Primordial helium entrained by the hottest mantle plumes

Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet's interior. Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high ³He/⁴He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower ³He/⁴He ratios identified in mid-ocean-ridge basalts that form by melting the upper mantle (about 8Ra; ref. 5). A long-standing hypothesis maintains that the high-³He/⁴He domain resides in the deep mantle, beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-³He/⁴He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have ³He/⁴He values ranging from low to high, other hotspots exhibit only low ³He/⁴He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum ³He/⁴He ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle, unlike plume-fed hotspots with only low maximum ³He/⁴He ratios. We interpret the relationships between ³He/⁴He values, hotspot buoyancy flux, and upper-mantle shear wave velocity to mean that hot plumes-which exhibit seismic low-velocity anomalies at depths of 200 kilometres-are more buoyant and entrain both high-³He/⁴He and low-³He/⁴He material. In contrast, cooler, less buoyant plumes do not entrain this high-³He/⁴He material. This can be explained if the high-³He/⁴He domain is denser than low-³He/⁴He mantle components hosted in plumes, and if high-³He/⁴He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-³He/⁴He domain could remain isolated from the convecting mantle, which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir.