Deep below the surface of our world, beyond our feeble reach, mysterious processes grind and sway.
Every now and then, Earth unleashes clues to its nature: tiny chthonic diamonds encasing a skate of rare minerals. From these little bits we can extract tidbits of information about our planet’s interior.
A diamond recently discovered in a diamond mine in Botswana is just such a stone. It is riddled with flaws containing traces of ringwoodite, ferropericlase, enstatite, and other minerals that indicate diamonds formed 660 kilometers (410 miles) below the Earth’s surface.
Moreover, they suggest that the environment in which they formed – a gap between the upper and lower mantle called the 660-kilometre discontinuity (or more simply, the transition zone) – is rich in water.
“The occurrence of ringwoodites with aquatic phases indicates a humid environment at this boundary,” Write a team of researchers It was 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 the ocean. However, given the thousands of kilometers between the surface and the core of the planet, it is hardly just a puddle. Even at its deepest point, the ocean is just shy of 11 kilometers (7 miles) thick, from the tops of the waves to the land.
But the Earth’s crust is something that is cracked and fragmented, with separate tectonic plates coming together and sliding under each other’s edges. In these subduction zones, water seeps deeper into the planet, until it reaches the lower mantle.
Over time, it returns to the surface by volcanic activity. This flow-through cycle is known as deep water cycleSeparate from the active water cycle on the surface. Knowing how it works, and how much water is there, is also important for understanding the geological activity of our planet. The presence of water can influence the eruption of a volcano, for example, and play a role in seismic activity.
Because we can’t get down there, we have to wait for evidence of water to come to us, as happens in the form of diamonds that form crystal cages at extreme temperatures and pressures.
Jo and her colleagues recently studied such a gem in detail, finding 12 metallic inclusions and a group that included milky. Using precise Raman spectroscopy and X-ray diffraction, the researchers examined these inclusions to determine their nature.
Among the inclusions, they found a group of ringwoodite (magnesium silicate) in contact with ferropyricylase (magnesium oxide / iron) and enstatite (other magnesium silicate with a different composition).
At high pressure in the transition zone, ringwoodite decomposes into ferropericlase, as well as another mineral called bridgemanite. At low pressures closer to the surface, bridgemanite becomes diffuse. Their presence in diamonds tells the story of a journey, indicating that the stone formed at depth before making its way back into the crust.
That wasn’t all. Ringwoodite in particular has features that suggest it is watery 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 indicate that the environment in which the diamonds formed was very humid.
Evidence of the presence of water in the transition zone Found by, but this evidence was not enough to measure how much water is there. Was it a chance containment of a small spot pocket of water, or was it positively sloppy in there? The work of Joe and her team is more indicative of indolence.
“Although diamond formation in the upper mantle is often associated with the presence of fluids, ultra-deep diamonds have rarely been observed with similar retreating mineral assemblies accompanied by hydrated minerals,” They write on their papers.
“Although a local H2O enrichment of the mantle transition region has been suggested based on the previous ringwoodite result, the aqueous-phase ringwoodite, reported here—representative of the aqueous enrichment environment at the transition zone boundary—indicates a more extensively hydrated transition region down to and crossing the 660-kilometre discontinuity.
Previous research found that the Earth absorbs way more water than we thought before. This could finally give us an answer to where it’s all headed.
The search was published in natural earth sciences.