Of all the things that can come out of an Australian sheep ranch, scientists probably weren't expecting to find the oldest positively identified piece of gem on the planet.
A tiny zircon crystal found in a ranch in the Jack Hills region of Western Australia has been conclusively identified as being 4.4 billion years old. This means that the tiny crystal is now the oldest known object on Earth.
To accurately date the zircon fragment, the scientists used two dating techniques. The crystal was formed during the early stages of the planet's development and the discovery has given scientists a glimpse into the early years of our planet. The researchers published their findings in the online journal Nature Geoscience last Sunday.
"This confirms our view of how the Earth cooled and became habitable," says the lead author John Valley, a geoscience professor at the University of Wisconsin-Madison. "This may also help us understand how other habitable planets would form."
At 4.4 billion years of age, the zircon crystal predates every other known object, rock, mineral and gem on the planet. The piece of zircon was formed back when the planet was still a molten ball of magma. In fact, it was formed a mere 160 million years after the formation of the entire solar system.
The successful dating of the crystal also lends credence to the speculations of a "cool early Earth." The theory states that shortly after the Earth's crust solidified from an ocean of magma, global temperatures decreased enough to allow the existence of water in its liquid form. This means that the hydrosphere may have formed early on in the planet's history.
The researchers who conducted the study used a combination of atom-probe tomography and secondary ion mass spectrometry to accurately identify the age of the microscopic zircon fragments.
"The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots," says Valley. "This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules."
Aside from using the latest available techniques to date the samples, the scientists also took measurements of oxygen isotope rations in the zircon fragments. The analysis of the measurements further supports their theory of a "cool early Earth."
"The Earth was assembled from a lot of heterogeneous material from the solar system," Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago "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."
