After a new analysis of super-deep diamonds, a surprisingly hard rock reservoir still lurks inside the planet, left over immediately after the Earth's creation.

Liquid trapped inside this diamond, forged hundreds of miles underground, bears the chemical signatures of rocks that have remained relatively undisturbed for billions of years. This remnant of ancient rock may be almost as old as Earth itself – making it one of the oldest surviving materials on the planet today, researchers report on August 16 science, Understanding the properties and preserving such an untouched part of the early Earth can provide new insights into the formation and evolution of the planet.

Past chemical analyzes of volcanic rock have shown that the Earth's mantle may contain a repository of extremely old material. However, scientists were not sure if such an untouched relic could withstand the constant stirring and mixing of material. Evidence of volcanic rock alone is hard to trust: molten rock tends to become contaminated as it is thrown through the crust, and it is difficult to pinpoint where specific rocks have been created, says Suzette Timmerman, geochemist at the Australian National University in Australia Acton.

To get a closer look at the Earth's interior, Timmerman and his colleagues examined 24 Superdeep diamonds from Brazil that were known to have originated 410 to 660 kilometers underground. As the diamonds crystallized, they swallowed microscopic liquid pouches from their environment (SN Online: 08.03.18). When a Superdeep diamond rises to the surface, its robust crystal structure protects these diamond inclusions from contamination, Timmerman says. "It's all about keeping the chemical composition in those really deep lows."

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Using mass spectrometry, researchers cataloged different isotopes or types of elements encased in super-deep diamond inclusions. "It's amazing how they managed to analyze a signal that needs to be very small," says Andrew Thomson, a geologist at University College London.

Timmerman's team compared the abundance of two helium isotopes, helium-3 and helium-4, in the diamond inclusions. Unlike helium-4, the Earth has not produced any new helium-3 since its formation, and any helium-3 that reaches the Earth's surface escapes into space, according to Thomson. Material that is relatively rich in helium-3 compared to helium-4 must have formed early on Earth and isolated from the surface for a very long time – and all other internal processes that could derive helium.

In the super-deep diamond inclusions richest in helium-3, the ratio of helium-3 to helium-4 was about 1 to 14,300. That may not seem like much, but is about 50 times higher than the ratio of helium-3 to helium-4 in the air: only 1 to about 714,000.

This suggests that the fluid inclusions date back to about 4.5 billion years ago. Based on the depths at which the diamonds crystallized, this reservoir is likely to be at least 410 kilometers underground, but it is unclear how much deeper it could be. Determining the exact location of the reservoir could explain how it formed and remained undisturbed for so long, says Thomson.

The overall chemical composition of the reservoir is still a mystery, "but it must be fairly dense to avoid mixing with the rest of the shell," says Timmerman, whose team will present the work at the Goldschmidt geochemistry conference in Barcelona in August 23 Investigation of other isotopes, such as metals enclosed in super-deep diamond inclusions, may provide further clues to the chemical mixture in this reservoir.

Another question is whether the reservoir contains a huge mass or several smaller pockets of ancient material. Seismic observations have discovered two particularly dense clumps of rock at the bottom of the mantle (SN: 6/11/16, p. 13), "But we're not sure if it's this reservoir or something else," Timmerman says.