Perth, our planet was born about 4.5 billion years ago. To understand this mind-bogglingly long history, we need to study the rocks and minerals they are made of.
Australia's oldest rocks, 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 ago.
In a new study, we 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.
Australia's ancient crust is crucial to understanding the early Earth, because it tells us how continental crust formed and evolved.
The continental crust forms the basis of the land masses 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, which makes 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 ancient hidden crust, scientists often turn to 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 look at the deep crust and hopefully even sample it.
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 emerge from depths of at least 50 kilometers (much deeper than even the deepest well, which extends just 12 kilometers).
These dams can collect small amounts of minerals from the depths and transport them to the surface, where we can examine them.
In our recent study, we have uncovered evidence of ancient buried rocks by dating zircon grains from one of these dykes.
Zircon contains traces of uranium, which over time decomposes and turns into lead. 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 in the dam date back 3.44 billion years.
titanite armor
Zircons are encapsulated in a different mineral, called titanite, which is chemically more stable than the zirconium in the dam. Think of a grain of salt, trapped inside a hard fudge, dropped into a hot cup of tea.
The stability of the titanite armor protected the ancient zircon crystals through changes in chemical, pressure, and temperature conditions as the dam ascended. The zircon crystals discovered in the dam were strongly modified during the trip, erasing their isotopic records.
However, the titanite-coated grains survived intact and provided a rare glimpse into Earth's early history.
The dyke, which dates back around 1.4 billion years, has offered a unique window into ancient crust that would otherwise have remained hidden. We also found similar ancient zircon grains further north in the sand of the Swan River, which runs through Perth and drains the same region, further corroborating the age and origin of these ancient materials.
The results expand the known area of ancient crust, previously recognized in the Narryer area of the Murchison district.
One reason it is important to understand the deep crust is because we often find metals at the boundaries between blocks of this crust. Mapping these blocks can help plot areas to investigate mining potential.
So, the next time you pick up a rock and some mineral grains stick to your hand, think about how long those grains may have existed.
To understand the time scale, let's imagine that the history of our planet lasted one year. The Earth formed from dust devils 12 months ago. Any handful of sand you pick up in Perth will contain one or two grains from about ten months ago. Most of Australia's gold was formed seven months ago and land plants arrived only a month ago.
Two weeks ago, the dinosaurs appeared. All of humanity has come in the last 30 minutes. And you? Surprisingly, on this scale, your life would last about half a second. (The conversation) RUP
Australia's oldest rocks, 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 ago.
In a new study, we 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.
Australia's ancient crust is crucial to understanding the early Earth, because it tells us how continental crust formed and evolved.
The continental crust forms the basis of the land masses 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, which makes 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 ancient hidden crust, scientists often turn to 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 look at the deep crust and hopefully even sample it.
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 emerge from depths of at least 50 kilometers (much deeper than even the deepest well, which extends just 12 kilometers).
These dams can collect small amounts of minerals from the depths and transport them to the surface, where we can examine them.
In our recent study, we have uncovered evidence of ancient buried rocks by dating zircon grains from one of these dykes.
Zircon contains traces of uranium, which over time decomposes and turns into lead. 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 in the dam date back 3.44 billion years.
titanite armor
Zircons are encapsulated in a different mineral, called titanite, which is chemically more stable than the zirconium in the dam. Think of a grain of salt, trapped inside a hard fudge, dropped into a hot cup of tea.
The stability of the titanite armor protected the ancient zircon crystals through changes in chemical, pressure, and temperature conditions as the dam ascended. The zircon crystals discovered in the dam were strongly modified during the trip, erasing their isotopic records.
However, the titanite-coated grains survived intact and provided a rare glimpse into Earth's early history.
The dyke, which dates back around 1.4 billion years, has offered a unique window into ancient crust that would otherwise have remained hidden. We also found similar ancient zircon grains further north in the sand of the Swan River, which runs through Perth and drains the same region, further corroborating the age and origin of these ancient materials.
The results expand the known area of ancient crust, previously recognized in the Narryer area of the Murchison district.
One reason it is important to understand the deep crust is because we often find metals at the boundaries between blocks of this crust. Mapping these blocks can help plot areas to investigate mining potential.
So, the next time you pick up a rock and some mineral grains stick to your hand, think about how long those grains may have existed.
To understand the time scale, let's imagine that the history of our planet lasted one year. The Earth formed from dust devils 12 months ago. Any handful of sand you pick up in Perth will contain one or two grains from about ten months ago. Most of Australia's gold was formed seven months ago and land plants arrived only a month ago.
Two weeks ago, the dinosaurs appeared. All of humanity has come in the last 30 minutes. And you? Surprisingly, on this scale, your life would last about half a second. (The conversation) RUP
