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

PY Zhang and LJ Man and L Yuan and X Wu and JF Zhang, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 130, e2025JB031906 (2025).

DOI: 10.1029/2025JB031906

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 similar to 15% and similar to 45%, respectively, and producing exceptionally low shear-wave velocities (similar to 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.

Return to Publications page