The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

Abstract

The composition of Earth's mantle is in part known through the chemical and isotopic analyses of lavas from different tectonic settings such as mid-ocean ridge basalts and ocean island basalts (OIB) (e.g., Hofmann, 2013). The variable isotopic compositions of OIB are usually interpreted to reflect mantle heterogeneity formed by recycling of surface material back into the mantle through subduction, the contribution of Earth's core into the deep source of certain lavas, or the survival of early formed heterogeneities in the mantle (e.g.

Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle

“…As shown in Fig. 3A, OIBs and CFBs investigated in this study show negative δ 202 Hg (−1.80 ± 0.42‰, 2 SD) and near-zero Δ 199 Hg values (−0.01 ± 0.08‰, 2 SD), which are within the range of the estimate of the primitive mantle (δ 202 Hg: −1.7 ± 1.2‰; Δ 199 Hg: 0 ± 0.1‰) based on Hg isotope analyses of 3 He-rich lavas 1 . The recently observed significant Δ 199 Hg signals in two EM-1 OIBs (−0.45 and −0.11‰), an EM-2 OIB (0.23‰), and a HIMU OIB (−0.14‰) 1 , imply that Hg in oceanic and continental crustal materials can be recycled into the lower mantle.…”

“…3A, OIBs and CFBs investigated in this study show negative δ 202 Hg (−1.80 ± 0.42‰, 2 SD) and near-zero Δ 199 Hg values (−0.01 ± 0.08‰, 2 SD), which are within the range of the estimate of the primitive mantle (δ 202 Hg: −1.7 ± 1.2‰; Δ 199 Hg: 0 ± 0.1‰) based on Hg isotope analyses of 3 He-rich lavas 1 . The recently observed significant Δ 199 Hg signals in two EM-1 OIBs (−0.45 and −0.11‰), an EM-2 OIB (0.23‰), and a HIMU OIB (−0.14‰) 1 , imply that Hg in oceanic and continental crustal materials can be recycled into the lower mantle. However, the amount of recycled Hg is likely small with respect to the total Hg pool in the lower mantle, given the fact that OIBs and CFBs in this study as well as 16 previously studied OIBs (including 11 3 He-rich lavas) 1 show near-zero Δ 199 Hg values (Fig.…”

“…Due to its toxicity to humans and wildlife, the geochemical cycle of Hg in Earth's surface environment (atmosphere, hydrosphere, and biosphere) has been extensively studied 2 . However, the cycling of Hg in Earth's interior reservoirs (e.g., crust and mantle) remains less studied 1,5 .…”

“…Mercury in oceanic reservoirs (e.g., sediments, seawater) may be carried by subducting ocean slabs into the asthenospheric mantle, and can then be emitted during arc magmatism to form Hg-enriched hydrothermal ore deposits in active continental margin settings 6 . A recent study, based on Hg isotopes in ocean island basalts, suggests that crustal Hg is also recycled into the lower mantle 1 .…”

“…On Earth's surface, photochemical processes result in negative Δ 199 Hg (−0.6 to 0‰) in terrestrial reservoirs (e.g., soil and vegetation) and positive Δ 199 Hg (0 to 0.4‰) in oceanic reservoirs (e.g., seawater and marine sediments) 7,8 . It has been estimated that the primitive mantle has near-zero Δ 199 Hg (Δ 199 Hg: 0.00 ± 0.10‰, 2 SD) and negative δ 202 Hg (−1.7 ± 1.2‰, 2 SD), based on Hg isotope analyses of 3 He-rich lavas 1 . A recent study also found significantly positive Δ 199 Hg (0 to 0.4‰) in hydrothermal ore deposits associated with arc magmatism, implying the recycling of Hg from marine sediments into arc lavas 6 .…”