Mantle Hg Isotopic Heterogeneity and Evidence of Oceanic Hg Recycling into the Mantle

The geochemical cycle of mercury in Earth's surface environment (atmosphere, hydrosphere, biosphere) has been extensively studied; however, the deep geological cycling of this element is less well known. Here we document distinct mass-independent mercury isotope fractionation (expressed as ΔHg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive ΔHg values up to 0.34‰ and 0.22‰, respectively, which deviate from recent estimates of the primitive mantle (ΔHg: 0.00 ± 0.10‰, 2 SD). The positive ΔHg values indicate recycling of marine Hg into the asthenospheric mantle. Such a crustal Hg isotope signature was not observed in our samples of ocean island basalts and continental flood basalts, but has recently been identified in canonical end-member samples of the deep mantle, therefore demonstrating that recycling of mercury can affect both the upper and lower mantle. Our study reveals large-scale translithospheric Hg recycling via plate tectonics.