Diamond

Rare diamonds indicate that water exists much deeper in the Earth’s interior than scientists previously thought.

Recently, clues about water in Earth’s deep interior were extracted from rare diamonds.

Water may be able to penetrate deeper into the Earth’s interior than scientists previously thought, according to a rare type of diamond.

Though water covers more than 70% of our planet, there is also water in minerals more than 200 miles (322 kilometers) underground, including the upper mantle, the semi-malleable layer on top of which the crust “floats.” Minerals can hold far less water as the upper mantle transitions into the hotter, denser lower mantle, according to scientists.

Diamond
Mineral inclusion in a diamond, containing ringwoodite, enstatite and ferropericlase.

Researchers discovered inclusions, or tiny bits of other minerals, that can hold more water and appear to have existed on the boundary between the upper and lower mantle in a new study published Sept. 26 in the journal Nature Geoscience. The findings suggest that there may be more water in the Earth than scientists previously thought, which could have an impact on our understanding of the deep water cycle and plate tectonics.

Tingting Gu, the study’s lead author, is now a mineral physicist at Purdue University in Indiana but was a researcher at the Gemological Institute of America in New York City at the time of the study.

Gu and her colleagues looked at type IaB diamonds, a rare type of diamond from Botswana’s Karowe mine that forms deep underground and can stay there for a long time. They used “nondestructive” methods of analysis to examine the diamond, such as Raman micro-spectroscopy, which uses a laser to reveal some of a material’s physical properties without cutting it open, and X-ray diffraction to examine the diamond’s internal structure without cutting it open.

The researchers discovered ringwoodite inside the diamond’s inclusions, which has the same chemical composition as olivine, the primary material of the upper mantle, but forms under such high temperature and pressure that scientists had only ever found it in a meteorite sample until 2008. Ringwoodite is typically found in the transition zone between the upper and lower mantles, between 255 and 410 miles (410 to 660 km) below the Earth’s surface, and can contain significantly more water than the minerals bridgmanite and ferropericlase, which are thought to dominate the lower mantle, according to the study authors.

However, instead of transition zone minerals, the minerals surrounding this ringwoodite were lower mantle minerals. Because the encasing diamond preserved the properties of these minerals as they appeared in deep Earth, the researchers were able to determine the temperatures and pressures they were subjected to; they estimated the minerals’ depth to be around 410 miles (660 km) below the surface, near the outer boundary of the transition zone. The ringwoodite was also found to be in the process of breaking down into more typical lower mantle minerals in a hydrous, or water-saturated, environment, implying that water could penetrate from the transition zone into the lower mantle.

Although previous research has discovered some forms of minerals from the lower mantle in diamond inclusions, the authors noted that the combination of materials in this inclusion is unique. According to the study’s authors, it was also unclear from previous findings whether these minerals indicated the presence of water-containing minerals in the lower mantle. Diamond inclusions are one of the few sources of minerals from Earth’s mantle because no one has directly sampled rock deeper than about 7 miles (11 km) beneath the planet’s surface.

According to Gu, the findings could have implications for understanding the deep water cycle, or the flow of water between the planet’s surface and deep interior.

“The timescale for the [water cycle] is actually much shorter if it can be stored at a deeper place,” Gu explained, implying that water would renew itself faster if it were stored deep underground.

diamond

The findings could also have an impact on plate tectonic models. Gu hopes that scientists will be able to incorporate the findings of this study into models of how water in the mantle influences processes such as the Earth’s internal convection current. This current drives plate tectonics by unevenly heating the Earth’s mantle over millions of years, causing hotter parts to rise and shift the Earth’s plates.

Although inclusions are sometimes regarded as blemishes in diamonds, they can provide valuable scientific information, according to Gu.

“Don’t be afraid to buy a diamond with an inclusion,” she advised, because you never know what it might contain.

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