The Electrical Conductivity of Liebermannite: Implications for Water Transport Into the Earth's Lower Mantle

Geeth Manthilake,Mohamed Ali Bouhifd,Laurent Jouffret,Mainak Mookherjee,Chengcheng Zhao,Federica Schiavi

doi:10.1029/2020jb020094

Abstract

AbstractLiebermannite (KAlSi3O8) is a principal mineral phase expected to be thermodynamically stable in deeply subducted continental and oceanic crusts. The crystal structure of liebermannite exhibits tunnels that are formed between the assemblies of double chains of edge‐sharing (Si, Al) O6 octahedral units, which act as a repository for large incompatible alkali ions. In this study, we investigate the electrical conductivity of liebermannite at 12, 15, and 24 GPa and temperature of 1500 K to track subduction pathways of continental sediments into the Earth's lower mantle. Further, we looked at whether liebermannite could sequester incompatible H2O at deep mantle conditions. We observe that the superionic conductivity of liebermannite due to the thermally activated hopping of K+ ions results in high electrical conductivity of more than 1 S/m. Infrared spectral features of hydrous liebermannite indicate the presence of both molecular H2O and hydroxyl (OH−) groups in its crystal structure. The observed high electrical conductivity in the mantle transition zone beneath Northeastern China and the lower mantle beneath the Philippine Sea can be attributed to the subduction pathways of continental sediments deep into the Earth's mantle. While major mineral phases in pyrolitic compositions are almost devoid of H2O under lower mantle conditions, our study demonstrates that liebermannite could be an important host of H2O in these conditions. We propose that the relatively high H2O contents of ocean island basalts derived from deep mantle plumes are primarily related to deeply subducted continental sediments, in which liebermannite is the principal H2O carrier.

Highlights

Trace quantities of H2O stored in the Earth’s mantle significantly influence the physical properties of mantle rocks (Smyth & Jacobsen, 2006)
We propose that the relatively high H2O contents of ocean island basalts derived from deep mantle plumes are primarily related to deeply subducted continental sediments, in which liebermannite is the principal H2O carrier
H2O plays an important role by affecting both mineral properties and mantle dynamics of the upper mantle, it is well known that the Earth’s lower mantle is relatively dry, as indicated by the low (< 1000 ppm wt.) H2O solubility in major lower mantle mineral phases (Bolfan-Casanova et al, 2003; Fu et al, 2019; Hirschmann, 2006)

Summary

Introduction

Trace quantities of H2O stored in the Earth’s mantle significantly influence the physical properties of mantle rocks (Smyth & Jacobsen, 2006). H2O plays an important role by affecting both mineral properties and mantle dynamics of the upper mantle, it is well known that the Earth’s lower mantle is relatively dry, as indicated by the low (< 1000 ppm wt.) H2O solubility in major lower mantle mineral phases (Bolfan-Casanova et al, 2003; Fu et al, 2019; Hirschmann, 2006). %, are fed by plumes rising from the lower mantle (Deschamps et al., 2011) These plumes are thought to originate from enriched reservoirs composed of recycled crustal materials that subducted into the deep mantle (Willbold & Stracke, 2006). Hydrous phases associated with subducting slabs descending into the lower mantle (van der Hilst et al, 1997) may provide a plausible mechanism for transporting H2O into the deep mantle. Dense hydrous magnesium silicates (DHMS), such as phase H and phase D, which are known to be stable in the depleted portion of the subducting slab at lower mantle conditions, are likely to transport H2O into the lower mantle and subsequently hydrate the subducted crustal components (Nishi et al, 2014; Pamato et al, 2014)

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Conclusion