“Water changes everything about the way a planet works.” With those words, Graham Pearson, a foremost expert on deep Earth minerals, conveyed the scale of a breakthrough that is revolutionizing our picture of the interior of the planet. Hidden deep beneath our feet, some 400 miles down, in the hot, high-pressure world of the mantle transition zone, researchers have discovered strong evidence for an enormous reservoir of water possibly equal to all of Earth’s surface oceans trapped in the form of a common mineral called ringwoodite.
The road to this epiphany has been decades long. Geophysicists have long theorized that the mantle transition zone, wedged between the upper and lower mantle at depths of 410 to 660 kilometers, could be a secret vault for Earth’s lost water. But only now have lab experiments, seismic imaging, and the serendipitous finding of a water-rich ringwoodite inclusion within a Brazilian diamond come together to validate the theory.
Ringwoodite, for Australian geologist Alfred Ringwood, is a blue, high-pressure variety of olivine that can take water into its crystal structure not as water, ice, or vapor, but as hydroxyl (OH) groups attached at the atomic level. Steve Jacobsen, a geophysicist at Northwestern University, clarified, “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.” This mineral has the ability to hold a great deal of water in the conditions of the deep mantle. Experiments in the lab indicate that ringwoodite has the capacity to store more than one percent of its weight in water a comparatively small amount, but one, when extrapolated to the huge volume of the transition zone, suggests a reservoir of water equaling, or possibly greater than, all the oceans on Earth.
The existence of this unseen ocean is not only a mineralogical anomaly. It is redefining our knowledge of the water cycle of Earth, tectonics, and even the conditions under which our planet is habitable. Since Jacobsen pointed out, “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.” The finding of water in the deep mantle indicates a whole-Earth water cycle, with water moving not only between atmosphere, oceans, and crust, but deep into the interior and back.
The secret to ending this cycle is found in the subduction process, where oceanic plates plunge under continents and transport water-bearing minerals into the mantle. As the slabs move downward, the high pressure and temperature of the transition zone permit ringwoodite formation and the entrapment of water. But that isn’t the end. When these rocks descend even further, into the lower mantle, ringwoodite is converted to minerals that are not able to retain water. The outcome: water is discharged, decreasing the melting point of the surrounding rock and creating bubbles of magma in a phenomenon referred to as dehydration melting.
Brandon Schmandt, a seismologist at the University of New Mexico, explained the importance: “Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles. 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.” Seismic evidence collected by the USArray network, an array of more than 2,000 seismometers, has offered a glimpse into those deep processes. By monitoring the velocity of seismic waves caused by earthquakes, scientists have identified areas of partial melting at depths of 400 miles under North America just where dehydration of ringwoodite would be predicted.
These discoveries are not theoretical. Experiments at the lab with diamond-anvil cells and synchrotron radiation mimicked the mantle’s crushing pressure and fiery heat, and sure enough, ringwoodite was shown to store and release water as had been previously theorized. As Jacobsen described, “Because the diamond windows are transparent, we can look into the high-pressure device and watch reactions occurring at conditions of the deep mantle,” he said. “We used intense beams of X-rays, electrons and infrared light to study the chemical reactions taking place in the diamond cell.”
The implications spread outward. Water stored in ringwoodite and released during dehydration melting not only affects the formation of magma and volcanism, but also impacts the physical characteristics of mantle rocks, including their viscosity and electrical conductivity. New models propose that these melting processes could buffer the water content in the mantle transition zone, by anywhere from 0.1 to 1 percent by weight sufficient to have intense impacts on mantle convection and long-term ocean stability on Earth.
More sophisticated seismic tomography has been instrumental in charting these deep structures. Methods like Reverse Time Migration Full Waveform Inversion (RTM-FWI) are now imaging sharp boundaries and mid-mantle discontinuities with unparalleled resolution, unveiling how cold subducting slabs and hot upwelling plumes interact with the hydrated transition zone. These imaging advances are enabling geophysicists to put dots on the connections between processes in the deep Earth and phenomena at the surface, from the evolution of continents to volcanic arc evolution.
With ongoing research, the secret ocean inside ringwoodite is causing a basic reevaluation of Earth’s water cycle, the physics of mantle convection, and the geological processes that sculpt our planet. The understanding that Earth’s interior contains as much water as all its surface oceans is revolutionizing not just our knowledge of the planet’s history, but also the scientific pursuit of untangling the secrets still deep beneath our feet.
