Super-deep diamonds shine new light on Earth’s ancient past

16 August 2019

A first-of-its-kind analysis of 'super-deep' diamonds from hundreds of kilometres below the Earth's surface has provided new clues about the material composition of our planet.

The study also found new evidence to support the idea of a 'primordial' reservoir of molten rock that has been around since, or very soon after, the Earth's formation.

The diamonds likely came from a depth of 410-660km (known as the 'transition zone'), according to lead researcher Dr Suzette Timmerman, who was recently awarded her PhD from The Australian National University.

"We analysed, for the first time, helium isotopes contained in microscopic bubbles inside the super-deep diamonds," Dr Timmerman said.

"We found high Helium-3 (3He) and Helium-4 ratios (4He).

"This indicates the existence of a 'primordial' reservoir, as 3He is a stable isotope, meaning it doesn't decay into other elements. All 3He inside the Earth has been there since the beginning of time."

Dr Timmerman and co-author Dr Lynton Jaques say diamonds offer a unique glimpse inside the Earth. 

"There are only a few places in the world where these super-deep diamonds are found but they provide a lot of information about the deeper parts of the Earth's mantle, which we never otherwise see," Dr Jaques said.

"Diamonds form perfect capsules so they retain the exact chemistry and the isotope composition of material from the part of the Earth where they formed.

"These diamonds in particular are some of our deepest direct samples of the Earth. They allow us to see some original material from the formation of the Earth, that doesn't seem to have changed much over the past four billion years."

Dr Jaques says scientists, including researchers at the ANU, have used high pressure experiments to replicate the unique conditions in the Earth's mantle - but these diamonds offer a natural sample. 

Despite the breakthrough, there are still plenty of unanswered questions about this ancient reservoir.

"People had long thought there was this high Helium-3 patch in the mantle and we can say a little bit more about it now, but we still don't know how big it is,  or how evenly distributed across the deeper parts of the mantle it is," said Dr Timmerman.

The research has been published in the journal Science.

Dr Timmerman will present her findings at the Goldschmidt Conference on 23 August, an annual conference organised by the European Association of Geochemistry and the Geochemical Society.