Groundbreaking discovery: How researchers discovered remains of Earth’s primordial crust near Perth

Our planet was born about 4.5 billion years ago. To understand this incredibly long history, we need to study the rocks and minerals they are made of.

Australia’s oldest rocks, some of the oldest on the planet, are found in the Murchison district of Western Australia, 700 kilometers north of Perth. They are dated almost 4 billion years ago.

In a new study published in Earth and Environment Communications, we found evidence of rocks of a similar age near Collie, south of Perth. This suggests that ancient Western Australian rocks cover a much larger area than we imagined, buried deep in the crust.

The ancient continental crust

Ancient Australian crust is crucial to understanding the early history of the Earth, because it tells us about the formation and evolution of continental crust.

The continental crust forms the base of the landmasses where humans live, supporting ecosystems and providing essential resources for civilization. Without it, there would be no fresh water. It is rich in mineral resources such as gold and iron, making it economically important.

However, exploring ancient continental crust is not easy. Most of it is deeply buried or has been intensely modified by its environment. There are only a few exposed areas where researchers can directly observe this ancient crust.

To understand the age and composition of this hidden ancient crust, scientists often rely on indirect methods, such as studying eroded minerals preserved in overlying basins, or remote sensing of sound waves, magnetism or gravity.

However, there may be another way to peer into the deep crust and, with any luck, even taste it.

Dragging Deep Crystals

Our planet’s crust is frequently cut by dark fingers of magma, rich in iron and magnesium, which can extend from the upper crust into the Earth’s mantle. These structures, called dikes, can come from depths of at least 50 kilometers (much deeper than even the deepest borehole, which extends only 12 kilometers).

These dykes can capture tiny amounts of minerals from deep below and transport them to the surface, where we can examine them.

In our recent study, we discovered evidence of ancient buried rocks by dating zircon grains from one of these dykes.

Zircon contains traces of uranium which, over time, decays to produce lead. By precisely measuring the lead/uranium ratio in zircon grains, we can determine how long ago the grain crystallized.

This method showed that the dyke’s zircon crystals date back 3.44 billion years.

Titanium armor

The zircons are encapsulated in a different mineral, called titanite, which is chemically more stable than the zircon found in the dyke. Think of a grain of salt, trapped in hard sugar, placed in a cup of hot tea.

The stability of the titanite armor protected the ancient zircon crystals from changes in chemical conditions, pressure, and temperature as the dike moved upward. Unprotected zircon crystals in the dyke were heavily modified during the journey, erasing their isotopic records.

However, the grains encased in titanite have survived intact and offer a rare glimpse into Earth’s early history.

The dike, itself dated to around 1.4 billion years old, opened a unique window into ancient crust that otherwise would have remained hidden. We also found similar ancient zircon grains further north in the sand of the Swan River, which flows through Perth and drains the same region, further corroborating the age and origin of these ancient materials.

The results extend the known area of ​​ancient crust, previously recognized in the Narryer area of ​​the Murchison district.

One of the reasons it is important to understand the deep crust is that we often find metals at the boundaries between blocks of this crust. Mapping these blocks can help map areas to study mining potential.

Remains of deep time

So the next time you pick up a rock and see mineral grains rubbing off on your hand, think about how long these grains may have existed.

To understand the time scale, imagine that the history of our planet lasted one year. The Earth formed from swirling dust 12 months ago. Any handful of sand you pick up around Perth will contain a grain or two from around ten months ago. Most of Australia’s gold was formed seven months ago and land plants arrived only a month ago.

Two weeks ago, dinosaurs appeared. All of humanity has come in the last 30 minutes. And you? Let’s be honest, on this scale your life would last about half a second.