A surprisingly hardy reservoir of rock left over from just after Earths formation still lurks deep inside the planet, according to a new analysis of superdeep diamonds.
Fluid trapped inside these diamonds, forged hundreds of kilometers underground in Earths mantle, bears the chemical signatures of rock that has remained relatively undisturbed for billions of years. This holdout of primordial rock may be nearly as ancient as Earth itself — making it some of the oldest preserved material on the planet today, researchers report in the Aug. 16 Science. Understanding the characteristics and preservation of such a pristine piece of early Earth may yield new insights into the formation and evolution of the planet.
Past chemical analyses of volcanic rock have hinted that Earths mantle might contain a repository of extremely old material. But scientists werent sure whether such an unspoiled relic could withstand the continual churning and mixing of material there. Evidence from volcanic rock is hard to trust on its own: Molten rock tends to get contaminated as it pushes up through the crust, and its difficult to pinpoint where specific bits of rock originated, says Suzette Timmerman, a geochemist at the Australian National University in Acton.
For a more direct glimpse into Earths interior, Timmerman and colleagues scrutinized 24 superdeep diamonds from Brazil that were known to have formed 410 to 660 kilometers underground. As the diamonds crystallized, they swallowed up microscopic pouches of fluid from their surroundings (SN Online: 3/8/18). When a superdeep diamond rises to Earths surface, its sturdy crystal structure shields these diamond inclusions from contamination, Timmerman says. “Its exactly preserving the chemical composition at those really deep depths.”
Using mass spectrometry, the researchers cataloged different isotopes, or types, of elements enclosed in superdeep diamond inclusions. “Its quite impressive how theyve managed to analyze a signal which must be very tiny” from fluid bubbles less than a micrometer across, says Andrew Thomson, a geologist at University College London.
Timmermans team compared the abundance of two helium isotopes, helium-3 and helium-4, in the diamond inclusions. Unlike helium-4, Earth has not generated any new helium-3 since its formation, and any helium-3 that reaches Earths surface escapes into space, Thomson says. So material that is relatively rich in helium-3 compared with helium-4 must have formed early in Earths existence and been isolated for a very long time from the surface — and any other internal processes that could siphon away helium.
In the superdeep diamond inclusions that were richest in helium-3, the ratio of helium-3 to helium-4 was about 1-to-14,300. That may not seem like mRead More – Source