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 (3)He/(4)He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower (3)He/(4)He 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-(3)He/(4)He 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-(3)He/(4)He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have (3)He/(4)He values ranging from low to high, other hotspots exhibit only low (3)He/(4)He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum (3)He/(4)He 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 (3)He/(4)He ratios. We interpret the relationships between (3)He/(4)He 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-(3)He/(4)He and low-(3)He/(4)He material. In contrast, cooler, less buoyant plumes do not entrain this high-(3)He/(4)He material. This can be explained if the high-(3)He/(4)He domain is denser than low-(3)He/(4)He mantle components hosted in plumes, and if high-(3)He/(4)He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-(3)He/(4)He 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.