Deep below the surface of our world, far beyond our feeble reach, enigmatic processes move and stir.
From time to time, Earth throws up clues about their nature: small chthonic diamonds containing skerricks of rare minerals. From these small fragments we can extract information about the interior of our planet.
A recently discovered diamond in a diamond mine in Botswana is a stone. It is full of flaws that contain traces of ringwoodite, ferropericlase, enstatite and other minerals that suggest the diamond formed 660 kilometers (410 miles) below the Earth’s surface.
They also suggest that the environment in which they formed – a divide between the upper and lower mantle called the 660 kilometer discontinuity (or, more simply, the transition zone) – is rich in water.
“The occurrence of ringwoodite together with hydrated phases indicates a humid environment at this boundary,” writes a team of researchers led by mineral physicist Tingting Gu of the Gemological Institute of New York and Purdue University.
Most of the Earth’s surface is covered by ocean. However, considering the thousands of kilometers between the surface and the core of the planet, they are hardly a puddle. Even at its deepest point, the ocean is only 11 kilometers (7 miles) thick from the waves to the ground.
But the Earth’s crust is a cracked, fragmented thing, with separate tectonic plates grinding and squishing beneath each other’s edges. In these subduction zones the water climbs deeper into the planet, reaching the lower mantle.
Over time it returns to the surface through volcanic activity. This cycle of intrusion and expulsion is known as the deep water cycle, separate from the water cycle active at the surface. Knowing how it works, and how much water is down there, is also important to understanding the geological activity of our planet. The presence of water can influence the explosiveness of a volcanic eruption, for example, and play a role in seismic activity.
Since we can’t go down there, however, we have to wait for evidence of water to come to us, as it does in the form of diamonds that form crystal cages in the extreme heat and pressure.
Gu and his colleagues recently studied this gem in detail, finding 12 mineral inclusions and a milky inclusion cluster. Using micro-Raman spectroscopy and X-ray diffraction, the researchers investigated these inclusions to determine their nature.
Among the inclusions they found a set of ringwoodite (magnesium silicate) in contact with ferropericlase (magnesium/iron oxide) and enstatite (another magnesium silicate with a different composition).
At the high pressures in the transition zone, ringwoodite decomposes into ferropericlase, as well as another mineral called bridgmanite. At lower pressures closer to the surface, bridgmanite is converted to enstatite. Its presence in the diamond tells the story of a journey, indicating the stone formed at depth before returning to the crust.
That wasn’t all. Ringwoodite in particular had characteristics that suggest it is hydrated in nature, a mineral that forms in the presence of water. Meanwhile, other minerals found in diamond, such as brucite, are also hydrated. These clues suggest that the environment in which the diamond formed was quite wet.
Evidence of water has been found in the transition zone before, but this evidence has not been sufficient to assess how much water is down there. Was it a chance inclusion of a small localized pocket of water, or is it positively slony down there? The work of Gu and his team points more towards the shoulder.
“Although upper mantle diamond formation is often associated with the presence of fluids, super-deep diamonds with similar regressed mineral assemblages accompanied by hydrous minerals have rarely been observed,” they write in their paper.
“Although a local enrichment of H2O was suggested for the mantle transition zone based on the previous finding of ringwoodite, the ringwoodite with hydrated phases, reported here, representative of a hydrated peridotitic environment at the boundary of the transition zone, indicates a more extensively hydrated transition zone downward. and cross the 660-kilometer discontinuity.”
Previous research has found that the Earth sucks up much more water than we previously thought. This could finally give us an answer as to where it all goes.
The research has been published in Nature Geoscience.