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

Abstract

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

Mantle plumes and their role in Earth processes

“…This extreme core value developed because tungsten is a moderately siderophile element that, during core formation, became enriched in the Earth's core relative to the short-lived, lithophile radioactive parent ( 182 Hf) that remained in the mantle 86,87 . It is possible that this core material is partitioned back into the mantle at appear in some plume-generated ocean island basalt volcanoes, with no relation to long-lived 87 Sr/ 86 Sr signatures, but a relation between μ 182 W and the presence of recycled oceanic and continental material does emerge from the data 239 . One interpretation of these observations is that some deeply sourced mantle plumes with strongly negative μ 182 W anomalies have inherited a W-isotopic signature of Earth's core 86,87 , but other interpretations exist, as discussed in the main text.…”

“…The grey band represents the 2σ reproducibility (±5 ppm) on the terrestrial tungsten isotope standard, which, by definition, has μ 182 W = 0. Data and errors shown are 2σ measurement errors summarized in REFS 87,239 . We note that the estimated μ 182 W is as low as −220 in Earth's core 328 .…”

“…Until the ULVZs have been conclusively mapped out in seismic images, it will not be possible to determine whether ULVZ-related plumes have a different composition than plumes that are not related to ULVZs, and this currently limits what can be inferred about the isotope geochemistry of the ULVZs. Ultimately, the evolution of these geochemical domains is linked to plate tectonic processes, including recycling of oceanic and continental crust and transport by plumes, and these processes control the composition, location, size, amount of chemical overprinting 239 and longevity of geochemical reservoirs in the Earth's interior. Chemical plume structures.…”

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

“…This would potentially result in preferential overprinting of Hadean µ 142 Nd compositions while leaving Hadean µ 182 W compositions relatively unchanged. Similarly, a mantle assimilant with a high intrinsic Nd/W ratio, such as depleted MORB mantle or an ancient, W-depleted reservoir, could also efficiently overprint µ 142 Nd while leaving µ 182 W unchanged (cf., Jackson et al, 2020).…”

Spatial Characteristics of Recycled and Primordial Reservoirs in the Deep Mantle

“…All data published in this manuscript are available in Jackson et al (2020), Spatial characteristics of recycled and primordial reservoirs in the deep mantle, Version 1.0. Interdisciplinary Earth Data Alliance (IEDA).…”