Native Hgliq in the metalliferous sediments of the East Pacific Rise (21°S)

Native Hg droplets, documented here for the first time in mid-ocean ridge metalliferous sediments, were found in the surface sediment of a magmatically robust and ultra-fast spreading mid-ocean ridge, the East Pacific Rise. The surface sediment and Fe–Mn micronodules in it are enriched in Hg. Two different processes can produce Hg0liq in this environment: hydrothermal and biogenic. Bio-transfer of ionic Hg dissolved in the hydrothermal fluids into Hg0 in the sediment can account for the eventual native Hg occurrences in the close vicinity of the hydrothermal vents. The Hg0liq occurrences in the distal metalliferous sediments most probably have originated directly from the hydrothermal fluids and been transported by hydrothermal plumes away from the vents. Condensation of Hg0liq from Hg0vap in the vent fluids subjected to near critical phase separation seems the most plausible mode of origin of the native Hg occurrences. The high-temperature vents at the global spreading ridge chain introduce an assessed amount of 1.62–4.86 × 103 t/yr Hg0liq into the ocean. With time, the deposited Hg0liq will likely be dissolved in the overlaying seawater and will not survive in the geologic record.