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

Abstract

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 δ202Hg values increase with rising temperatures (WH black shale: 333–89 ppb, −1.34 to −0.79‰, 250–700 °C; GSS-4: 545–265 ppb, −1.39 to −1.01‰, 400–700 °C), suggesting the loss of isotopically light Hg isotopes under high-temperature conditions. Under constant temperatures of both 200 °C and 500 °C, with increasing pressure (0.5–1.4 GPa), GSS-4 shows only a slight decrease in Hg concentration with no variation in δ202Hg, suggesting that high-pressure conditions restrain the loss of isotopically lighter isotopes. Consistent Δ199Hg and Δ200Hg 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.