Spinning in a vast cosmos, our planet is unique with its blue color covered by liquid water on its surface. Beneath the gorgeous landscape sculpted by Earth’s processes, scientists are curious about what is happening thousands of kilometers deeper in the Earth. Unfortunately, we can't view the enigmatic inner realm directly and scientists apply seismic waves as an indirect approach to probe the inner structures of the invisible deep Earth. The process is comparable to having an ultrasound examination at a medical facility: 3D images of the biological structure are created by converting the various sound wave speeds as they pass through the body. Similarly, scientists may understand the components that compose Earth's interior by "listening" how seismic waves move through different materials at varying depths.
From seismic investigations made over the past few decades, scientists learned that a global discontinuity exists at a depth of around 660 km below the surface. This discontinuity has been defined as the boundary between Earth’s lower and upper mantle. Above the boundary, the Earth is made up of crust, upper mantle and transition zone, which connect with the biosphere and atmosphere; while below the boundary, the rocks behave like flows in the lower mantle that encircles Earth’s core under extreme pressure and temperature. However, whether the chemical composition at this boundary changes abruptly or stays the same has long been a matter of debate.
Although geodynamic models predict that parts of Earth’s upper and lower mantle should be mixed and stirred together, geochemical analyses of the basaltic products of mantle melting in the upper mantle display inconsistent signature with that from the region believed to represent the lower mantle, which means the upper and lower mantle could be chemically distinct. The different stories revealed by geophysics and geochemistry cast fundamental debates on the inner work of Earth: whether the materials at this 660 km discontinuity are blocked from mixing or penetrate through this region? More importantly, how do volatiles, such as water, cross the upper mantle through the transition zone and into the lower mantle boundary if there is a block there?
The ultimate solution to these questions is to collect direct samples from below the transition zone or ca. 660 km. How can we obtain the rocks in a region where drilling is impossible? Luckily, the interior of our Earth is dynamic: the solid body is moving, bringing materials from deep to its surface. However, over a long geological time, most of the geological signatures of these materials from deep Earth has been overwritten, and the chemical information of the rocks from the deep interior have been lost via their interactions with the surroundings.
That’s why inclusions in diamonds are so important. As a chemically inert vessel, diamond can envelop fragments of other minerals when they were in the deep Earth and, amazingly, protect them from chemical modifications with their surroundings and preserve their original structures when they formed under an extreme pressure inside the Earth.
Therefore, although an inclusion in a diamond can be a concern for its clarity and reduce its commercial value, the geological story behind the harbored crystal endows them a unique virtue allowing us to interpret the invisible interior of the Earth.
In the most recent issue of Nature Geoscience (https://www.nature.com/articles/s41561-022-01024-y), by persevering hard work and a little bit of luck, scientists made some exceptional discoveries in a 1.5 ct gem quality diamond, which could otherwise end up in someone's engagement ring if a scientist (Dr. Tingting Gu) from GIA hadn't given it a second glance.
“It was a beautiful morning in Manhattan, when my colleague Ulrika passed me this stone and asked if I need more type IaB diamonds. This stone stood out by its superior D grade color and lots of interesting inclusions.” said Tingting Gu, the first author of this paper. “When I began examining the diamond inclusions under microscope and with Raman spectroscopy, I found a weird inclusion with a translucent rim but a dark and iridescent center, which resembled an eye. When I shot the red laser gently at the edge of the dark center, a distinct spectrum with a doublet at 803 and 839 cm-1 popped up. I continued to take the high-frequency spectrum, and noticed it was accompanied by a hydrogen vibration at around 3673 cm-1. Then I took the entire spectrum and observed both the hydrogen peak and the doublet.”
The spectrum observed was later determined to be ringwoodite. Ringwoodite has the same chemical composition with olivine, but has a different structure because it formed under high pressure; this structure is the same as spinel. It was first synthesized by Dr. Alfred Ringwood, an eminent Australian experimental geophysicist and geochemist, who predicted that a phase change of the mantle olivine to its spinel polymorph should occur in the transition zone. Later, this high-pressure olivine polymorph was found in a meteorite and named after Dr. Ringwood in honor of his contribution. More importantly, ringwoodite is recognized for its capacity to store hydrogen in its structure, therefore, a massive amount of water on Earth’s interior could be locked up by this mineral.
The presence of ringwoodite in this diamond suggests that it has an unusual origin. To determine the identities of the related mineral inclusions, Tingting investigated the entire set of mineral inclusions in this diamond with Raman spectroscopy. She collaborated with Prof. Fabrizio Nestola at University of Padova (Italy) on X-ray analyses to confirm the identification of the associated minerals.
“Using single-crystal X-ray diffraction, the structure of the mineral inclusions can be identified without cutting or destroying their diamond host” said Fabrizio, the head of the Department of Geosciences at University of Padova, “this is a non-destructive method that can allow us to have a precise identification of the mineral species included by this diamond”.
X-ray diffraction confirmed the observation of Raman spectroscopy, and combined with these techniques, ringwoodite was discovered for the first time in a polyphasic inclusion coexisting with ferropericlase and enstatite (this last could be a regressed bridgmanite, thought to be the most dominant mineral in the lower mantle). The equilibrium of these phases is crucial because a typical pressure and temperature condition can be established with a defined chemical composition.
Remarkably, despite ringwoodite has been discovered in a diamond in 2014, unprecedentedly, Tingting and her team were able to expose four micrometre-sized ringwoodite grains in this sample for the first time. This enabled the retrieval of their chemical composition directly from the incredible depth, as well as the interpretation of the thermodynamic history of the mineral assemblage, which falls within the range corresponding to about 660 km below the Earth’s surface, right at the boundary between the upper and lower mantle. More crucially, a few unusual Raman peaks were observed with hydroxyl vibration. With a persistent searching of X-ray diffraction patterns, diffraction sports of this phase were discovered that potentially belong to phase D, a dense hydrous magnesium silicate phase that is thought to be the main host for water in the lower mantle.
“Together with the hydrous environment revealed by the ringwoodite mineral assemblage, this sample not only demonstrated the chemical composition of phases that may exist in the lower mantle and tranverse through the transition zone, but it also demonstrated that water, in the form of hydroxyl, can penetrate across this boundary beyond the 660 km discontinuity. Diamonds are unique vehicles for sampling the deep Earth.” Said Prof. Barb Dutrow, a mineralogist at Louisiana State University.
It’s a great privilege to find and study on this beautiful diamond. Nature samples are awe-inspiring because they were generated by Earth millions of years ago in a process that is well beyond the power of a scientific equipment to precisely recreate. Nature's wisdom always demonstrates its majesty and how modest we should be.
(Caption of the front image: Mineral inclusion containing the assemblage of ringwoodite, enstatite and ferropericlase. Field view is 0.91 mm. Taken by Nathan D. Renfro and Tingting Gu at GIA Carlsbad.)