Evidence for the survival of the oldest terrestrial mantle reservoir

Helium is a powerful tracer of primitive material in Earth's mantle. Extremely high He-3/He-4 ratios in some ocean-island basalts suggest the presence of relatively undegassed and undifferentiated material preserved in Earth's mantle. However, terrestrial lavas with high He-3/He-4 ratios have never been observed to host the primitive lead-isotopic compositions that are required for an early (roughly 4.5 Gyr ago) formation age(1,2). Here we show that Cenozoic-era Baffin Island and West Greenland lavas, previously found to host the highest terrestrial-mantle He-3/He-4 ratios(3-5), exhibit primitive lead-isotope ratios that are consistent with an ancient mantle source age of 4.55-4.45 Gyr. The Baffin Island and West Greenland lavas also exhibit Nd-143/Nd-144 ratios similar to values recently proposed for an early-formed (roughly 4.5 Gyr ago) terrestrial mantle reservoir(6,7). The combined helium-, lead- and Nd-isotopic compositions in Baffin Island and West Greenland lavas therefore suggest that their source is the most ancient accessible reservoir in the Earth's mantle, and it may be parental to all mantle reservoirs that give rise to modern volcanism.