There could potentially be up to six times more water, albeit in a bound state rather than freely flowing, within the transition zone that lies between the upper and lower mantles of Earth, compared to the total volume of water in all of its seas combined. The discovery of tiny ringwoodite inclusions inside a diamond located at a depth of 410 miles (660 km) has provided evidence for this. This finding serves to confirm the layer’s substantial, water-rich composition. In a recent publication in Nature Geoscience, researchers revealed how the considerable water content in the transition zone significantly influences both plate tectonics and the formation of volcanic mantle plumes.
More Liquid Than Previously Thought
Jules Verne depicted a vast ocean situated deep within the Earth’s interior. Surprisingly, his idea wasn’t entirely inaccurate. Despite the belief that extreme temperatures and pressures would prevent the presence of water, recent findings indicate that the Earth’s mantle might contain more liquid than previously thought.
Wadsleyite and ringwoodite, two common transition zone minerals, have the potential to hold six times as much water as the Earth’s seas do.
The initial clues came from seismic observations, revealing instances of rock melting at the juncture between the upper and lower mantles. A significant breakthrough occurred in 2014 when scientists discovered the presence of the hydrous mineral ringwoodite within diamonds sourced from this region, which lies at a depth of 410 miles (660 km).
Because of the diamond’s size, scientists couldn’t collect data about the mineral composition of the area where it was found. It remains uncertain whether the presence of crystallized water in ringwoodite was an uncommon occurrence or indicative of the entire transitional zone within Earth’s mantle.
Ultra-Rare Diamonds Inside the Earth
Another larger diamond has recently been found from the same depth, providing further insights into the situation. Researchers, led by Tingting Gu from the Gemological Institute of America in New York, examined a 1.5-carat diamond that was discovered at the Karow mine in Botswana.
The team analyzed its composition and mineral inclusions. These inclusions, which are found within the diamond’s structure, could contain minerals originating from deep within the Earth.
The findings indicate that this diamond, similar to other famous gemstones, did not form in the upper mantle of the Earth. Instead, it originated at the boundary between the upper and lower mantles.
This places the Karow diamond’s creation at a depth of around 400 miles, where extreme conditions of 3,000 degrees Fahrenheit (1,650 °C) temperature and 23.5 gigapascals pressure prevailed.
Including Usual Mantle Minerals and Bound Water
The inclusions found within the diamond offer insights into the harsh conditions that led to its formation. The diamond’s mineral composition suggests a situation where ringwoodite transforms into ferropericlase and bridgmanite at a specific boundary, a transformation common in the transition zone to the lower mantle.
These characteristics, combined with other chemical traits, indicate that the diamond and its mineral inclusions are not exceptional anomalies. Instead, the research team determined that they are representative of the typical conditions in this area.
After the discovery of bound water within diamond inclusions, it has become evident that the minerals comprising this boundary layer contain substantial water content, like an ocean.
Contrastingly, studies utilizing laser-based Raman spectroscopy have revealed that the inclusions contain water that is held in place (bound water). Two minerals, ringwoodite from the Earth’s crust and brucite from the mantle, are partially trapping this water.
Using diamond cells, it has been shown that brucite forms from water-soaked ringwoodite in an environment with a lot of water. In simpler terms, this diamond and its inclusions couldn’t have originated from a dry source layer; instead, they must have come from a region rich in water, potentially a hidden ocean within the Earth’s mantle.
A Hidden Ocean Inside the Earth’s Mantle
The recent discovery of a diamond from Botswana has provided compelling evidence for the presence of an extensive underground water reservoir beneath the Earth’s mantle.
This groundbreaking research indicates that the transition zone, previously thought to be devoid of moisture, is actually capable of absorbing and retaining substantial amounts of water.
Notably, minerals like wadsleyite and ringwoodite, commonly found in the transition zone, have the remarkable potential to hold up to six times more water than the Earth’s oceans.
The identification of bound water within diamond inclusions has solidified the understanding that minerals within this boundary layer possess significant water content. This discovery supports Jules Verne’s depiction, which suggests the existence of a sizable subterranean ocean.
However, unlike conventional bodies of water, this concealed reservoir remains unseen due to its confinement within rock structures, rather than flowing freely through the Earth’s core.
Relevance on a Wide Scale
The presence of high water content in the Earth’s mantle transition zone has significant implications. The water alters the physical properties of the rock, making it more flexible compared to dry rock.
As a result, both the barrier layer and the mantle regions above it become softer and more dynamic than they would be without water. This, in turn, affects how mantle rock masses move, allowing plumes of material from lower levels to more easily rise into the upper mantle and the Earth’s crust.
By examining the water content in the transition zone, we can now better understand plate tectonics and what happens to Earth’s sinking tectonic plates at subduction zones.
These descending plates carry oceanic sediments as they go deeper into the Earth. These sediments can contain substantial amounts of water and carbon dioxide.
However, it was previously uncertain how much of these materials actually made it to the transition zone in the form of stable hydrous minerals and carbonates. Recent scientific findings indicate that a significant portion of this water does indeed make its way down to the ocean floor.
References
- Gu, T., Pamato, M.G., Novella, D. et al. Hydrous peridotitic fragments of Earth’s mantle 660 km discontinuity sampled by a diamond. Nat. Geosci. 15, 950–954 (2022). https://doi.org/10.1038/s41561-022-01024-y
