Deep Hydrology: Discovering H2O in the Earth’s Mantle

Dec. 1, 2016

Ultra-deep diamonds aren’t the type of glittering gems that jewelers covet. Instead, they are rough, oftentimes milky-looking minerals that travel up from the transition zone, a region hundreds of miles deep in the earth’s mantle. These precious stones are not only historical artifacts, formed some 90 million years ago, they represent a window into the deep Earth. 

Ever wonder what lies below the crust? A flaw in a diamond found in Juina, Brazil has recently offered researchers a perfectly preserved specimen from the deep mantle. A small inclusion, which was sealed off when the diamond was formed, contains residual minerals and unmistakable evidence of the presence of hydroxyl ions, which indicate the presence of water.

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Steve Jacobsen, associate professor of Earth and Planetary Sciences at Northwestern University, and his team examined the diamond with infrared microscopy and analyzed the inclusion. In doing so, they discovered that it is made of a ferropericlase mineral, which is composed of iron and magnesium oxide, and can also absorb other metals such as chromium, aluminium, and titanium at ultra-high temperatures and pressures.

The residual indicates that water exists much deeper than ever seen before, a third of the way to the edge of Earth’s core. It also suggests that the planet’s water cycle is bigger than scientists thought, extending into the deep mantle. “Based on the composition of the trapped mineral, we speculate that the depth was around 1000 kilometers,” Jacobsen told New Scientist. “This implies a bigger reservoir of water on the planet than previously thought.”  

While it’s still not clear how water penetrated the miles of rock and earth, scientists agree that it’s most likely not residing in subterranean seas, rather, intermixed with sediment and stored within layers of rock. And some speculate that it may have arrived at those depths even earlier than 90 million years ago, perhaps by filtering through the Earth’s sedimentary oceanic crust as tectonic plates shifted.

“Water clearly has a role in plate tectonics, and we didn’t know before how deep these effects could reach,” he says. “Water mixes with ocean crust and gets subducted at convergent plate boundaries,” he says. “Introducing water into the mantle promotes melting and weakens rock, likely helping out the motions of plates like grease.” Jacobsen believes that this discovery may help explain the Earth’s plate tectonicshow plates move, form mountains and volcanoes. But could it also help assuage future concerns of water scarcity?
About the Author

Laura Sanchez

Laura Sanchez is the editor of Distributed Energy and Water Efficiency magazines.

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