Oldest Known Rocks Give New Clues to the Earth’s History

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At 4.4 billion years old, this zircon crystal is the oldest known. Image credit: John Valley

At 4.4 billion years old, this zircon crystal is the oldest known. Image credit: John Valley

Geologists call it ‘deep time’ – a period back in the very earliest history of our planet, a time from which virtually nothing remains of the churning, dynamic early Earth.

But our understanding of the very ancient history of our planet is improving – and now research into some of the oldest rocks is suggests that the planet may have formed a solid crust – and thus begun the process by which it eventually became habitable – even earlier than was previously thought.

Dating the Earth

Science estimates that the Earth is around 4.5 billion years (Ga) old. We know this through radiometric dating of minerals which function, effectively, as mineralogical ‘clocks’. Radioactive decay converts isotopes (types of element) of one element – the so-called parent isotope – to another, which is known as the daughter.

High precision measuring of the relative proportions of particular sets of isotopes in mineral grains, along with knowledge of how long the process takes (the half life) allows scientists to calculate the age of particular rocks.

But going back so far in time, into a period known as the Hadean, inevitably involves a degree of uncertainty, as rocks can be destroyed as well as created. The most suitable dating material comes from tiny grains of a durable mineral known as zircon, which we can date by the decay relationship between uranium, thorium and lead (U-Th-Pb). This material, as the researchers note, provides: “The only physical evidence from the earliest phases of Earth’s evolution.”

The Jack Hills Zircon

Australia's Jack Hills contain some of the planet's oldest rocks. Image credit: NASA

Australia’s Jack Hills contain some of the planet’s oldest rocks. Image credit: NASA

In their study, published in Nature Geoscience, the research team applied dating methods to the oldest known rock on the planet – a zircon grain from the ancient Jack Hills region of Australia.

Using new technology, atom-probe tomography, in conjunction with more conventional dating methods of mass spectrometry, they dated the zircon grains at a staggering 4.4 Ga old.

The most significant finding of the study, according to lead author Professor John Valley, lies in the accuracy of the dating process, through which they were able to provide what he described as “the oldest reliably dated sample from Earth.

The Hadean zircon suite shows that the earth had crust by 4.4 Ga and liquid water oceans by 4.3 Ga,” he told Decoded Science. “Thus conditions were habitable for life  over 800 million years before the oldest known microscopic fossils.”

Early Earth: Implications of the Study

So what does the study of a tiny grain of rock tell us about the early Earth? The planet formed from coalescence of matter following the formation of the solar system and the crust solidified as the planet cooled. Very early in the planet’s existence, a collision with a large planetary object created the Moon and the force of the impact rested a so-called ‘magma ocean’ over the surface.

Clearly, without cooling and without the creation of the earth, the planet could not be habitable. The dating of the Jack Hills zircon, with its ancient core and a later growth around it (dated at 3.4 Ga), provides a fixed point in time at which the Hadean Earth had land. “this is the oldest known crystal from earth,” said Professor Valley. “There could be others slightly older, but not older than 4.5 Ga when the earth was a magma ocean.”

We know, then, when there was solid land and when there was water. Does this have any implications for the initiation of life? The oldest known fossils at present date from somewhere around 3.5-3.8 Ga, indicating a long gap between the emergence of land and the beginnings of life on Earth. But, as Professor Valley says: “We don’t have evidence that life existed then, but there is no evidence that it didn’t.”

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