Ultra-Low-Velocity Disordered CaCO3 May Explain Mid-Lithospheric Discontinuities

Seismology reveals vertical heterogeneities in the thick cratonic lithospheric mantle (CLM), yet limited mineral elasticity data hinder our understanding of their origin, as well as continental structure and evolution. As a major carbon reservoir, the CLM stores carbon primarily as carbonates including CaCO₃. Using ab initio machine learning-accelerated molecular dynamics at time–length scales beyond standard simulations, we identify a new phase transition in orientationally disordered crystalline CaCO₃ under mid-lithospheric discontinuity (MLD) conditions (3–5 GPa, 1300–1500 K). This transition, strongly supported by recent in situ X-ray diffraction experiments (ACS Earth Space Chem. 2022, 6, 6, 1506–1513), induces significant elastic softening, reducing bulk and shear moduli by ∼15% and ∼45%, respectively, and producing exceptionally low shear-wave velocities (∼2.04 km/s). Seismic low-velocity anomalies and high electrical resistance at MLDs, which cannot be fully explained by hydrous minerals alone, may instead result from small amounts (2–10 vol%) of ultra-low-velocity CaCO₃.