A tiny sealed inclusion in a diamond from Juína held goethite, a water-bearing iron mineral, offering the first direct natural evidence that it can survive the plunge into Earth’s deep interior.

Most diamonds form around 150 kilometers beneath the surface. A rare few, known as super-deep diamonds, crystallize far lower, below 300 kilometers, and are carried upward by volcanic eruptions. As they grow, they can trap specks of the surrounding minerals, sealing them off like insects in amber. For geologists, those inclusions are among the only physical samples of Earth’s deep interior.

A team of Brazilian researchers has now found something unusual inside one such diamond. Writing in Scientific Reports, they report that a microscopic inclusion in a three-millimeter diamond from Juína, in the state of Mato Grosso, contains goethite, an iron oxyhydroxide that carries water in its chemical structure. The finding offers the first direct evidence that the mineral can survive the extreme pressures and temperatures of the deep Earth, and supports the idea that it helps transport water into the lower mantle.

“Ours is the first study conducted entirely by a Brazilian team using Brazilian instruments,” said Fernanda Gervasoni, a geologist at the Federal University of Pelotas who carried out the work as a postdoctoral fellow at the Brazilian Synchrotron Light Laboratory (LNLS).

An inclusion others might have ignored

Goethite is common at Earth’s surface. It forms when iron-bearing minerals weather and is a major component of rust and of many soils. Near the surface, it breaks down into hematite and water at temperatures above about 200 degrees Celsius. That fragility is one reason researchers are often suspicious when they find it inside diamonds: it could have formed much later, from fluids seeping in through cracks.

“When researchers find iron hydroxides, they usually dismiss the inclusion,” said Carolina Camarda, a co-author who is now a doctoral candidate at the European XFEL in Germany.

The team wanted to rule out that possibility. Using X-ray tomography, they showed that the inclusion containing goethite was completely enclosed by diamond. “Since tomography proved the inclusion had no connection to the outside, we decided to investigate,” Camarda said.

Inside the single inclusion, the researchers found goethite together with hematite and magnetite, two iron oxides. Another inclusion in the same diamond contained ferropericlase, a magnesium-iron oxide that is abundant in the lower mantle, consistent with the diamond’s deep origin.

A Brazilian synchrotron at work

The analysis relied on Sirius, the synchrotron light source operated by the Brazilian Center for Research in Energy and Materials (CNPEM) in Campinas. Synchrotrons accelerate electrons to nearly the speed of light and use them to generate extremely bright X-rays, which can reveal the structure and chemistry of materials in fine detail.

The team used three of the facility’s beamlines. At Mogno, they mapped about 100 inclusions in the diamond using tomography. At Carnaúba, they measured the inclusions’ chemical composition. At Ema, they identified the minerals present. The diamonds from Juína had been donated for research in July 2018.

Gervasoni, Camarda, Tiago Jalowitzki and Hélio Tolentino were joined by 11 other researchers from CNPEM, the University of Brasília and the Federal University of Rio Grande do Sul.

How water travels down

Earth’s crust is thin compared with the planet as a whole, roughly 20 to 80 kilometers thick beneath the continents and 5 to 10 kilometers beneath the oceans. Below it, the mantle extends to a depth of about 2,900 kilometers. Water from the surface reaches this interior mainly through subduction, the process by which oceanic plates sink beneath other plates and descend into the mantle.

Many water-bearing minerals break down at relatively shallow depths, releasing their water. Earlier work has traced water from the crust down to about 660 kilometers, the boundary between the mantle transition zone and the lower mantle. A 2014 study in Nature reported ringwoodite, a high-pressure mineral containing water, in another diamond from Juína, providing evidence that the transition zone holds significant amounts of water.

Whether water can travel deeper has been less clear. Laboratory experiments have suggested that goethite might do the job. Shock-compression experiments at Sichuan University reported in 2021 found the mineral stable at pressures of 35 to 57 gigapascals and temperatures of 877 to 1,827 degrees Celsius, conditions corresponding to depths of roughly 900 to 1,250 kilometers. A study the same year at the University of Bayreuth, using a diamond anvil cell to squeeze samples to high pressures, reported similar stability.

The new diamond provides natural evidence to go with those experiments. The team proposes that goethite can survive subduction within relatively cool oceanic plates to depths greater than 400 kilometers. Deeper down, it would eventually break down, releasing its water into the lower mantle.

Why deep water matters

Water in the mantle has consequences far beyond its quantity. Even small amounts can change how rocks behave. “The release of water lowers the melting point of these rocks,” Gervasoni said.

Lowering the melting point can trigger the formation of magma, and water also makes mantle rocks weaker and easier to deform, affecting how the mantle flows. Gervasoni noted that some theories link the volcanic rocks that carry super-deep diamonds to the surface to the region near the transition zone, where such processes may play a role.

Over geological time, the exchange of water between the surface and the deep interior may help regulate the size of the oceans. Estimates of how much water is stored in the mantle vary widely, and some researchers have suggested it could hold as much as or more than the oceans themselves.

One diamond, many questions

The study is based on a single inclusion in one diamond, so it cannot show how common goethite is in the deep mantle or how much water it delivers. Super-deep diamonds are rare, and only a small fraction contain inclusions suitable for analysis. Additional samples, along with further experiments, will be needed to determine how important this pathway is compared with other possible water carriers.

The work was supported by the Brazilian agencies CAPES and CNPq, the National Institute of Science and Technology for Tectonic Studies, the Serrapilheira Institute and the Women in Research program at the University of Münster. FAPESP funded Gervasoni’s postdoctoral fellowship.

For now, the speck of goethite offers a rare glimpse of how Earth recycles its water. Carried down by sinking plates and locked inside a diamond for the return journey, it suggests that some of the planet’s surface water may reach much deeper than once thought.

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Notes

1. “Iron oxyhydroxide as water carrier to the Earth’s mantle.” Scientific Reports, 11 May 2026. DOI: 10.1038/s41598-026-46683-8.
2. Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). “Analysis of a super-deep diamond reveals how water travels to the Earth’s interior.” EurekAlert!, 31 July 2026. https://www.eurekalert.org/news-releases/1138448.

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