Mercury isotopic compositions of the Precambrian rocks and implications for tracing mercury cycling in Earth's interior

Mercury isotopes undergo unique mass-independent fractionation (MIF) during photochemical processes on Earth's surface. Studies have observed pronounced Hg-MIF signals in sedimentary and magmatic rocks, suggesting recycling of Hg from Earth's surface systems into the lithosphere via sedimentation and magmatism. However, the isotopic signature of Hg in metamorphic rocks and the geochemical fate of Hg during metamorphism remain unclear. Precambrian basements are important components of cratons or orogenic belts on Earth. Here, we study the Hg concentration and isotopic composition of Precambrian metamorphic and sedimentary rocks from the eastern Central Asian Orogenic Belt, and North and South China cratons. Metamorphic rocks show much lower Hg contents (0.21-7.8 ppb) than sedimentary rocks (2.6-694 ppb), indicating a substantial loss of Hg during metamorphism. The lack of correlation between delta Hg-202 values (-2.41 to 0.18 parts per thousand) and metamorphic grades indicates no systematic mass-dependent fractionation (MDF) of Hg isotopes during metamorphism. The Delta Hg-199/Delta Hg-201 ratios of similar to 1.0 for both metamorphic and sedimentary rocks indicate Hg was sourced from Earth's surface systems. The coupling of Hg-MIF signals between the metasedimentary rocks and the sedimentary settings of their protolith suggests no Hg-MIF during metamorphism. The negative Delta Hg-199 values (-0.30 to -0.02 parts per thousand) in the Precambrian coastal sedimentary rocks imply the input of Hg into coastal regions via soil erosion. The positive Delta Hg-199 values (0.06 to 0.31 parts per thousand) in the Precambrian marine sedimentary rocks suggest deposition of atmospheric Hg(II) to open oceans via wet deposition. The lack of significant Hg-MIF during metamorphism and other underground geological processes shows that Hg-MIF signals can work as a reliable tracer for indicating material cycling in Earth's interior.