Oceanic mercury recycled into the mantle: Evidence from positive Δ199Hg in lamprophyres

Due to mercury (Hg) is a global pollutant, the biogeochemistry of Hg in Earth's surface environment has been well studied. However, the cycling of Hg in interior reservoirs remains less clear, limiting our understanding of the global Hg geochemical cycle. Here, we use Hg isotopes to constrain the sources of Hg in typical alkaline lamprophyres in South China: the Early Ordovician lampmphyres in the Zhenyuan area and the Late Cretaceous lamprophyres in the Zhenfeng area. Lampmphyres from both areas are enriched in large-ion-lithophile elements (LILEs) and light rare earth elements (LREEs) but depleted in high-field-strength elements (HSFEs), typical of subduction-related igneous rocks. The Early Ordovician and Late Cretaceous lamprophyres show significantly positive to near-zero Delta Hg-199 values of 0.18 to 0.37 parts per thousand and - 0.08 to 0.36 parts per thousand, respectively. The positive Delta Hg-199 values are consistent with that reported for marine sediments and seawaters, suggesting Hg in these lamprophyres was sourced from marine reservoirs. This study suggests a heterogeneous distribution of Hg in the mantle and demonstrates that subducted marine materials can have contributed a substantial amount of Hg into the mantle.