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

Abstract The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ 199 Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ 199 Hg for terrestrial sub‐aerially‐derived materials and positive Δ 199 Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐ 3 He/ 4 He lavas from Samoa and Iceland, providing an average of Δ 199 Hg = 0.00 ± 0.10, Δ 201 Hg = −0.02 ± 0.0.09, δ 202 Hg = −1.7 ± 1.2; 2SD, N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ 199 Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ 199 Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ 199 Hg which correlate with 87 Sr/ 86 Sr, consistent with variable proportions of continental (low Δ 199 Hg and high 87 Sr/ 86 Sr) and oceanic (high Δ 199 Hg and low 87 Sr/ 86 Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions.