Plus vieilles roches connues, côte baie d'Hudson.

[JacquesL]

http://www.sciencedaily.com/releases/2008/09/080925144624.htm



Ça ne se voit pas, hein ! Elles ne font pas leur âge, comme cela, à première vue...

Oldest Known Rocks On Earth Discovered: 4.28 Billion Years Old

ScienceDaily (Sep. 26, 2008) — The discovery of rocks as old as 4.28 billion years pushes back age of most ancient remnant of Earth's crust by 300 million years.

McGill University researchers have discovered the oldest rocks on Earth – a discovery which sheds more light on our planet's mysterious beginnings. These rocks, known as "faux-amphibolites", may be remnants of a portion of Earth's primordial crust – the first crust that formed at the surface of our planet.

The ancient rocks were found in Northern Quebec, along the Hudson's Bay coast, 40 km south of Inukjuak in an area known as the Nuvvuagittuq greenstone belt.

The discovery was made by Jonathan O'Neil, a Ph.D. candidate at McGill's Department of Earth and Planetary Sciences, Richard W. Carlson, a researcher at the Carnegie Institution for Science in Washington, D.C., Don Francis, a McGill professor in the Department of Earth and Planetary Sciences, and Ross K. Stevenson, a professor at the Université du Québec à Montréal (UQAM).

O'Neil and colleagues estimated the age of the rocks using isotopic dating, which analyzes the decay of the radioactive element neodymium-142 contained within them. This technique can only be used to date rocks roughly 4.1 billion years old or older; this is the first time it has ever been used to date terrestrial rocks, because nothing this old has ever been discovered before.

"There have been older dates from Western Australia for isolated resistant mineral grains called zircons," says Carlson, "but these are the oldest whole rocks found so far." The oldest zircon dates are 4.36 billion years. Before this study, the oldest dated rocks were from a body of rock known as the Acasta Gneiss in the Northwest Territories, which are 4.03 billion years old. The Earth is 4.6 billion years old, and remnants of its early crust are extremely rare—most of it has been mashed and recycled into Earth's interior several times over by plate tectonics since the Earth formed.

The data from these findings will give researchers a new window on the early separation of Earth's mantle from the crust in the Hadean Era, said O'Neil.

"Our discovery not only opens the door to further unlock the secrets of the Earth's beginnings," he continued. "Geologists now have a new playground to explore how and when life began, what the atmosphere may have looked like, and when the first continent formed."

Their results are published in the September 26 issue of the journal Science.
Adapted from materials provided by McGill University

Il nous manque notamment la densité, qui a pu en faire des flotteurs depuis ce temps-là.
J'ignore aussi ce que qu'est la "fausse amphibolite" : nous n'avons pas de renseignements minéralogiques clairs dans cet article de vulgarisation sensationnalisante. Et pas non plus la référence scientifique d'origine.
Nous ignorons vers quelle profondeur cette roche a été formée, etc.