A rare diamond found in the mines of Botswana has provided more details about the region between the Earth’s upper and lower mantle. Also called the transition zone or the 660 km discontinuity, the region is likely to be rich in water, according to a recent study.
Finding large amounts of water underground on a planet whose 71 percent surface is water may not sound like a big revelation. Yet it is. Liquid water on the Earth’s surface may seem like a lot but it is merely a puddle when compared to the water content under the crust.
The outermost layer of the planet, the crust, is fragmented, and the tectonic plates constantly grind against each other and at times slip under each other. Over here, water slips deeper into the planet and goes as far as the lower mantle, Science Alert said in its report.
The deep water cycle
Over a period of time, this water that has slipped below the Earth’s surface is sent back to the surface during volcanic activities. This cycling of water occurs independently of the cycle that occurs every year on the surface and is therefore referred to as the ‘deep water cycle’.
Geologists study the deep water cycle to understand how it works; also it can provide an assessment of the quantity of water under the surface. This is important since the water quantity plays a role in the geological activities on the planet. However, there is one major problem.
Even with the advances made in technology, we don’t have the tools to go under the Earth’s crust and, therefore, must wait for evidence to pop out and then study them to make conclusions.
Diamond from the transition zone
Mineral physicist Tingting Gu of the Gemological Institute of New York and Purdue University is a researcher who waits for such rare gems to pop out of the Earth. The diamond found in the Botswana mine was one such rare find and Gu and her colleagues set to work on it to find out more about its origins.
The researchers used micro-Raman spectroscopy and X-ray diffraction to probe the 12 mineral inclusions and a milky inclusion cluster found on the diamond. They found a mix of minerals in these inclusions ranging from ringwoodite (magnesium silicate) to ferropericlase (magnesium/iron oxide) as well as enstatite (a form of magnesium silicate).
The contents suggest that the diamond was formed in the transition zone, 410 miles (660 km) below the Earth’s surface. Scientists also know that at high pressures, ringwoodite decomposes into ferropericlase and bridgmanite, another mineral. At lower pressures, bridgmanite converts into enstatite, confirming the diamond’s journey from the depths of the Earth to the crust.
Additionally, the ringwoodite and another mineral brucite found in the diamond are hydrous in nature, which suggests that the region where it was formed was rich in water. There is enough to show that Earth’s interiors have taken in much more water than previously thought, we might now be beginning to learn where it is going.
The research was published in the journal Nature Geoscience.
Abstract
The internal structure and dynamics of Earth have been shaped by the 660 km boundary between the mantle transition zone and lower mantle. However, due to the paucity of natural samples from this depth, the nature of this boundary—its composition and volatile fluxes across it—remain debated. Here we analyse the mineral inclusions in a rare type IaB gem diamond from the Karowe mine (Botswana). We discovered recovered lower-mantle minerals ringwoodite + ferropericlase + low-Ni enstatite (MgSiO3) in a polyphase inclusion, together with other principal lower-mantle minerals and hydrous phases, place its origin at ~23.5 GPa and ~1,650 °C, corresponding to the depth at the 660 km discontinuity. The petrological character of the inclusions indicates that ringwoodite (∼Mg1.84Fe0.15SiO4) breaks down into bridgmanite (∼Mg0.93Fe0.07SiO3) and ferropericlase (∼Mg0.84Fe0.16O) in a water-saturated environment at the 660 km discontinuity and reveals that the peridotitic composition and hydrous conditions extend at least across the transition zone and into the lower mantle.
ABOUT THE EDITOR
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
0