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To resolve whether the viscosity minimum in basaltic melts is an experimental artifact, the authors performed machine learning-accelerated molecular dynamics based on quantum mechanics across mantle pressures. They reveal a robust ~20% viscosity minimum at ~3 GPa driven by pressure-induced aluminum coordination restructuring that accelerates shear relaxation while the silica network remains rigid. This depth-dependent rheological transition explains why melt mobilizes rapidly below 150 km yet stalls beneath the lithosphere, providing a physical mechanism for asthenospheric softening.
Long dismissed as an experimental artifact, a 20% viscosity drop in basaltic melt at 3 GPa is physically real and explains why magma pools directly beneath tectonic plates.
The asthenosphere's mechanical weakness enables plate tectonics, but its origin is debated. Partial melt has been proposed to cause this softening, yet recent studies suggest that the measured viscosity minimum in basaltic melts, an essential control on melt mobility, is an experimental artifact. Using quantum mechanics-based, machine learning-accelerated molecular dynamics, we extend simulation timescales by more than a factor of 1000 and achieve percent-level precision. We show that basaltic melt exhibits a robust viscosity minimum (approximately 20% below 1-bar values) at approximately 3 GPa, driven by pressure-induced reorganization of aluminum coordination that facilitates shear relaxation while silicon-oxygen polyhedra remain structurally rigid. Our results reveal a depth-dependent rheological transition: melt mobility peaks below approximately 150 km, promoting efficient extraction, but declines sharply during ascent, causing melt to stagnate beneath the lithosphere. This mechanism provides a physical basis for the dual seismic signatures of a melt-depleted deep asthenosphere and a melt-enriched layer near the lithosphere-asthenosphere boundary.
