Earth has a vast interior ocean, 400-miles under our feet, that creates "ringwoodite" gems

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In a remarkable discovery that challenges our understanding of Earth's water distribution, aided by a mineral called ringwoodite, researchers found evidence suggesting the presence of several oceans worth of water trapped deep beneath the United States.

The discovery, made by geophysicist Steve Jacobsen from Northwestern University and seismologist Brandon Schmandt from the University of New Mexico, may represent the planet's largest water reservoir.

Rethinking Earth's water cycle

Published in the journal Science, their findings shed light on the Earth's formation, composition, and the amount of water trapped in mantle rock.

"Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," explained Jacobsen, a co-author of the paper.

"I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades," Jacobsen continued.

Scientists have long speculated about the existence of water trapped in the Earth's mantle, located between the lower and upper mantle at depths of 250 to 410 miles.

Jacobsen and Schmandt are the first to provide direct evidence of water in this "transition zone" on a regional scale, extending across most of the interior of the United States.

"Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles," said Schmandt, a co-author of the paper.

"If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found."

Liquid, ice, vapor and a fourth form of water

The water discovered in the mantle is not in a form familiar to us - it is not liquid, ice, or vapor. Instead, it is trapped inside the molecular structure of the minerals in the mantle rock.

The immense pressure created by 250 miles of solid rock, along with temperatures above 2,000 degrees Fahrenheit, causes water molecules to split and form hydroxyl radicals (OH) that can be bound into a mineral's crystal structure.

"Whether or not this unique sample is representative of the Earth's interior composition is not known, however," Jacobsen said. "Now we have found evidence for extensive melting beneath North America at the same depths corresponding to the dehydration of ringwoodite, which is exactly what has been happening in my experiments."

Following the ringwoodite

Ringwoodite, a high-pressure mineral with a captivating blue hue, was named after the Australian geologist Alfred Ringwood, forms deep within the Earth's mantle at depths between 410 and 660 kilometers (250-410 miles). What makes ringwoodite truly remarkable is its ability to store water within its crystal structure.

Synthesizing ringwoodite in the lab

Jacobsen has been synthesizing ringwoodite, a sapphire-like blue mineral, in his Northwestern lab by reacting the green mineral olivine with water at high-pressure conditions. He found that more than one percent of the weight of ringwoodite's crystal structure can consist of water.

Under the immense pressures and temperatures found in the mantle's transition zone, water molecules split into hydroxyl radicals (OH), which can then be incorporated into ringwoodite's structure. This mineral acts like a sponge, soaking up water and storing it in the deep Earth.

"There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water. This mineral can contain a lot of water under conditions of the deep mantle," Jacobsen noted.

Ringwoodite implications for Earth's formation and habitability

The presence of water-rich ringwoodite in the Earth's mantle has implications for our understanding of the planet's formation and its potential for habitability.

As scientists continue to study this intriguing mineral, they may uncover new insights into the role of water in the Earth's deep interior and its influence on the geologic processes that shape our world.

Ringwoodite's ability to store and transport water within the mantle could also have implications for the search for life on other planets, as the presence of water is considered a key factor in the development and sustainability of life as we know it.

Detecting magma beneath North America

Using seismic waves, Schmandt detected the presence of magma beneath North America, which aligned with Jacobsen's findings of partial melt when subjecting synthesized ringwoodite to conditions around 400 miles below the Earth's surface.

Additionally, seismic studies have detected regions of partial melting in the mantle's transition zone, which could be attributed to the release of water from ringwoodite as it transforms into other minerals at greater depths.

"Seismic data from the USArray are giving us a clearer picture than ever before of the Earth's internal structure beneath North America," Schmandt explained. "The melting we see appears to be driven by subduction — the downwelling of mantle material from the surface."

Process of dehydration melting

The melting detected by the researchers is called dehydration melting. When ringwoodite in the transition zone moves deeper into the lower mantle, it forms a higher-pressure mineral called silicate perovskite, which cannot absorb water. This causes the rock at the boundary between the transition zone and lower mantle to partially melt.

"When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit," Schmandt said. "This is called dehydration melting."

"Once the water is released, much of it may become trapped there in the transition zone," Jacobsen added.

How ringwoodite was accidentally discovered

The international team of scientists responsible for this remarkable find was led by Graham Pearson, Canada Excellence Research Chair in Arctic Resources at the University of Alberta.

Analysis of the mineral sample revealed that it contains a significant amount of water — 1.5 per cent of its weight — a finding that proved to have far-reaching implications for our understanding of the Earth's interior and its dynamic nature.

Elusive ringwoodite

Ringwoodite, a form of the mineral peridot, is believed to exist in large quantities under high pressures in the transition zone between the Earth's upper and lower mantle.

While ringwoodite has been found in meteorites, no terrestrial sample had ever been discovered before Graham Pearson came along. Due to the extreme depths at which it is formed, this elusive mineral is inaccessible for direct fieldwork.

Contribution of artisan miners in Brazil

The ringwoodite sample was found in 2008 in the Juina area of Mato Grosso, Brazil, where artisan miners unearthed a diamond from shallow river gravels.

The diamond, which had been brought to the Earth's surface by a deeply derived volcanic rock known as kimberlite, contained the ringwoodite inclusion.

Pearson's team had been searching for another mineral when they purchased the three-millimeter-wide, dirty-looking, commercially worthless brown diamond.

The ringwoodite itself, invisible to the naked eye and buried beneath the surface, was fortunately discovered by Pearson's graduate student, John McNeill, in 2009.

"It's so small, this inclusion, it's extremely difficult to find, never mind work on," Pearson said, "so it was a bit of a piece of luck, this discovery, as are many scientific discoveries."

Confirming the Find

The sample underwent years of rigorous analysis using Raman and infrared spectroscopy and X-ray diffraction before it was officially confirmed as ringwoodite.

The critical water measurements were performed at Pearson's Arctic Resources Geochemistry Laboratory at the University of Alberta, which forms part of the world-renowned Canadian Centre for Isotopic Microanalysis, also home to the world's largest academic diamond research group.

Implications for Earth's Dynamics

For Pearson, one of the world's leading authorities in the study of deep Earth diamond host rocks, this discovery proved to be the most significant of his career.

It confirmed about 50 years of theoretical and experimental work by geophysicists, seismologists, and other scientists trying to understand the makeup of the Earth's interior.

"One of the reasons the Earth is such a dynamic planet is the presence of some water in its interior," Pearson said. "Water changes everything about the way a planet works."

Ringwoodite, Earth's water cycle, and the future

In summary, the crucial discovery of a vast water reservoir deep within the Earth's mantle by Steve Jacobsen and Brandon Schmandt has revolutionized our understanding of the planet's formation, composition, and water cycle.

As researchers continued to explore the complex processes occurring far beneath the Earth's surface, their ringwoodite findings provided a new understanding of the delicate balance that makes our planet habitable.

The presence of this hidden ocean, trapped within the crystals of mantle rock, serves as a testament to the incredible forces at work within the Earth and the countless mysteries that still lie beneath our feet.

With each new revelation, we move closer to unraveling the secrets of our planet's past, present, and future, and to appreciating the remarkable world we call home.

The full study was published in the journal Science and the journal Nature

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