4.4 Billion Years Old Zircon Crystal, Earth's Oldest Fragment, Found

Scientists have solved one of geology's biggest mysteries by confirming that Earth's crust was first formed some 4.4 billion years ago, 160 million years after the formation of our Sun system.

In a new finding, researchers from the University of Wisconsin confirm that the zircon crystal, dating from some 4.374 billion years ago, discovered 15 years back from the Jack Hills region of Australia, is the oldest fragment of Earth's crust ever discovered. This tiny piece of Zircon offers clues on how our planet became habitable some 4.4 billion years ago.

In this study, Professor John Valley of University of Wisconsin-Madison geosciences, used a novel technique called atom-probe tomography to precisely establish the age and the thermal history of the zircon crystal by determining the mass of each individual atom of lead in the sample. Using this he successfully confirmed that the Earth's crust was initially formed some 4.4. billion years ago, when our planet was a scorching ball blanketed in a magma ocean, indicating the crust formed relatively soon.

"We've proved that the chemical record inside these zircons is trustworthy," Valley was quoted in LiveScience.

Zircons are the oldest known terrestrial materials that occur in sedimentary, igneous and metamorphic rocks. They occur in several colors and can also be colorless. They contain fine traces of uranium and thorium. These crystals are bundled with rich and varied records of geological processes because they have survived erosion and metamorphism.

This new finding builds on the previous studies that dated the Australian zircons using the lead isotopes.

Valley had previously conducted uranium-lead radioactive dating on a zircon sample. It is known that uranium radioactively decays into lead making it easy to determine its age based on the ratio of uranium and lead in the sample. But this radioactive decay leads to the release of alpha particles that destroy the crystals. Due to this defect there are chances that the fluids and elements present outside can penetrate into the crystal making any conclusion dubious. Also, since uranium and lead move within the crystal or even break away, the number of lead isotopes required to calculate the age also changes.

"If there's a process by where lead can move from one part of the crystal to another place, then the place where lead is concentrated will have an older apparent age and the place from where it moves will have a younger apparent age," Valley was quoted in LiveScience.

In this study, the team showed that even if the oldest fragment of Earth curst underwent radiation damage, the number of lead atoms within it was intact. The number of lead atoms in the oldest known zircon crystal was counted using atom-probe tomography. They calculated the distances the lead atom move and realized that the distance was too small to affect any analysis made regarding  the age.

"We capture both the lead-depleted and lead-enriched domains, so the ratio we measure is averaged out," Valley said in ABC Science. "We're getting the true ratio of the parent uranium to daughter lead, and therefore we're getting the true age."

Valley explains when the zircon formed 4.4 billion years ago, the lead that existed then were all concentrated in these hotspots.

"The Earth was assembled from a lot of heterogeneous material from the solar system," Valley explains "that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early."

The finding was documented in the journal Nature.