Mercury loss and isotope fractionation during high-pressure and high-temperature processing of sediments: Implication for the behaviors of mercury during metamorphism

Metamorphic rocks show much lower mercury (Hg) levels than sedimentary rocks, which may be due to the loss of Hg during high-pressure and high-temperature conditions during metamorphism. To test this hypothesis, we conduct high-pressure and high-temperature experiments on ancient and modern sediments (WH black shale and GSS-4 soil). Under 0.3 GPa, the Hg concentrations decrease while the delta Hg-202 values increase with rising temperatures (WH black shale: 333-89 ppb, -1.34 to -0.79 parts per thousand, 250-700 degrees C; GSS-4: 545-265 ppb, -1.39 to -1.01 parts per thousand, 400-700 degrees C), suggesting the loss of isotopically light Hg isotopes under high-temperature conditions. Under constant temperatures of both 200 degrees C and 500 degrees C, with increasing pressure (0.5-1.4 GPa), GSS-4 shows only a slight decrease in Hg concentration with no variation in delta Hg-202, suggesting that high-pressure conditions restrain the loss of isotopically lighter isotopes. Consistent delta Hg-199 and delta Hg-200 values were observed in both samples during our experiment, implying no Hg isotope mass-independent fractionation (Hg-MIF) under high-temperature and high-pressure conditions. While results of this imply that metamorphism may lead to the emission of isotopically lighter Hg from sedimentary rocks to the surface environment, the lack of Hg-MIF during metamorphism provides important support for the use of Hg isotopes for paleoenvironment reconstruction. (C) 2022 Elsevier Ltd. All rights reserved.