How One Diamond Disproved Everything We Thought About Earth’s Mantle!

Scientists have confirmed for the first time that goethite—a common iron oxyhydroxide found in soils and on ocean floors—can survive subduction and reach depths exceeding 900 kilometers within Earth’s lower mantle. The discovery emerged from analysis of a 3-millimeter diamond recovered near Juína in Brazil’s Mato Grosso state.

How synchrotron light revealed a sealed pocket

Researchers employed high-resolution X-ray microtomography at the Mogno beamline and X-ray spectroscopy at the Carnaúba beamline of the Sirius particle accelerator, operated by the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, São Paulo. These instruments mapped approximately 100 mineral inclusions inside the diamond and identified their elemental composition. One inclusion stood out: it contained iron hydroxide, an unexpected finding given the deep-mantle environment’s low oxidation potential and scarcity of hydrated minerals.

“When researchers find iron hydroxides, they usually dismiss the inclusion, assuming that some microscopic fracture in the diamond caused oxidation through contact with air,” explained Carolina Camarda, a Ph.D. candidate at the European XFEL laboratory in Germany. “Since tomography proved the inclusion had no connection to the outside, we decided to investigate.”

At the Ema beamline, X-ray diffraction confirmed the presence of goethite (FeOOH), hematite (Fe₂O₃), and magnetite (Fe₃O₄) within the same sealed, microscopic pocket. All three minerals are abundant at Earth’s surface but cannot coexist under surface temperature and pressure conditions—indicating formation at extreme depth.

Why goethite defies earlier assumptions

Prior models held that goethite decomposes rapidly during subduction, converting to hematite and water at temperatures above 200 °C—well before reaching the lower mantle. Yet the diffraction pattern measured at Sirius closely matched results from a 2021 shock compression experiment conducted at Sichuan University in China, which demonstrated goethite stability at pressures of 35–57 gigapascals (GPa) and temperatures between 877 °C and 1,827 °C. Those conditions correspond to depths of roughly 900–1,250 kilometers in the lower mantle.

A separate 2021 study led by the University of Bayreuth in Germany reinforced those findings using diamond anvil cells—devices that more closely simulate mantle conditions than shock compression. While other diamond anvil experiments suggested goethite loses water at shallower depths, the new evidence points to a context-dependent survival pathway, possibly involving protection within cool, fractured oceanic plates descending beneath continental crust.

The Juína diamond and its geological significance

The diamond originated in the Chapadão region near Juína, where super-deep diamonds—those formed more than 300 kilometers below the surface—were first identified in the late 1980s. Unlike typical diamonds, which form at about 150 kilometers depth, these ultra-deep specimens crystallize in magma associated with volcanic eruptions and often contain irregular shapes and dark mineral inclusions.

Those inclusions remain chemically unaltered due to the diamond’s protective enclosure, preserving records of deep-Earth physical and chemical conditions from hundreds of millions of years ago. Another inclusion in the same diamond contained ferropericlase ((Mg, Fe)O), a mineral abundant in the lower mantle—further confirming the stone’s origin at great depth.

“Ours is the first study conducted entirely by a Brazilian team using Brazilian instruments,” noted Fernanda Gervasoni, a geologist at the Federal University of Pelotas (UFPel) and postdoctoral fellow at LNLS. The project began with a July 2018 visit to a gold miners’ cooperative in Chapadão, where Gervasoni and Tiago Jalowitzki of the University of Brasília (UnB) received a donation of ultra-deep diamonds.

Water’s path into Earth’s interior

The findings support a revised model in which goethite, shielded inside relatively cool oceanic plates, descends beyond 400 kilometers before transforming into hematite and water—or magnetite, oxygen, and water. Those reactions could continue into the lower mantle, delivering surface-derived water to regions previously considered incapable of substantial water storage.

“The release of water lowers the melting point of these rocks, potentially generating small amounts of magma that tend to rise slowly to the surface,” Gervasoni explained. “Some theories suggest that volcanic rocks that bring super-deep diamonds to the surface form near the transition zone between the upper and lower mantle, around 400 km deep.”

This mechanism complements earlier evidence from Juína: a 2014 international study published in Nature reported ringwoodite (Mg₂SiO₄)—a water-bearing mineral common in the transition zone—in another Juína diamond. Together, the discoveries indicate that Earth’s upper mantle may hold substantial water carried downward from the surface, while goethite now offers a plausible carrier for deeper transport.

The study, titled “Iron oxyhydroxide as water carrier to the Earth’s mantle,” was published on 11 May 2026 in Scientific Reports. DOI: 10.1038/s41598-026-46683-8. Funding came from the National Council for Scientific and Technological Development (CNPq), affiliated with Brazil’s Ministry of Science, Technology, and Innovation; the National Institute of Science and Technology for Tectonic Studies; the Serrapilheira Institute; and the Women in Research program at the University of Münster in Germany.