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

• Published in: Geochimica et cosmochimica acta 2022-10, Vol.334, p.231-240
• Main Authors: Chen, Di, Ren, Dongsheng, Deng, Changzhou, Tian, Zhendong, Yin, Runsheng
• Format: Article
• Language: English
• Subjects: High-pressure and high-temperature; Isotope fractionation; Mercury; Metamorphism
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2022.08.010

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.