The Earth «was born» about 4.5 billion years ago. To understand its long history, scientists study the rocks and minerals that make it up. The rocks in Australia, some of the oldest on Earth, are 700 km north of the city of Perth — they are almost 4 billion years old. In a new study, scientists have found evidence of rocks of the same age south of Perth. This indicates that the ancient rocks cover a much larger area than was previously known.
The ancient rocks of Australia are crucial to understanding the early Earth. They tell us how the continental crust, which forms the basis of the land, was formed and evolved. Without it, there would be no fresh water, and it is rich in mineral resources.
Most of the ancient continental crust is deeply buried or heavily modified by the environment. There are only a few open areas where researchers can directly observe it. To understand the age and composition of the hidden crust, scientists rely on indirect methods, such as studying eroded minerals preserved in upper basins or using remote sensing. However, there is another way to look into the deep crust and, if you’re lucky, even get a sample.
The Earth’s crust is cut by magma veins that can extend from the top of the crust all the way to the Earth’s mantle. These structures, known as dikes, can emerge from a depth of at least 50 km. They can transport small amounts of minerals from the depths to the surface where they can be found and studied. Scientists have found evidence of ancient buried rocks by dating zircon grains from one of these dikes.
Zircon contains trace amounts of uranium, which decays over time to form lead. Accurate measurement of the ratio of lead to uranium in zircon grains allows us to determine how long ago the grain crystallized. The method showed that the studied zircon crystals date back to 3.44 billion years.
Zircon is encapsulated in another mineral, titanite, which is more chemically stable than zircon. The stability of the titanite armor protected the ancient zircon crystals from changes in chemicals, pressure, and temperature as the dam moved upward. The unshielded zircon crystals in the dam were heavily altered during the journey and lost their isotopic records.
The dike, itself dated to about 1.4 billion years old, offered a unique window into ancient crust that would otherwise have remained hidden. One of the reasons understanding the deep crust is important is that people often find metals at the boundaries between blocks of this crust. Mapping these blocks can help identify areas for exploration of their mining potential.
Source: The Conversation