Deep beneath the red soils of Western Australia, an ancient secret lay locked in stone. And through the stubborn durability of a single mineral (zircon), scientists have now cracked open a new chapter in Earth’s oldest history.

In a recent study, researchers from Curtin University and the Geological Survey of Western Australia unearthed compelling evidence that a portion of the Yilgarn Craton is over 3.4 billion years old. Their smoking gun were tiny zircon crystals, preserved within the durable mineral titanite.

Most of the crust we see around us is relatively new. Relatively compared to the age of the Earth, that is. Plate tectonics, the unwavering mechanism that creates mountains and sinks continents, constantly pushes landmass around. It destroys the existing crust and creates new land area. So, the vast majority of the crust currently on the surface isn’t the original one from when the Earth was created. Most of it was destroyed and reformed in the mantle and brought back up to the surface.

The exception is places called cratons. Cratons are the ancient, stable cores of continents that have survived billions of years of geological activity. They’re typically made of very old metamorphic and igneous rocks and form the foundation upon which younger geological layers are built.

Ancient Rocks

“Our planet’s most ancient rocks are often overwritten by later geological processes,” the researchers write. But sometimes, minerals are spared. In this case, zircons — often referred to as Earth’s oldest timekeepers — were discovered safely tucked inside titanite, a mineral resistant to heat, pressure, and time.

In this case, researchers studied an area in the Yilgarn Craton; more specifically, at a dyke within the craton. Dykes are sheets of rock that form when magma intrudes into a crack and solidifies, cutting across older layers of rock. They’re often more resistant to erosion and can preserve ancient materials carried up from deep within the Earth.

Most of this area is old, but not that old. The rocks date from around 1.4 billion years to 2.7 billion years. But when researchers dated individual minerals, as opposed to the surrounding rock, they found a much longer history, stretching back to 3.4 billion years.

It’s a finding that mirrors the distant Jack Hills, famed for housing some of Earth’s oldest known zircon crystals. But unlike the Jack Hills, where zircons were found in ancient sedimentary rocks, the zircon here was discovered in place — encased in an igneous intrusion that cut through older rock and froze its contents in time.

In other words, this is a window into a hidden world beneath the crust.

Why zircon?

Zircon, familiar to most people as a cheap imitation to diamond, is an extremely sturdy mineral that can withstand billions of years under the right conditions.

The researchers didn’t just analyze zircon. They also looked at titanite and apatite — two other minerals capable of preserving radioactive isotopes that can be dated. Each mineral offers a timestamp from a different chapter in Earth’s story.

The titanite told a story of dyke formation around 1,390 million years ago, a time when ancient crustal fragments were swept up and embedded in rising magma. Later, around 1,000 million years ago, this rock was metamorphosed again, likely during the so-called Pinjarra Orogeny — a mountain-building event along what is now Australia’s western margin.

But the zircon is the oldest. It’s basically a time capsule from one of the oldest pieces of the crust that’s still around.

There could be other ancient zircons across the world

Minerals preserved in magmatic dykes may now emerge as a new frontier in geological research. Unlike sedimentary layers, dykes can sample materials from deep within the Earth, delivering relics of the past directly to the surface. They could help us understand what’s happening in the depths of the Earth, as well as what changes our planet underwent across the aeons.

Their approach — analyzing tiny mineral inclusions protected inside more durable hosts — could be applied across ancient terrain worldwide. It’s an elegant method for peering into the past without drilling miles into the crust.

For now, the 3.44-billion-year-old zircon in a titanite grain remains one of the oldest voices yet heard from Australia’s deep geological history. And it suggests that long before the outback turned red, before the cratons collided, before life itself found its footing — there was rock. And it remembers.

Journal Reference: Christopher L. Kirkland et al, Cryptic geological histories accessed through entombed and matrix geochronometers in dykes, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01469-6