Super-deep diamond haul confirms subsurface reservoir older than the moon

16 Aug 2019

Diamonds from the Juina area, most of which are super-deep diamonds. Image: Graham Pearson

The discovery of diamonds from the deepest depths of the Earth has confirmed the existence of a vast, ancient reservoir undisturbed by planetary formation.

For decades, scientists have speculated that there exists a reservoir of rock – somewhere between the Earth’s crust and core – that has remained totally undisturbed since its very beginning. Not long after forming, the planet underwent violent geological activity and experienced a number of extra-terrestrial impacts, which means almost none of its original structure remains.

However, in new research published to Science, an international team of researchers has shown gases found in microscopic inclusions in diamonds prove the existence of this elusive reservoir more than 410km below sea level in Earth’s mantle.

Unlike most diamonds that form between 150km and 230km below the Earth’s crust and eventually rise to the surface, super-deep diamonds form between 230km and 800km deep. Not only do they look remarkably different to regular diamonds, but because they come from so much deeper down in the Earth they’re an incredibly rare find.

Their importance first came to light in the 1980s when geochemists noted that in some basalt lavas from particular locations the ratio of the helium-3 to helium-4 isotope was higher than expected, mirroring the isotope ratio found in extremely old meteorites that had fallen to Earth.

‘The oldest remaining undisturbed material on Earth’

Suzette Timmerman of the Australian National University explained that she and her fellow researchers measured the helium isotope ratios in 23 super-deep diamonds from the Juina region of Brazil.

“These showed the characteristic isotopic composition that we would expect from a very ancient reservoir, confirming that the gases are remnants of a time at, or even before, the moon and Earth collided,” Timmerman said.

“From the geochemistry of the diamonds, we know that they formed in an area called the ‘transition zone’, which is between 410km and 660 km below the surface of the Earth. This means that this unseen reservoir, left over from the Earth’s beginnings, must be in this area or below it.”

However, a number of questions remain, such as whether there is one single, enormous reservoir or multiple reservoirs, as well as what is its chemical composition and where is it even located?

“With this work we are beginning to home in on what is probably the oldest remaining comparatively undisturbed material on Earth,” Timmerman added.

Colm Gorey was a senior journalist with Silicon Republic