A rare type of diamond may suggest that water may penetrate deeper into the Earth’s interior than scientists previously thought.
Although more than 70% of our planet is covered in water, there is also water in minerals more than 200 miles (322 kilometers) below the earth, including in the upper mantle, the semi-ductile layer in which the crust “floats.” Scientists have long believed that as the upper mantle transitions into the warmer, denser lower mantle, the minerals can hold much less water.
But in a new study, published September 26 in the journal Geoscience of nature (opens in new tab)researchers found that a diamond it contained inclusions or tiny pieces of other minerals, which can hold more water and appear to have existed at the boundary between the upper and lower mantle. The results suggest that there may be water deeper in the Earth than scientists thought, which could affect our understanding of the deep-sea cycle and tectonic plates.
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The results were unexpected, said study lead author Tingting Gu, who is currently a mineral physicist at Purdue University in Indiana but was a researcher at the Gemological Institute of America in New York at the time of the study.
Gu and her colleagues looked at Type IaB diamonds, a rare type of diamond from the Karowe mine in Botswana that form deep underground and are often in the Earth for a long time. To study the diamond, they used “non-destructive” forms of analysis, including Raman microspectroscopy, which uses lasers to non-invasively reveal some of a material’s physical properties, and X-ray diffraction to examine the diamond’s internal structure without breaking it. cut .
Inside the diamond inclusions, the researchers found a mineral called ringwoodite, which has the same chemical composition as olivine, the main material of the upper mantle, but formed under such intense temperature and pressure that, until 2014, scientists had only found in a meteorite sample, Gu said. Ringwoodite is typically found in the transition zone between the upper and lower mantle, between about 255 and 410 miles (410 to 660 km) below the Earth’s surface, and may contain much more water than the minerals bridgmanite and ferropericlase, which are thought to that dominate the lower mantle, the study authors noted.
But instead of minerals typically found in the transition zone, around this ringwood were forms of minerals characteristic of the lower mantle. Because the mantle diamond preserved the properties of these minerals as they appeared in the deep Earth, the researchers were able to find the temperatures these minerals endured and the pressures they were under. they estimated the depth of the minerals to be about 410 miles (660 km) below the surface, near the outer boundary of the transition zone. The analysis further revealed that ringwoodite was likely in the process of breaking down into more typical lower mantle minerals in a hydrated or water-saturated environment, implying that water could have infiltrated the transition zone into the lower mantle.
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Although previous research has found some forms of minerals from the lower mantle in diamond inclusions, the combination of materials in this inclusion is unique, the authors noted. It was also unclear from previous findings whether these minerals indicated the presence of water-bearing minerals in the lower mantle, the study authors said. Because no one has directly sampled rocks deeper than about 7 miles (11 kilometers) below the planet’s surface, diamond inclusions are one of the few sources of minerals from the Earth’s mantle.
The results could have implications for understanding the deep-water cycle, or the water cycle between the planet’s surface and the deep interior, Gu said.
“The timeline for the [water cycle] it’s actually much more if it can be stored in a deeper place,” Gu said, meaning it would take longer to replenish the water if it was stored deep underground.
The findings may also affect models of plate tectonics. Gu said she hopes scientists will be able to incorporate the findings of this study into models of how water in the mantle can affect processes such as Earth’s internal convection currents. This current fuels plate tectonics by unevenly heating the Earth’s mantle, causing the hottest parts to rise and the Earth’s plates to shift over millions of years.
Although inclusions are sometimes seen as flaws in diamonds that make them less desirable, Gu said, they can provide valuable scientific information.
“Don’t be afraid to buy a diamond with an inclusion,” he said – you never know what they might contain.
EDITOR’S NOTE: This article was updated on September 28 to correct the year scientists first identified ringwoodite in mantle minerals (2014, not 2008) and to amend the timeline for the mantle water cycle ( greater at greater depths).
Originally published in Live Science.