Groundbreaking discovery: how researchers found remains of Earth’s original crust near Perth

Our planet was born about 4.5 billion years ago. To understand this mind-bogglingly long history, we must study rocks and the minerals from which they are made.

The oldest rocks in Australia, which are among the oldest on Earth, are found in the Murchison District of Western Australia, 700 kilometers north of Perth. They have been dated to almost 4 billion years old.

From a new study published in Communication Earth & Environmentwe have found evidence of rocks of a similar age near 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.

The ancient continental crust

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

Continental crust forms the basis of landmasses where people 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, making it economically significant.

However, exploring the ancient continental crust is not easy. Most of it is deeply buried, or has been greatly modified by the 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 any luck, even taste it.

Dragging crystals up 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 dikes, can originate from depths of at least 50 kilometers (much deeper than even the deepest borehole, which extends only 12 kilometers).

These dikes can pick up small amounts of minerals from deep and transport them all the way to the surface, where we can examine them.

In our recent research, we found evidence of ancient buried rock by dating zircon grains from one of these dikes.

Zircon contains traces of uranium, which decays to 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 are 3.44 billion years old.

Titanite armor

The zircons are encased in another mineral called titanite, which is chemically more stable than zircon in the dike. Think of a grain of salt trapped in a hard-boiled sugar 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 rose. Unshielded zircon crystals in the dike were heavily modified during the voyage, destroying their isotopic records.

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

The dike, itself about 1.4 billion years old, has provided a unique window into the ancient crust that would otherwise have remained hidden. We have also found similar ancient zircon grains further north in the sands of the Swan River, which runs through Perth and drains the same region, further confirming 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 reason why it is important to understand the deep crust is because we often find metals at the boundaries between the blocks of this crust. By mapping these blocks you can map out zones that you can investigate for mining potential.

Remnants of deep time

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

To get a handle on the time scale, imagine that the history of our planet lasted one year. The Earth was formed twelve months ago from swirling dust. Every handful of sand you pick up around Perth contains one or two grains from about ten months ago. Most of Australia’s gold originated seven months ago, while land plants only arrived a month ago.

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

This article is republished from The Conversation under a Creative Commons license. Read the original article.