Single‐Crystal Elasticity of Lawsonite at High Pressure: Implications for High Poisson's Ratio and V P / V S
Shuo Gao, Xinyang Li, Jiacheng Zhang, Sergio Speziale, Haotian Zhang, Zihan Zhang, You Tian, Xiaohui Yu, Youyue Zhang, Liang Li, Zhaodong Liu, Fangfei Li, Qiang ZhouAbstract
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 (CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O). Our high‐pressure and room temperature experiments reveal anomalous changes in the elastic tensor of lawsonite at ∼4.5 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 overpressurized, fluid‐saturated oceanic crust. In colder subduction systems such as NE Japan, the presence of 20–40 vol.% lawsonite can account for the seismic anomalies observed at 50–90 km depth. Furthermore, lawsonite exhibits strong velocity anisotropy, which may help interpret the observed seismic anisotropy. 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.