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Physical Chemistry

Isotopes Help Home In On Ancient Rock

Arctic region lava samples provide an unprecedented view of nascent Earth's geochemical evolution

by Elizabeth K. Wilson
August 16, 2010 | A version of this story appeared in Volume 88, Issue 33

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Credit: Don Francis
Scientists found samples of the Earth's earliest known mantle reservoir on Canada's Baffin Island.
Credit: Don Francis
Scientists found samples of the Earth's earliest known mantle reservoir on Canada's Baffin Island.

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.

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