Epigraph:
Do they not then look at the camel, how it is created?
And at the heaven, how it is raised high?
And at the mountains, how they are set up?
And at the earth, how it is spread out? (Al Quran 88:18-21)
Cathodoluminescence image of a 400-micrometer Jack Hills zircon. | John Valley, University of Wisconsin
Source: The Huffington Post
LiveScience | by Becky Oskin
Ever heard this life advice? 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.
“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. The findings were published today (Feb. 23) in the journal Nature Geoscience.
Confirmation of the zircon age holds enormous implications for models of early Earth. 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. [How Was The Moon Formed?]
“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.
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.
Timeline showing major events in Earth history.
Just three of the very oldest zircons have been found, ones that date back to almost 4.4 billion years ago. Their extreme age always makes the dates suspect, because of possible radiation damage. 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. Counting the number of lead isotopes is how scientists date the crystals. But as the uranium kicks out lead atoms, the radioactive decay releases alpha particles, which can damage the crystals, creating defects. These defects mean fluids and outside elements can infiltrate the crystals, casting doubt on any conclusions about early Earth based on the zircons.
More important, uranium and lead can move around within a crystal, or even escape or enter the zircon. This mobility can throw off the lead isotope count used to calculate the zircon ages, and is the source of the decades-long controversy over the Jack Hills zircons’ Methuselah lifespan.
Categories: Europe and Australia, Evolution, Geology