Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Jackson, Matthew G., Blichert-Toft, Janne, Halldórsson, Saemundur A., Mundl-Petermeier, Andrea, Bizimis, Michael, Kurz, Mark D., Price, Allison A., Harðardóttir, Sunna, Willhite, Lori N., Breddam, Kresten, Becker, Thorsten W., Fischer, Rebecca A.
• Format: Text
• Language: English
• Published: National Academy of Sciences 2020
• Subjects: Physical Sciences; Iceland; Ocean Island
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7733831/; http://www.ncbi.nlm.nih.gov/pubmed/33229590; https://doi.org/10.1073/pnas.2009663117

Rare high-(3)He/(4)He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-(3)He/(4)He OIB exhibit anomalous (182)W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high (3)He/(4)He. However, it is not understood why some OIB host anomalous (182)W while others do not. We provide geochemical data for the highest-(3)He/(4)He lavas from Iceland (up to 42.9 times atmospheric) with anomalous (182)W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high (3)He/(4)He and anomalous (182)W. These data, together with data on global OIB, show that the highest-(3)He/(4)He and the largest-magnitude (182)W anomalies are found only in geochemically depleted mantle domains—with high (143)Nd/(144)Nd and low (206)Pb/(204)Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low (3)He/(4)He and lack anomalous (182)W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-(3)He/(4)He mantle domains with anomalous (182)W have low W and (4)He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low (3)He/(4)He and normal (not anomalous) (182)W characteristic of subducted crust. Thus, high (3)He/(4)He and anomalous (182)W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high (3)He/(4)He and anomalous (182)W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.