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Ancient Helium and Tungsten Isotopic Signatures Preserved in Mantle Domains Least Modified by Crustal Recycling

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Ancient Helium and Tungsten Isotopic Signatures Preserved in Mantle Domains Least Modified by Crustal Recycling Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling Matthew G. Jacksona,1, Janne Blichert-Toftb,2, Saemundur A. Halldórssonc,2, Andrea Mundl-Petermeierd, Michael Bizimise, Mark D. Kurzf, Allison A. Pricea, Sunna Harðardóttirc, Lori N. Willhitea,g, Kresten Breddamh, Thorsten W. Beckeri,j, and Rebecca A. Fischerk aDepartment of Earth Science, University of California, Santa Barbara, CA 93106-9630; bLaboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon, CNRS, and Université de Lyon, 69007 Lyon, France; cNordVulk, Institute of Earth Sciences, University of Iceland, 102 Reykjavík, Iceland; dDepartment of Lithospheric Research, University of Vienna, 1090 Vienna, Austria; eSchool of the Earth, Ocean and Environment, University of South Carolina, Columbia, SC 29208; fDepartment of Marine Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; gDepartment of Geology, University of Maryland, College Park, MD 20742; hRadiation Protection, Danish Health Authority, 2300 Copenhagen, Denmark; iInstitute for Geophysics, The Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78713; jDepartment of Geological Sciences, The Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78713; and kDepartment of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138 Edited by Albrecht W. Hofmann, Max Planck Institute for Chemistry, Mainz, Germany, and approved October 22, 2020 (received for review May 14, 2020) Rare high-3He/4He signatures in ocean island basalts (OIB) erupted Sr-Nd-Pb isotopic space, these four geochemical endmembers can at volcanic hotspots derive from deep-seated domains preserved in be used to define the apices of a tetrahedron (8) (Fig. 1) and have Earth’s interior. Only high-3He/4He OIB exhibit anomalous 182W—an low 3He/4He, similar to, or lower than, MORB (8 ± 2 Ra, ratio to isotopic signature inherited during the earliest history of Earth— atmosphere) (9). An additional mantle component is found in- supporting an ancient origin of high 3He/4He. However, it is not 182 ternal to the other four and, like DM, is characterized by geo- understood why some OIB host anomalous W while others do chemically depleted compositions that relate to long-term crustal not. We provide geochemical data for the highest-3He/4He lavas extraction from the mantle, which changes the parent–daughter from Iceland (up to 42.9 times atmospheric) with anomalous 182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for trac- ratios that govern key radiogenic isotopic systems, resulting in high 143 144 176 177 87 86 206 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES ing subducted, recycled crust—relate to high 3He/4He and anoma- Nd/ Nd and Hf/ Hf coupled with low Sr/ Sr and Pb/ 204 lous 182W. These data, together with data on global OIB, show that Pb. Existing long-lived radiogenic isotopic data show that this the highest-3He/4He and the largest-magnitude 182W anomalies are fifth component is geochemically depleted (8, 10) and has been found only in geochemically depleted mantle domains—with high variously referred to as FOZO [focus zone (8)], PHEM [primor- 143Nd/144Nd and low 206Pb/204Pb—lacking strong signatures of dial helium mantle (11)], or C [common (12)]. In contrast to DM, recycled materials. In contrast, OIB with the strongest signatures this geochemically depleted mantle composition was argued to associated with recycled materials have low 3He/4He and lack anom- exhibit elevated 3He/4He (8), signaling the preservation of an 182 alous W. These observations provide important clues regarding ancient reservoir in the deep Earth (13). the survival of the ancient He and W signatures in Earth’smantle. 3 4 182 We show that high- He/ He mantle domains with anomalous W Significance have low W and 4He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low 182 3He/4He and normal (not anomalous) 182W characteristic of sub- The recent discovery of anomalous W signatures in modern, ducted crust. Thus, high 3He/4He and anomalous 182W are preserved plume-derived hotspot lavas provides evidence for survival of domains in Earth’s interior since the early Hadean. Only lavas exclusively in mantle domains least modified by recycled crust. This 3 4 3 4 with ancient, high- He/ He signatures preserve anomalous model places the long-term preservation of ancient high He/ He 182 3 182 W signatures. However, it is not known why only high- He/ and anomalous W in the geodynamic context of crustal subduc- 4 182 tion and recycling and informs on survival of other early-formed He lavas have anomalous W, while other hotspot lavas do heterogeneities in Earth’s interior. not. We show that only hotspot lavas lacking strong recycled crust signatures exhibit high 3He/4He and anomalous 182W. This 4 182 3 4 observation is explained by subduction of W- and He-rich crust mantle geochemistry | hotspot volcanism | W | He/ He | Hadean that has low 3He/4He and normal 182W: Following subduction, the crust masks high-3He/4He and anomalous 182W signatures late tectonic motions deliver oceanic and continental crust into characteristic of ancient mantle domains. Our model links de- Pthe mantle at subduction zones. Following residence times of struction of Hadean geochemical signatures with the geo- ∼ 2 Ga, subducted crustal material is returned to the shallow dynamic process of plate subduction and recycling. mantle in buoyantly upwelling conduits of hot mantle material, known as mantle plumes, where they are partially melted and Author contributions: M.G.J. designed research; M.G.J., J.B.-T., S.A.H., A.M.-P., M.B., erupted as ocean island basalts (OIB) at volcanic hotspots (1–3). M.D.K., A.A.P., S.H., L.N.W., K.B., T.W.B., and R.A.F. performed research; M.G.J., J.B.-T., S.A.H., A.M.-P., M.B., M.D.K., A.A.P., S.H., L.N.W., K.B., T.W.B., and R.A.F. analyzed data; Recycled materials contribute to a variety of geochemically dis- M.G.J. wrote the paper with contributions from all authors; and all co-authors contrib- tinct mantle components often referred to as EM1 (enriched uted ideas for interpretation. mantle I), EM2 (enriched mantle II), and HIMU (high “μ” or high The authors declare no competing interest. 238 204 U/ Pb) (1–3). While there is debate surrounding the exact This article is a PNAS Direct Submission. nature of the recycled protoliths responsible for these mantle Published under the PNAS license. endmembers (4–7), they are likely generated by input of shallow 1To whom correspondence may be addressed. Email: [email protected]. geochemical reservoirs into the deep Earth. The three mantle 2J.B.-T. and S.A.H. contributed equally to this work. 87 86 143 144 176 177 206 components have Sr/ Sr, Nd/ Nd, Hf/ Hf, and Pb/ This article contains supporting information online at https://www.pnas.org/lookup/suppl/ 204 Pb distinct from the depleted mantle (DM) sampled by basalts doi:10.1073/pnas.2009663117/-/DCSupplemental. erupted at midocean ridges (MORB). In three-dimensional (3D) www.pnas.org/cgi/doi/10.1073/pnas.2009663117 PNAS Latest Articles | 1of9 Downloaded by guest on September 27, 2021 are 182W systematics somehow insensitive to these geodynamic processes such that Sr-Nd-Pb isotope systematics are decoupled EM2 from 182W? Sr Iceland hotspot high He/ He: EM1 In order to address these outstanding questions, we provide a 3 4 Pb large geochemical dataset on high- He/ He lavas from the Ice- Iceland (ERZ, 25.9 Ra) land hotspot and examine the data in the context of new global Iceland (SIVZ, 25.7 Ra) OIB datasets combining He, Sr, Nd, Hf, and Pb isotopes with W Nd Samoa isotopes. The data and analysis provide insights into the mech- Global hotspot high He/ He: anisms for preservation of anomalous 182W and high-3He/4He Samoa (Ofu, 33.8 Ra) signatures in the mantle. Hawaii Galapagos (Fernandina, 30.3 Ra) Hawaii (Loihi, 32.3 Ra) Results Iceland We present He-Sr-Nd-Hf-Pb-W isotopic data (Materials and Methods) on a suite of 28 basalt samples from all Icelandic lo- c calities known to have high 3He/4He (>25 Ra): mid-Miocene lavas Common from northwest Iceland (Vestfirðir) as well as Neovolcanic zone Mantle lavas from the Eastern Rift Zone (ERZ), the South Iceland Galapagos Component Volcanic Zone (SIVZ), and Vaðalda shield volcano in the MORB HIMU Northern Rift Zone (NRZ) (20–22) (SI Appendix,Fig.S1and Datasets S1, S2,andS3). These data are presented together with 3 4 Fig. 1. Mantle tetrahedron showing that most of the highest He/ He lavas published data (23) from ∼60 Ma Baffin Island and West in Iceland, and global hotspots, do not overlap with the common mantle Greenland (BIWG) flood basalt lavas, an additional high-3He/4He component [shown as ellipsoid (27)]. Four different high-3He/4He (>25 Ra) [up to 50 Ra (23–26)] locality on the Iceland hotspot track. He- components in Iceland (Vestfirðir, ERZ, SIVZ, and Baffin Island) are shown. = The highest 3He/4He (>30 Ra) samples from each global oceanic hotspot with lium, Pb, and W isotopic data on a small subset (n 8, 9, and 10, available Sr-Nd-Pb isotopes—Galapagos, Hawaii, and Samoa—are also respectively) of the Iceland samples examined here (14, 15), as 3 4 shown (data sources for these additional lavas are in SI Appendix, Fig. S4). well as He/ He data on a different subset of 10 samples (20, 22), EM1, EM2, and HIMU apices of the tetrahedron are published elsewhere were published previously. The dataset presented here includes (27), as is MORB (63), but EM2 extends to more extreme compositions (36) 87Sr/86Sr and 143Nd/144Nd for all 28 samples and 176Hf/177Hf and 3 4 (indicated by arrow).
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