The oldest remaining grain of early Earth’s original solid rock crust has now been confirmed to be a 4.374-billion-year-old zircon crystal from Jack Hills, Australia.
That age should settle a scientific debate over the accuracy of that mineral’s internal clock, and cuts the time from when Earth was hit by a Mars-sized body (which led to the formation of the Moon) and the cooling and creation of Earth’s first solid crust from 600 million years to 100 million years.
Some rock formations that are upwards of 3.5 billion years old persist in places such as Canada, but the vast majority of Earth’s surface rock is modern, less than a few hundred million years old. The zircons found at Jack Hills are tough pieces of old rock that have been incorporated into the newer, reworked material. Although they are barely visible to the naked eye, zircons still retain insights on the conditions under which they originally solidified.
Scientists dated the crystal by studying its uranium and lead atoms. Uranium decays into the latter very slowly over time and can be used like a clock.
“This, I believe, is the oldest zircon that’s ever been dated on Earth,” said Professor John Valley, of the University of Wisconsin in Madison, United States. He and his research team have published their findings in the journal Nature Geoscience.
“Previous dating of the ancient Jack Hills zircons about ten years ago had arrived at ages of about 4.4 billion years,” Valley explained. But there were doubts about whether some of the atomic elements used for the dating had moved about in the crystals and thrown off the age by a few hundred million years.
The element in question is a lead isotope, created by the radioactive decay of uranium in zircons. The age of a grain is figured by measuring the amounts of the parent uranium isotopes compared to the daughter lead isotopes. However, this is only accurate if no uranium or lead gets into or escapes from the zircon.
The previous research had indicated the Jack Hills zircon to be very ancient, but scientists had concerns that some of its lead atoms might have been lost or even migrated inside the crystal over time. This would have given the impression the zircon was older than it really is.
“Whether a grain is 4.3 or 4.4 billion years old, and whether this reflects a primary age, is not a trivial matter,” explained Sam Bowring of the Massachusetts Institute of Technology. “In the context of the 4.4- to 4.5-billion-year age of the Earth, a difference in age of 0.1 or 0.2 billion years (100 or 200 million years) is enormous in terms of modelling the geochemical evolution of Earth and the formation and recycling of the first continental crust.”
Researchers were concerned that the very process of uranium decaying, which fires out a high-speed alpha particle, might have kicked around lead atoms in the zircons and messed things up.
“If the lead leaves, it can be concentrated somewhere else,” said lead author Prof Valley. “The apparent age where it goes will appear older and where it has left will appear younger. What we’ve done is solve the lead mobility problem.”
The team did it by laboriously counting and mapping clusters of lead atoms in the zircon, using a technique called atom-probe tomography.
Atom probes are not like conventional optical or electron microscopes, in that the magnification effect comes from the magnification provided by a highly curved electric field, rather than by the manipulation of radiation paths. Technically, the method is destructive in nature, removing ions from a sample surface in order to image and identify them and generating magnifications sufficient to observe individual atoms as they are removed from the sample surface.
Through successive evaporation of material, layers of atoms are removed from a specimen, allowing for probing not only of the surface, but also through the material itself. Computer methods are then utilised to build a three-dimensional model of the sample, prior to it being evaporated, providing atomic scale information on the structure of a sample, as well as providing the type atomic species information. The instrument allows the three-dimensional reconstruction of up to hundreds of millions of atoms from a sharp tip (corresponding to specimen volumes of 10,000-1,000,000 nm3).
“It’s astonishing to be able to do this,” said Valley, “literally counting atoms with an atomic probe.”
Valley and his colleagues found that the lead was indeed getting kicked around, but it wasn’t going far enough to throw off the age of the zircon.
“I think it settles it, as far as it’s possible,” said Valley. “It also points the way for techniques that can be used to better study zircons from beyond Earth. There are, for instance, zircons from meteorites that are older than the Earth – up to perhaps 4.6 billion years old.”
Implications for the History of Life
The implication of those findings is that the Earth formed a solid crust much sooner after its formation 4.6 billion years ago than was previously thought, and very quickly following the great collision with a Mars-sized body that is thought to have produced the Moon just a few tens of millions of years after that. Prior to this time, Earth would have been a seething ball of molten magma.
Plate tectonics and weathering have ensured that very little of the Earth’s early surface remains to be studied. The study suggests strongly a continental crust was present on Earth about 100 million years after the planet formed. And by implication, it tells us that if temperatures were low enough, it could have perhaps even sustained liquid water at its surface. Knowledge that the Earth’s surface hardened so early raises the tantalising prospect that our world became ready to host life very early on in its history.
“This confirms our view of how the Earth cooled and became habitable,” says John Valley. “We have no evidence that life existed then. We have no evidence that it didn’t. But there is no reason why life could not have existed on Earth 4.3 billion years ago,” he told the Reuters news agency.