Single-crystal elasticity of lawsonite at high pressure: Implications for high Poisson's ratio and VP/VS zones in subduction zones

Conventional models attribute anomalously high Poisson’s ratios in Earth’s interior to the presence of fluids or melts. Here, we propose a fundamentally different mechanism, which is rooted in the unique crystal chemistry of lawsonite (CaAl₂Si₂O₇(OH)₂·H₂O). Our high-pressure experiments reveal anomalous changes in the elastic tensor of lawsonite at ~4 GPa and ~9 GPa, which are linked to phase transitions involving hydrogen-bond reorganization. Compared to typical mantle minerals, lawsonite exhibits moderately low P-wave velocity (V_P) and very low S-wave velocity (V_S). More notably, it is characterized by exceptionally high isotropic aggregate Poisson’s ratio (0.32–0.38) and V_P/V_S ratio (1.92–2.27), which serve as diagnostic identifiers in the interpretation of seismic models. In hot subduction zones, such as Cascadia and Southwest Japan, lawsonite provides a key mineralogical mechanism for the high-Poisson’s-ratio anomalies observed at 20–50 km depth, presenting a viable explanation distinct from conventional models that invoke overpressured fluid-saturated oceanic crust. In colder subduction systems such as NE Japan, the presence of 20–40 vol.% lawsonite can account for the regional-scale seismic anomalies observed at 50–90 km depth. Furthermore, we find that the localized ultra-low shear-wave velocity zone at 50–60 km depth in oceanic crust is most likely caused by lawsonite enrichment. The seismologically unique signature of lawsonite and its compatibility with seismic models underscore how this mineral could have a critical role in facilitating water transport into the deep mantle.