Isotopes Help Home In On Ancient Rock

Isotopic evidence found in Arctic region lava samples has led scientists to a pristine reservoir of Earth’s mantle that has remained unchanged since nearly the formation of the planet (Nature 2010, 466, 853). Such a find offers scientists an unprecedented view of nascent Earth and a chance to shed light on early geochemical evolution. Scientists had believed the original character of most primordial mantle reservoirs would have been destroyed long ago by constant geological churning and upheaval, note Boston University’s Matthew G. Jackson and colleagues. The group studied isotopic ratios of helium, lead, and neodymium in ancient lava samples from Canada’s Baffin Island and West Greenland. The combined characteristic ratios—in particular, high ³He/⁴He ratios paired with ratios of a series of lead isotopes—indicate that these lavas welled up from a mantle reservoir that formed nearly 4.5 billion years ago, only tens of millions of years after Earth’s crust had formed. “The possible survival of primitive mantle relicts needs full consideration in future models of mantle structure and evolution,” notes Oregon State University geochemist David W. Graham in a commentary about the work.