Image: Nature.

Geochemical evidence from rocks collected on Baffin Island in the Canadian Arctic suggests that beneath lies a mantle reservoir that has largely escaped the billions of years of melting affecting the rest of the planet. This reservoir likely represents the composition of the mantle shortly after the formation of the core, but before the 4.5 billion years of crustal formation and recycling modified the composition of much of the rest of the Earth’s interior. The study, led by Rick Carlson, former postdoctoral fellow Matt Jackson, and colleagues, appears in this week’s issue of Nature.

The group targeted the Baffin island rocks because they are the earliest expression of a mantle hotspot-- one now feeding volcanic eruptions on Iceland--because previous study of helium isotopes in these rocks showed them to have anomalously high ratios of helium-3 to helium-4. Helium-3 is rare in the Earth, as most of the mantle’s supply has been outgassed by volcanic eruptions. In contrast, helium-4 has been constantly replenished within the Earth by the decay of radioactive uranium and thorium. The high proportion of helium-3 suggests that the Baffin Island lavas came from a reservoir in the mantle that had never previously outgassed its original helium-3.

For more information, see the Nature paper or the CIW press release.