the mid‐mantle. Key Points: We calculate thermodynamic and elastic properties of phase H at mid‐mantle conditions using first‐principles calculations Dehydration of phase H causes significant density jump and moderate V S jump and provides an explanation for mid‐mantle discontinuities Some cold slabs may enrich water in the mid‐mantle by the dehydration of phase H