A new study resolves a long-standing puzzle in Earth's interior dynamics: how trace amounts of melt can produce the distinct geophysical signals observed at the base of the upper mantle. It shows that volatile-rich carbonate melts can form interconnected networks and act as a "super-wetting agent" at depth, ultimately reconciling laboratory experiments with real-world geophysical data.

Located above the 410 km seismic discontinuity, the low-velocity layer (LVL) exhibits slower seismic wave velocities and higher electrical conductivity than the surrounding mantle rocks. These features have long been interpreted as evidence for an interconnected melt network, yet a persistent contradiction remains. While carbonate melts derived from subducted slabs represent a prime candidate, prior experiments have demonstrated that "dry" carbonate melts fail to effectively wet olivine—the most abundant mineral in the mantle. Models indicate that such dry melts would need to exceed 2 vol% to form a connected network, a value far higher than the trace melt fractions (< 0.1 vol%) inferred from geophysical observations.

Published in Science Advances and led by Associate Prof. HUANG Yongsheng from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences, this study identifies volatiles as the missing ingredient. By simulating mantle conditions (1–13 GPa, 1100–1400 °C), the team found that even small volatile additions—such as H₂O and NaCl—can trigger a "super-wetting" effect. While dry carbonate melts form isolated pockets with a dihedral angle near 30°, adding just 0.25 wt% NaCl or more than 20 wt% H₂O reduces the angle to nearly 0°, allowing melts to fully coat olivine grains and form an extensive three-dimensional network.

Electron microprobe analysis revealed the underlying mechanism: volatile-bearing melts dissolve silicate components (Si, Mg, Fe) from olivine, drastically lowering the melt–mineral interfacial energy.

Using this complete-wetting model, the team calculated that a volatile-rich carbonate melt representing only 0.02–0.08% of total rock volume is sufficient to raise electrical conductivity to the observed range (0.02–0.05 S/m) and produce the corresponding seismic velocity reduction, consistent with geophysical observations of the LVL. This extremely low melt fraction—orders of magnitude lower than predicted by dry models—finally reconciles laboratory experiments with geophysical observations.

Beyond resolving a scientific enigma, this study highlights volatiles as key regulators of melt connectivity in the deep Earth. The resulting super-wetting carbonate network functions as a low-viscosity channel, efficiently transporting carbon, water, and trace elements between Earth's interior and surface. This process also drives mantle metasomatism and may be associated with intraplate volcanism, the formation of deep-source diamonds, and the genesis of certain rare-earth element deposits.

This work was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and other funding agencies.

Schematic illustration of a pervasive volatile‑charged carbonate melt network percolating through the upper mantle, promoting global material cycling. (Image by HUANG Yongsheng et al.)