How researchers found remnants of the Earth’s primordial crust near Perth

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

The oldest rocks in Australia, which are some of the oldest on Earth, are located in the Murchison district of Western Australia, 700 kilometers north of Perth. They are dated to almost 4 billion years.

In a new study published in Earth and environment communication, we found evidence of rocks of a similar age at Collie, south of Perth. This suggests that the ancient rocks of Western Australia cover a much larger area than we knew, buried deep in the crust.

Ancient continental crust

The ancient crust of Australia is crucial to understanding the early Earth because it tells us how the continental crust formed and evolved.

The continental crust forms the basis of landmasses where humans live, supports ecosystems, and provides essential resources for civilization. Without it, there would be no fresh water. It is rich in mineral resources such as gold and iron and is therefore economically significant.

However, the study of ancient continental crust is not easy. Most of them are deeply buried or have been intensively changed by their 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 using remote sensing of sound waves, magnetism, or gravity.

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

Pulling crystals from the depths

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

These dikes can pick up tiny amounts of minerals from the depths and bring them up to the surface where we can examine them.

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

Zircon contains trace amounts of uranium, which decays into lead over time. By accurately measuring the ratio of lead to uranium in zircon grains, we can tell how long ago the grain crystallized.

This method showed that the zircon crystals from the dike were 3.44 billion years old.

Titanite armor

The zircons are encased in another mineral, called titanite, which is chemically more stable than the zircon in the dike. Think of a grain of salt, trapped in a hard-boiled candy, dropped into a cup of hot tea.

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

However, the titanite-armored grains survived intact, providing a rare glimpse into Earth’s early history.

The dyke, about 1.4 billion years old, offered a unique window into the ancient crust that would otherwise have remained hidden. We also found similar ancient zircon grains further north in sand from the Swan River, which flows through Perth and drains the same area, further confirming the age and provenance of these ancient materials.

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

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 out zones for exploring mining potential.

Relics of deep time

So the next time you pick up a rock and some mineral grains rub off your hand, think about how long it might have been there.

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

Two weeks ago, the dinosaurs appeared. All of humanity has come in the last 30 minutes. and you? Sober, on this scale, your life would last about half a second.