High-temperature deuterium tracks the thermal stability of hydroxyl in epidote and zoisite

Studying the thermal stability of the -OH groups in epidote-group minerals is of significant importance for understanding the deep-water cycle of the Earth. Epidote group minerals are among the common silicate minerals in the mafic oceanic crust of subducting lithospheric plates and are important water carriers in the deep mantle as well as in the Earth's deep-water cycle. Deuteration reactions offer significant advantages in tracing the thermal stability of the -OH groups in minerals, allowing for the labeling of hydrogen without affecting the mineral structure. The structural properties of epidote and zoisite with different Fe³⁺ contents at various temperatures were studied using X-ray diffraction, Raman spectroscopy, thermogravimetry, and deuterium tracing techniques to understand the influence of Fe³⁺/Al³⁺ substitution on the water-carrying capacity and hydroxyl group thermal stability of epidote group minerals. The results suggest that the axial thermal expansion coefficients of epidote are α₀(a) = 1.42(8)*10^-5/K^-1, α₀(b) = 1.77(9)*10^-5/K^-1, α₀(c) = 2.7(6)*10^-5/K^-1, and zoisite are α₀(a) = 0.85(5)*10^-5/K^-1, α₀(b) = 2.13(8)*10^-5/K^-1, α₀(c) = 4.16(8)*10^-5/K^-1. In addition, the activation temperatures of the epidote and zoisite hydroxyl groups are similar at approximately 573(1) K, and the decomposition temperatures of epidote and zoisite are 1170 K and 1198 K, respectively. The deuteration process of epidote and zoisite before decomposition is divided into two stages: 573-773 K and 823-1123 K, the deuteration degrees of both increased with increasing temperature in each stage. The integral area growth rate of -OD peak in infrared spectroscopy is found to be as follows: 0.0239 K^-1 in 573-773 K and 0.1938 K^-1 in 823-1123 K for epidote, and 0.0374 K^-1 in 573-773 K and 0.1812 K^-1 in 823-1123 K for zoisite. Moreover, owing to the Fe³⁺/Al³⁺ substitution, the structural characteristics of zoisite gradually evolve to epidote at high temperatures, and the stability of the hydroxyl group decreases. Therefore, in the geothermal environment of plate subduction, the hydroxyl groups in epidote and zoisite are first activated when the temperature rises to 573 K, followed by the exchange and transport of H⁺ from the surrounding environment or minerals, leading to the dehydration and decomposition of epidote and zoisite. Our results provide key insights into water storage and migration in subduction zones.