Basalts from the Snake River Plain (pictured) in Yellowstone National Park in the northern United States look like they originate from deep in Earth's mantle. But they have the characteristic isotopic signature of rocks from shallower reaches — from the continental lithosphere. This curiosity has been explained by Barry Hanan of San Diego State University, California, and his colleagues.
Using mass spectrometry on samples of volcanic rock, the authors showed that deep-mantle magma can inherit the lithosphere's isotopic signature when it rises and picks up contamination.
The findings warn geologists about assuming too much from isotopic signatures.