The Earth occasionally spews clues to their nature: tiny chthonic diamonds encasing skerricks of rare mineral. We can glean tidbits of information about our planet's interior from these tiny fragments. A diamond of this type was recently discovered in a diamond mine in Botswana. It has flaws that contain traces of ringwoodite, ferropericlase, enstatite, and other minerals, indicating that the diamond formed 660 kilometres (410 miles) below Earth's surface.
Furthermore, they imply that the environment in which they formed - a 660-kilometer discontinuity (or, more simply, the transition zone) between the upper and lower mantle - is rich in water. "The presence of ringwoodite, along with the hydrous phases, indicates a wet environment at this boundary," write researchers led by mineral physicist Tingting Gu of the Gemological Institute of New York and Purdue University. The ocean covers the majority of the Earth's surface. Considering the thousands of kilometres between the planet's surface and its core, they're barely a puddle. Even at its deepest point, the ocean is only about 11 kilometres (7 miles) thick from the surface to the bottom.
However, the Earth's crust is cracked and fragmented, with separate tectonic plates grinding together and slipping under each other's edges. Water seeps deeper into the planet at these subduction zones, reaching as far as the lower mantle. It eventually makes its way back to the surface due to volcanic activity. This slurp-down, spew-out cycle is known as the deep water cycle, and it is distinct from the surface water cycle. Understanding how it works and how much water is down there is also important for understanding our planet's geological activity. The presence of water, for example, can influence the explosiveness of a volcanic eruption and play a role in seismic activity.
We have to wait for evidence of the water to come to us because we can't get down there, as it does in the form of diamonds that form crystal cages in the extreme heat and pressure. Gu and her colleagues recently investigated such a gem in depth, discovering 12 mineral inclusions and a cluster of milky inclusions. The researchers investigated these inclusions using micro-Raman spectroscopy and X-ray diffraction to determine their nature.
They discovered an assemblage of ringwoodite (magnesium silicate) in contact with ferropericlase (magnesium/iron oxide) and enstatite among the inclusions (another magnesium silicate with a different composition). Ringwoodite decomposes into ferropericlase and another mineral called bridgmanite at high pressures in the transition zone. Bridgmanite transforms into enstatite at lower pressures near the surface. Their presence in the diamond tells the tale of a journey, indicating that the stone formed at depth before rising to the crust.
"Although the formation of upper-mantle diamonds is frequently associated with the presence of fluids," they write in their paper, "super-deep diamonds with similar retrogressed mineral assemblages have rarely been observed accompanied by hydrous minerals." "Even though the previous ringwoodite finding suggested a local H2O enrichment for the mantle transition zone, 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 crossing the 660-kilometer discontinuity."
Previous research has revealed that the Earth is sucking in far more water than previously thought. This could finally tell us where everything is going.
