Mercury abundance and isotopic composition in granitic rocks: Implications for Hg cycling in the upper continental crust

Mercury (Hg) is a highly volatile metal exhibiting both isotope mass-dependent (MDF, defined as δ²⁰²Hg) and mass-independent (MIF, expressed as Δ¹⁹⁹Hg) fractionation. Assessing the Hg isotopic composition of the upper continental crust is crucial for the use of this new geochemical tool for petrogenetic tracing, particularly for understanding crust-mantle interaction. Here, we report Hg isotopic compositions of granitic rocks with distinct source affinity (I-, A-, and S-type) and of metasedimentary enclaves from the South China Craton. The results of more than 100 rock samples show large δ²⁰²Hg variations of -2.03 to 0.47 ‰ and relatively small Δ¹⁹⁹Hg variations of -0.16 to 0.09 ‰. The S-type granites studied here exhibit higher δ²⁰²Hg values (-0.58 to 0.00 ‰; n=57) than their sedimentary protoliths (metasedimentary enclaves; -2.03 to -0.91 ‰; n=7), suggesting significant Hg-MDF during magmatic processes. The Δ¹⁹⁹Hg values of S-type granites (-0.08 to 0.09 ‰) are indistinguishable from their sedimentary protoliths (-0.16 to 0.02 ‰), suggestive of the absence of Hg-MIF during crustal anatexis and subsequent magmatic differentiation. The I- and A-type granites studied have smaller Δ¹⁹⁹Hg variations of -0.11 to 0.04 ‰ (n=28) and -0.07 to 0.04 ‰ (n=15), respectively, but are within the range of the S-type granites. The compositional range of all granite variants may be explained by the variable intensity of large-scale MASH (melting-assimilation-storage-homogenization) processes. Our new data combined with previously published data of igneous, sedimentary and metamorphic rocks allow us to estimate that the upper continental crust (UCC) has a Hg abundance of 13.5 ppb and a weighted average δ²⁰²Hg of -1.15 ± 1.01 ‰ (2SD), which are higher than the corresponding value of the primitive mantle, indicating the enrichment of Hg and δ²⁰²Hg in the UCC. However, the weighted average Δ¹⁹⁹Hg value of the UCC (0.03 ± 0.15 ‰; 2SD) is undistinguished from the primitive mantle, suggesting no obvious Hg-MIF during the formation and differentiation of the continental crust.