February 24, 2014
Utilizing a new instrument that takes 3D images of individual atoms within a crystal, an international team of investigators has proved a rare sample of zircon is the oldest piece of Earth yet discovered at 4.4 billion years old.
The research team, which included Western University's Desmond Moser, performed the first-ever atom-probe measurement of a natural mineral: a crystal of zircon from the Hadean age. The sample was formed shortly after the crystallization of Earth's first crust from a global ocean of red hot magma.
In the findings, published today in Nature Geoscience, Moser and his collaborators revealed how they employed atom-probe tomography to image the vast amounts of geologic or 'deep' time preserved as radioactive uranium and decayed lead atoms within the crystal.
The ultimate survival mineral, zircons are microscopic grains found everywhere from the pink granite rocks of cottage country to the sandy parts of driveways and gardens. Able to persist through erosion, continent collisions and meteorite impacts, zircons are also excellent for recording the time passage of such events.
Zircons that have survived since the earliest period of Earth's history are extraordinarily rare and to date, the oldest samples are only found as sedimentary particles in a rock formation in Western Australia.
The particular sample of zircon investigated by the research team was discovered by the University of Wisconsin's John Valley in 2001. Valley sent the sample to Western to have it checked for signs of meteorite impact and crystal damage by Moser and technical scientist Ivan Barker with the specialized equipment at Western's Zircon and Accessory Phase Laboratory (ZAPLab).
Moser says, "Our role in this discovery was to identify the best preserved parts of the crystal for atom probe sampling, the ones that had the best chance of preserving chemical information from the Earth's beginning."
Last year, a competing team of researchers published a study suggesting the ages of the Western Australia zircon crystals were incorrect due to a change in lead atom distribution and that the samples were not as old as previously thought. These new findings from Moser, Valley and their collaborators established that while the lead atoms within the crystals had deviated to form nanoclusters, this reconfiguration did not change the atomic deep time measurements, meaning the age measurement of the sample remained accurate.
With its antiquity confirmed, oxygen atoms found within the crystal also confirm that the Earth was well-mixed chemically by 4.4 billion years ago and the corresponding chemical data procured earlier still supports the existence of oceans, and possibly life, on Earth as early as 4.3 billion years ago.
"While this is the first atom-probe study of an Earth zircon, the ZAPLab recently performed atom-probe measurements on ancient zircons from Mars," explains Moser. "It will be interesting to see how the early history of these two planets compare and it's really amazing that we can unravel these mysteries using a pin prick of material from these special crystals."
For more on the ZAPLab's Mars study, please visit http://communications.uwo.ca/media/agepuzzle/
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