Taken together, "The world's largest volcanic events in the last 250 million years all seem to have compositions that are consistent with sampling an early Earth reservoir," Jackson said.
The findings make sense, he notes, because the undisturbed, never-melted primitive mantle material would have retained high concentrations of radioactive elements to provide a lot of heat, as well as relatively high levels of easily melted materials, compared to already-depleted mantle reservoirs.
"Together these are the perfect recipe for generating enormous quantities of melt," Jackson said.
Recent work by others indicates that the largest volcanic events all erupted over one of two "superplumes" in the mantle, one under Africa and one in the South Pacific. It may be that these represent the locations of primitive mantle reservoirs, Jackson noted.
The findings also raise questions about what the Earth is made of, Jackson said. "We've always thought that Earth's primitive mantle would be the same as chondrites," carbon-containing meteorites thought to contain material formed in the early solar system. "People always thought the earth was just an agglomeration of chondrites."