Enigmatic processes operate below the surface and far above our reach.
Sometimes, clues to the Earth’s nature are revealed by tiny chthonic gemstones that protect rare mineral skerricks. We can learn a few bits about the interior of our planet from these small fragments.
This is exactly the kind of stone that was recently found in Botswana’s Diamond Mine. It has flaws with traces ferropericlase (enstatite), ringwoodite and other minerals. This suggests that the diamond formed 660 km (410 mi) below Earth’s Surface.
Moreover, they 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 presence of ringwoodite along with the hydrous phase indicates that there is a moist environment at this border,” Write a group of researchersTingting Gu, a mineral physicist at the Gemological Institute of New York, and Purdue University, is leading the charge.
The ocean covers most of the Earth’s surface. They are barely a puddle considering how many kilometers separate the surface from the core of the planet. The ocean, at its deepest point, is barely 11 kilometers (7 miles), thick from floor to wave-tops.
The Earth’s crust, however, is cracked and fractured. It has separate tectonic plates which grind together and slide under each other. Water seeps deeper into the earth’s crust at these subduction zones, reaching as far down as the lower mantle.
It eventually returns to the surface through volcanic activity over time. This is called the slurp down, spew out cycle. Deep water cycleThis is separate from the surface water cycle. Understanding how the water cycle works and how much water exists down there is crucial for understanding the geological activity on our planet. Water can have an impact on the explosiveness of volcanic eruptions, as well as play a role during seismic activity.
However, because we cannot get down there, we have to wait until evidence of the water comes to us. As it does with diamonds, which form crystal cages under extreme heat or pressure.
Gu and her coworkers recently studied this gem in depth. They discovered 12 inclusions and a cluster of inclusions. The researchers examined these inclusions using micro-Raman spectrum and Xray diffraction to determine their true nature.
They found an assemblage consisting of ringwoodite, a magnesium silicate, in contact with ferropericlase and enstatite, another magnesium silicate with a different structure.
At high pressures near the transition zone, ringwoodite is decomposed into ferropericlase as well as another mineral called Bridgmanite. Bridgmanite is transformed into enstatite at lower pressures. Their presence in diamonds tells the story of a journey. It indicates that the stone was formed at depth and then made its way back to the crust.
This wasn’t the end. The ringwoodite in particular had features suggesting it is hydrous in nature – a mineral that forms in the presence of water. Hydrous minerals can also be found in diamonds, like brucite. These clues indicate that the environment where the diamond was formed was quite wet.
Water evidence in the transition zone It has been used beforeThis evidence does not provide enough information to estimate the amount of water down there. Is it possible that it was a random inclusion from a tiny, localized water source? Or is it completely sloshy down there. Gu and her colleagues’ work suggests more sloshiness.
“Although upper-mantle-diamond formation is often associated with fluids, super deep diamonds with similar retrogressed minerals assemblages have rarely been observed with hydrous minerals.” They write in their paper.
“Even though a local H2O enrichment was suggested for the mantle transition zone based on the previous ringwoodite finding, the ringwoodite with hydrous phases, reported here – representative of a hydrous peridotitic environment at the transition zone boundary – indicates a more broadly hydrated transition zone down to and cross the 660-kilometer discontinuity.”
Research has shown that Earth is sinking. Way more waterWe were able to see more than we thought. This could be the answer to our questions about where it all is going.
The research was published in Nature Geoscience.
