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

Abstract

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 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), both of which are higher than the corresponding values of the primitive mantle. However, the weighted average Δ199Hg value of the UCC (0.03 ± 0.15 ‰; 2SD) is nondistinguished from the primitive mantle, suggesting no obvious Hg-MIF during the formation and differentiation of the continental crust.