Ugly diamonds hold more than a billion years of Earth’s history

Cloudy and yellowish “fibrous diamonds” are too unsightly for most jewelers. But for scientists, their crystal structure holds precious secrets dating back a billion years or more.

Yaakov Weiss, an Earth scientist at the Hebrew University of Jerusalem, and his colleagues crushed portions of South African fibrous diamonds to extract tiny pockets of fluid trapped inside. This fluid, from which diamonds once formed, holds a unique record of conditions long ago deep in the Earth. It also contains uranium and thorium, which decay into the helium 4 isotope and gradually escape from the crystal lattice of diamond. But no one knew the precise leak rate, which would be needed to determine the age of the diamonds and unlock the story inside.

By modeling this decay and the amount of helium-4 leakage possible over time, Weiss and his colleagues determined a broad age range for the calculations. They then ruled out ages that would be impossible based on the known tectonic and thermal conditions in the Earth’s mantle and crust at the site of diamond formation. The combination of this data gave an upper limit of leakage, which the researchers were able to apply to all the fibrous diamonds they studied. They recently described their results in Nature Communication.

The team dated the fluid to three distinct periods, each coinciding with large changes on the surface. The oldest diamonds were between 750 million and 2.6 billion years old; scientists reduced their creation to about a billion years ago, when tectonic forces built craggy mountains in what is now South Africa. The second oldest formed 300-540 million years ago, coinciding with the formation of the Naukluft Mountains in Namibia. The youngest formed between 85 million and 118 million years ago, just before an underground eruption propelled them into the upper crust.

In addition, the fluid was rich in carbon in the older diamonds, heavy in silica in the next older, and rich in salt in the younger. It could also echo important geological changes: for example, the younger fluids may come from the oceanic crust pushed deep into the Earth when the oceanic plate slid under the continental crust.

No other rock or mineral in the deep Earth reaches the surface with as little interior alteration as a diamond, says University of Alberta scientist Suzette Timmerman, who was not involved in the study . The fluids thus provide a rare direct window onto the deep lithosphere (crust) and the upper mantle. “Everything inside is basically a time capsule,” says Timmerman.

Next, the researchers plan to check diamonds from other regions for similar correlations between formation and large surface events, Weiss said: “We’ll have to think about what exactly this says about mantle and mantle evolution. lithosphere. “

About Lucille Thompson

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