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

• Published in: Geophysical Research Letters
• Main Authors: Moynier, Frederic, Jackson, Matthew, Zhang, Ke, Cai, Hongming, Halldórsson, Saemundur, Pik, Raphael, Day, James, Chen, Jiubin
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-IPG PARIS-Université de La Réunion (UR)-Centre National de la Recherche Scientifique (CNRS)-Université de Paris (UP)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Iceland; Ocean Island
• Online Access: https://hal.archives-ouvertes.fr/hal-03321622; https://hal.archives-ouvertes.fr/hal-03321622/document; https://hal.archives-ouvertes.fr/hal-03321622/file/MercuryGRL-HAL.pdf; https://doi.org/10.1029/2021GL094301

International audience 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. Plain language summary: While Earth's mantle is continuously chemically and isotopically stirred by convection, some ocean island lavas preserve isotopic anomalies. Their most likely origin is the recycling of crustal material into Earth's mantle by subduction. A question is then whether these crustal materials originate from the ocean or the continents. By using mercury stable isotopic compositions, which have specific signatures in ocean and continent materials, we identify whether these anomalies are due to continental or oceanic crustal material in various ocean island basalts.