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Mineral Reservoirs

Water, water everywhere, but it’s all locked underground. Wendy Panero, Ph.D., and The Ohio State University Mineral Physics Research Group have found that minerals within the earth’s mantle potentially contain a vast amount of water. The group is harnessing the computing power of the Ohio Supercomputer Center to find out where and just how much of this water sits hundreds of kilometers beneath our feet.

As we know, water is the keystone for life’s existence on a planet. How it came to sit on Earth’s surface is a point of contention among geologists. One hypothesis is that after the formation of a dry Earth, icy comets and space debris collided with it, effectively delivering water. Panero’s team is focusing on another hypothesis that could coexist with the former: The earth created its own water in the planet’s interior, and continues to do so, through plate tectonics.

Water within the earth’s mantle does come with some assembly required. Hydrogen atoms are locked within the structure of solid minerals. Certain chemical reactions below the earth’s crust allow these hydrogen atoms to bond with oxygen, creating water molecules. Though this may sound like a tedious, small-scale process, researchers predict that the mass of water stored in the earth’s mantle is approximately 50 percent greater than that on earth’s surface.

“It’s a lot of water,” Panero said. “And it’s completely inaccessible to humans– the earth has to give us the water from that depth through plate tectonics.”

As a result of plate tectonics, mantle rock circulates and will melt when it reaches a certain depth if it contains too much water. Molten rock then surfaces as magma through mid-ocean ridges where the water is flushed out into the ocean. By this process, the earth regulates the volume of its oceans. So, why isn’t sub-surface water included in our basic understanding of the water cycle?

“Part of it is timescale. A lot of [the water cycle] happens over days or weeks,” Panero said. “The process that I’m talking about is more of hundreds of millions of years.”

Because the minerals that Panero’s group is working with are hundreds of kilometers deep, they are using supercomputers to model the structure of the hydrogen-containing minerals as well as the energy cost of incorporating the hydrogen atoms. From there, they can calculate how much hydrogen is likely to exist in a mineral as a function of pressure and temperature.

Their calculations are used to complement experiments on physical mineral samples in the lab. Using a device called a diamond anvil cell, they squeeze a miniscule mineral sample between two gem-quality diamonds as it is heated with a laser to simulate the pressure and temperature deep within the earth. As the mineral’s composition changes, the team can hypothesize the amount of hydrogen it could store.

Many minerals stored within the earth undergo structural changes between 410 and 660 kilometers deep due to heat and pressure. If they contain water more water than the new minerals can store, the rocks should melt at these depths, Panero said.

“What is kind of enchanting to think about yet really difficult to look at in detail is that those minerals that are stable between that 410 and 660 kilometer depth range can take on a huge amount of water,” Panero said.

Panero’s group has found that the minerals garnet and the lesser-known ringwoodite have the greatest potential to store water in the earth’s mantle at 660 km depth. Ringwoodite is the dominant mineral in the mantle at these depths and, luckily, can be synthesized in the lab. Garnet is of interest due to its stability in the earth’s mantle down to 750 kilometers.

“The amount of water that garnet holds at that depth is going to be what tells us exactly how much water is in the earth because new data show us that the earth’s mantle might be melting at 750 kilometers depth,” Panero said.

The implications of this study could literally extend out of this world. If hydrogen is a key ingredient in the earth, it could be possible that it exists similarly within the interior of other planets. Studies have suggested that water has existed on the surface of Mars, for example, but there is not yet an explanation of its origins. Perhaps, trapped deep beneath the surface, the key to life exists.

Project Lead: Wendy Panero, Ph.D., The Ohio State University
Research Title: Water partitioning at the base of the transition zone: No need for a lower mantle water filter
Funding Source: National Science Foundation
Website: u.osu.edu/panero.1