Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet's interior(1-3). Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high He-3/He-4 isotopic ratios (up to 50 times(4) the present atmospheric ratio, Ra) compared to the lower He-3/He-4 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-He-3/He-4 domain resides in the deep mantle(6-8), beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-He-3/He-4 material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have He-3/He-4 values ranging from low to high, other hotspots exhibit only low He-3/He-4 ratios. Here we show that, among hotspots suggested to overlie mantle plumes(9,10), those with the highest maximum He-3/He-4 ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle(11), unlike plume-fed hotspots with only low maximum He-3/He-4 ratios. We interpret the relationships between He-3/He-4 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-He-3/He-4 and low-He-3/He-4 material. In contrast, cooler, less buoyant plumes do not entrain this high-He-3/He-4 material. This can be explained if the high-He-3/He-4 domain is denser than low-He-3/He-4 mantle components hosted in plumes, and if high-He-3/He-4 material is entrained from the deep mantle only by the hottest, most buoyant plumes(12). Such a dense, deep-mantle high-He-3/He-4 domain could remain isolated from the convecting mantle(13,14), which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir(1-3).