We knew for a while that there was some water below the Earth's surface, but we never knew how much. Scientists theorized that the water levels in the Earth's interior might be comparable to those on the surface, but a recent study now gives rock-hard proof.
The rock is ringwoodite and the proof is its melting capabilities. Ringwoodite is a mineral that can trap water in its molecular structure and is formed under high pressures and temperatures, such as those inside our planet. Specifically, ringwoodite is formed under conditions such as those found in the transition zone of the planet, approximately 255 to 410 miles below the surface. This zone is the boundary between the upper and lower mantle, and is hot and bothered, constantly shifting and producing seismic waves. It turns out that this zone is also a deep reservoir of water.
In March, a research group found a unique diamond from the mantle that was packed in ringwoodite containing water. Until then ringwoodite specimens were either collected from meteorite material or created in the laboratory. Brandon Schmandt and Steven Jacobsen, geophysicists from Northwestern University decided to test mantle ringwoodite to see if it could trap and sustain water.
Schmandt and Jacobsen knew that the lower mantle, which the contents of the transition zone slide into, couldn't hold water in its material structures. Thus the water-containing rocks of the transition zone must somehow remove the water before it enters the lower mantle. Schmandt and Jacobsen proposed a melting mechanism that allows the transition zone rock to get rid of its water before entering the lower mantle.
They simulated the traveling of transition zone ringwoodite in the laboratory, recreating the pressures and temperatures of Earth's interior with lasers and diamond compressions. As they imitated the conditions at the boundary between the transition zone and the lower mantle, the ringwoodite transformed into silicate perovskite that contained silicate melt. The results suggest that Schmandt's theory of ringwoodite-producing melt may be correct. If so, the amount of water in the ringwoodite in the transition zone can be measured.
The production of melt, which we now know is a result of the ringwoodite in the transition zone moving into the lower mantle, produces seismic waves, which can be measured by seismometers. Using these tools, Schmandt found that the melt material likely travels downward and then back up into the transition zone, creating the possibility (maybe even the probability) that the zone has a stable, unchanging supply of water. An ocean's worth, in fact.
Researchers are looking at Schmandt and Jacobsen's results for long-awaited answers to questions about the amount of water resources and water recycling on our planet.
"It provides an important multidisciplinary perspective on this problem. It has important implications on our understanding of the behavior of subducting slabs deep in the mantle, and on our understanding of [the] overall water budget/distribution in the Earth," says Anna Kelbert, a geophysicist at Oregon State University.
