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.