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 Δ(199)Hg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive Δ(199)Hg values up to 0.34‰ and 0.22‰, respectively, which deviate from recent estimates of the primitive mantle (Δ(199)Hg: 0.00 ± 0.10‰, 2 SD)(1). The positive Δ(199)Hg 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(1), 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.