Water Transport into the Earth’s Lower Mantle: Insights from the Electrical Conductivity of Liebermannite

Abstract:

Liebermannite (KAlSi₃O₈) is a high-pressure polymorph of potassium feldspar and is an important mineral phase that is expected to be thermodynamically stable in the deeply subducted continental crust and is also found to be stable in potassium-rich oceanic crusts. The crystal structure of liebermannite consists of edge-sharing (Si, Al)O₆ that share edges and form tunnels, where large potassium ions or other incompatible elements often reside. In order to track pressure and temperature pathways of subducted continental crust/sediments into the Earth’s lower mantle, we investigated the electrical conductivity of liebermannite at 12, 15, and 24 GPa and temperature of 1500 K. We also explored liebermannite’s possible role in sequestration of incompatible H₂O at deep mantle conditions. Our experimental results show a high electrical conductivity of more than 1 S/m. This high electrical conductivity in a condensed matter is likely due to the superionic conductivity of liebermannite due to the thermally activated hopping of fast K⁺ ions along the tunnels. We also characterized libermannite samples using infrared spectroscopy and the noticeable presence of hydroxyl (OH^-) and also bending modes due to the presence of molecular H₂O which are likely to be hosted in the large tunnels. 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 deeply subducted continental sediments 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.