Geologists have carefully sorted out more than 100,000 microscopic Jack Hills zircons that date back to Earth's early epochs, from 3 billion to nearly 4.4 billion years ago.(The planet is 4.54 billion years old.) The crystals contain microscopic inclusions, such as gas bubbles, that provide a unique window into conditions on Earth as life arose and the first continents formed.
The radiation damage means the zircons could have been contaminated during their long lifetime.
Zircons hold minute amounts of two naturally occurring uranium isotopes — isotopes are atoms of the same element with different numbers of neutrons.
Uranium radioactively decays to lead at a steady rate.
When solving a big problem seems impossible, break it into smaller steps.
Well, scientists just took one of geology's biggest controversies and shrunk it down to atomic size.
By zapping single atoms of lead in a tiny zircon crystal from Australia, researchers have confirmed the crystal is the oldest rock fragment ever found on Earth — 4.375 billion years old, plus or minus 6 million years. Confirmation of the zircon age holds enormous implications for models of early Earth.
"We've proved that the chemical record inside these zircons is trustworthy," said John Valley, lead study author and a geochemist at the University of Wisconsin, Madison. Trace elements in the oldest zircons from Australia's Jack Hills range suggest they came from water-rich, granite-like rocks such as granodiorite or tonalite, other studies have reported.
That means Earth cooled quickly enough for surface water and continental-type rocks just 100 million years after the moon impact, the massive collision that formed the Earth-moon system. ] "The zircons show us the earliest Earth was more like the Earth we know today," Valley said.
"It wasn't an inhospitable place." Dubious history Zircons are one of the toughest minerals on the planet.
The ancient Australian crystals date back to just 165 million years after Earth formed, and have survived tumbling trips down rivers, burial deep in the crust, heating, squeezing and a tectonic ride back to the surface.
The Australian zircons, from the Jack Hills, aren't the oldest rocks on Earth — those are in Canada — but about 3 billion years ago, the minerals eroded out some of Earth's first continental crust and became part of a riverbed.