Thermoelasticity and stability of natural epidote at high pressure and high temperature: Implications for water transport during cold slab subduction

Epidote is a typical hydrous mineral in subduction zones. Here, we report a synchrotron-based single-crystal X-ray diffraction (XRD) study of natural epidote Ca_1.97Al_2.15Fe_0.84(SiO₄)(Si₂O₇)O(OH) under simultaneously high pressure-temperature (high P-T) conditions to ~17.7 GPa and 700 K. No phase transition occurs over this P-T range. Using the third-order Birch-Murnaghan equation of state (EoS), we fitted the pressure-volume-temperature (P-V-T) data and obtained the zero-pressure bulk modulus K₀=138(2) GPa, its pressure derivative K₀'=3.0(3), the temperature derivative of the bulk modulus ((∂K/∂T)_P=-0.004(1) GPa/K), and the thermal expansion coefficient at 300 K (α₀=3.8(5) ×10^-5 K^-1), as the zero-pressure unit-cell volume V₀ was fixed at 465.2(2) ų (obtained by a single-crystal XRD experiment at ambient conditions). This study reveals that the bulk moduli of epidote show nonlinear compositional dependence. By discussing the stabilization of epidote and comparing its density with those of other hydrous minerals, we find that epidote, as a significant water transporter in subduction zones, may maintain a metastable state to ~14 GPa along the coldest subducting slab geotherm and promote slab subduction into the upper mantle while favoring slab stagnation above the 410 km discontinuity. Furthermore, the water released from epidote near 410 km may potentially affect the properties of the 410 km seismic discontinuity.