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 anomalous182W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high3He/4He. However, it is not understood why some OIB host anomalous182W while others do not. We provide geochemical data for the highest-3He/4He lavas from Iceland (up to 42.9 times atmospheric) with anomalous182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high3He/4He and anomalous182W. These data, together with data on global OIB, show that the highest-3He/4He and the largest-magnitude182W anomalies are found only in geochemically depleted mantle domains—with high143Nd/144Nd and low206Pb/204Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low3He/4He and lack anomalous182W. 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 anomalous182W have low W and4He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low3He/4He and normal (not anomalous)182W characteristic of subducted crust. Thus, high3He/4He and anomalous182W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high3He/4He and anomalous182W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.