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What secrets lie deep beneath the Earth's surface that continue to elude scientific understanding?
Due to technological constraints, humanity remains unable to fully comprehend the world deep beneath our planet's surface.
Scientists have long believed that as the upper mantle transitions to the lower mantle, it becomes hotter, denser, and potentially less hydrated.
However, the discovery of 'ambassadors' residing hundreds of kilometers beneath Earth's surface has somewhat provided scientists with a rare glimpse into this mysterious world.
One such 'ambassador' is a diamond recently found at a depth of 660 km.
Specifically, a recently excavated diamond from the Karowe diamond mine in Botswana (a country in southern Africa) exhibits numerous inclusions of ringwoodite, ferropericlase, enstatite, and other minerals, indicating diamond formation approximately 660 km beneath Earth's surface.
THE SECRET OF THE DIAMOND AT 660 KM DEPTH
What's even more remarkable, according to a team of researchers led by mineral physicist Tingting Gu from the Gemological Institute of America in New York, the environment where this diamond formed - the boundary between the upper and lower mantle, known as the 660 km discontinuity (or simply the transition zone) - IS VERY RICH IN WATER.
'The presence of ringwoodite along with hydrous phases indicates a very wet environment at this boundary. This is a remarkable discovery,' the team of scientists wrote in the journal Nature Geoscience.
The results suggest that there may be more water deep within Earth than scientists previously thought, which could impact our understanding of deep water cycles and planetary evolution.
Scientific Analysis: Most of Earth's surface is covered by oceans. However, considering the thousands of kilometers between the surface and the planet's core, they are essentially just small puddles. Even at the deepest point, the ocean is only about 11 km thick.
The Earth's crust is fractured and fragmented, with distinct tectonic plates grinding against and sliding beneath each other. In these subduction zones, water seeps deeper into the planet, reaching even lower layers.
Over time, it resurfaces through volcanic activity. This downward flow, upward eruption cycle is known as the deep water cycle, distinct from the surface water cycle.
Understanding the mechanisms and quantity of water below is also crucial to grasp the geological workings of our planet. For instance, the presence of water can impact the eruption of a volcano and play a role in seismic activity.
However, since we cannot venture down there, we must await evidence of water coming to us, much like it forms into diamonds under extreme temperature and pressure conditions.
Mineral physicist Tingting Gu and her colleagues recently conducted a detailed study on such a gemstone, finding 12 inclusions containing minerals and a cluster of milky white bodies.
To study the diamond, they employed 'non-destructive' analysis techniques, including micro-Raman spectroscopy; using laser beams to non-invasively reveal some physical properties of the material; and X-ray diffraction to examine the internal structure of the diamond, probing these impurities to determine their nature without cutting it open.
Among the mineral inclusions, they discovered a combination of ringwoodite (a magnesium silicate at high temperature and pressure) interacting with ferropericlase (magnesium/iron oxide) and enstatite (another magnesium silicate with a different composition).
Ringwoodite shares a chemical composition similar to olivine - the main material of the upper mantle but forms under extreme temperature and pressure conditions. Until 2008, scientists had only found it in a meteorite sample.
Ringwoodite is commonly found in the transition zone between the upper and lower mantle, between depths of 410 to 660 km beneath the Earth's surface and may contain much more water than minerals like Bridgmanite and ferropericlase.
At high pressure in the transition zone, ringwoodite decomposes into ferropericlase, as well as another mineral named Bridgmanite. At lower pressures closer to the surface, Bridgmanite transforms into enstatite. Their presence in the diamond tells a story of a journey, indicating the stone formed at depth before returning to the crust.
But that's not all.
Ringwoodite particularly exhibits characteristics suggesting it is hydrous - a type of mineral formed in the presence of water. Meanwhile, other minerals found in diamonds, such as brucite, also display hydrous properties. These clues indicate that the environment in which the diamond formed was quite moist.
Previous evidence of water in the transition zone has been found, but it's insufficient to assess the amount of water there.
A group of scientists wrote in their paper: 'While diamond formation in the upper mantle often involves the presence of fluids, ultra-deep diamonds with assemblages of minerals undergoing similar metasomatism are rarely observed accompanied by hydrous minerals.'
This discovery could also impact mantle convection models. Tingting Gu expressed hope that scientists would be able to integrate the findings of this study into models of how water in the mantle may affect processes like Earth's internal convection currents. This could drive mantle convection by unevenly heating Earth's mantle, causing hotter portions to rise and shifting Earth's tectonic plates over millions of years.
The research was published on September 26, 2022, in the journal Nature Geoscience.
Article sources: Sciencealert, Livescience
