Subduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high‐temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low‐temperature polymorph of serpentine, is stable under the P‐T conditions of cold subduction slabs (<260°C at 2 GPa), and its high‐temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P‐T conditions of subduction zones. Our static results agree with previous studies. Its high‐temperature velocities are much higher than previous experimental‐based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear‐wave splitting in some cold slabs such as Tonga.

Subduction of slabs couples the Earth's interior with its exterior, and powers the unique plate tectonics on our planet. The temperature of the slabs controls a series of subduction processes. Slab temperature influences the composition and the density of subducting slabs, which determines the fate of the slabs (Ganguly et al., 2009;Litasov et al., 2004). Low temperature can kinetically inhibit the gradual dissolution of pyroxene into garnet (Nishi et al., 2013;van Mierlo et al., 2013), which plays a critical role for the stagnation of the relative cold slabs above the depth of 660 km (King et al., 2015) because pyroxene is less dense than majoritic garnet. The phase transformations of the surviving pyroxene and pyrope could account for multiple discontinuities at the depths of ∼600-750 km in some cold subduction zones such as Northeast China and Tonga (Ai et al., 2003;Deuss et al., 2006;Zang et al., 2006). Temperature also controls slab devolatilization and affects the component of arc magma and the amount of volatiles recycling to the surface (Hsieh et al., 2022;Rüpke et al., 2004;van Keken et al., 2011). The partition coefficients of key elements that define arc geochemical signatures also exhibit strong temperature dependence (Kessel et al., 2005). The thermal structure of the slabs is believed to have a large effect on seismicity (