Ancient diamond deposit discovered near Earth's core

The team looked at helium isotopes in superdeep diamonds. Photo:Independent

Diamonds that had lain dormant for 4.5 billion years hundreds of kilometers underground were brought to the surface by a powerful volcanic eruption in Brazil, according to a study published on August 16 in the journal Science. The research team led by Dr. Suzette Timmerman of the Australian National University measured helium isotopes in the diamonds to determine the location of the ancient diamond cache.

The team says the diamonds act as a “perfect time capsule” to help them delve deeper into the chaotic period after Earth’s formation, during which geological activity was so intense that little of the young planet’s original structure remained.

However, scientists have long suspected that there is a relatively undisturbed zone in the mantle between the crust and the core. They have not found evidence to support this hypothesis. The first clue came in the 1980s when scientists noticed that some basaltic lava samples from several locations had higher than normal levels of helium-3 and helium-4 isotopes. Interestingly, these ratios were similar to the isotopic ratios found in the first meteorites to hit Earth.

The similarities suggest that the lava comes from a region deep within the Earth that has not changed over billions of years. "The problem is that although basalt rocks have been brought to the surface, we only know a glimpse of their history. We don't know much about the mantle where they form," Dr Timmerman said.

Dr Timmerman and his colleagues studied the helium isotope ratios in diamonds formed 150-230km below the Earth's crust. "Diamonds are the hardest and most indestructible natural materials known to man. They are the perfect time capsule for exploring the Earth's interior. We extracted helium from 23 ultra-deep diamonds in the Juina region of Brazil. These diamonds had the isotope ratios we expected from ancient reservoirs, confirming that the gases were from a period before the Moon's impact with the Earth," said Dr Timmerman.