First‐Principles Investigations of Antigorite Polysomatism Under Pressure.

Antigorite is the high‐temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in the subducting oceanic lithosphere. It has a wavy crystal structure along its a‐axis and several polysomes with different m‐values (m = 13–24) have been identified in nature. The m‐value is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit primarily depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of pressure and temperature. However, it is not well understood which m‐values of antigorite are stable at different pressure and temperature conditions. To investigate the pressure dependence of the stability of different m‐values in antigorite, we performed first‐principles calculations for several polysomes (m = 14–19) at high pressure from 0 to 14 GPa and compared their enthalpies at static 0 K. We found that although the energy differences between polysomes are small, polysomes with larger m‐values are more stable at ambient pressure, while polysomes with smaller m‐values are more stable at elevated pressures. This suggests that the structure of antigorite in the oceanic lithosphere subducting into the deep Earth may gradually evolve into a different polysome structure than the antigorite samples observed at ambient or near‐surface pressure conditions. These changes in the m‐values are accompanied by a minor dehydration reaction. By modulating the available amount of free water in the system, antigorite polysomatism may influence the distribution of intermediate‐depth seismicity, such as the observance of double seismic zones. Plain Language Summary: Antigorite is a hydrous mineral known for being an important carrier of water into the Earth's interior and a seismicity trigger at intermediate‐depths down to ∼200 km. However, its crystal structure is complex (e.g., Uehara, 1998), and thus it is not well understood which structural variant (polysome) of antigorite is stable or dominant at high pressures and temperatures during subduction. In this study, several crystal structures of antigorite were calculated by first‐principles calculations, and their enthalpies (i.e., Gibbs free energy of a system at static 0 K) were compared to determine the stable structure as a function of pressure. The results indicate that the structure and chemical composition of antigorite likely changes under high pressure, resulting in the gradual release of water during the subduction process. As antigorite is thought to be the main trigger of seismicity in the subducting lithospheric mantle, its metasomatism may contribute to explain the reported distributions of intermediate‐depth earthquakes. Key Points: Antigorite crystal structures with various values of m (14–19) were determined by first‐principles calculation under pressureThe relative enthalpy shows that antigorite with smaller m‐values are stabilized with increased pressureAntigorite in suducting slabs may gradually dehydrate under high pressure as a result of changes to stable m‐values [ABSTRACT FROM AUTHOR]