A team of Brazilian researchers discovered the first direct evidence that goethite – the mineral responsible for the brown color of soils – can withstand extreme pressures and temperatures all the way to the planet's interior inside a diamond just 3 millimeters long, within a microscopic impurity. Goethite forms in soil and on the ocean floor from iron-rich minerals in the presence of water. It incorporates some of these water molecules into its mineral structure. The study suggests that goethite may transport and release water into a region known as the lower mantle.
Previous studies have confirmed that water from the Earth's crust, the outermost layer of rock 20 to 80 kilometers thick on continents and 5 to 10 kilometers thick in oceans, can reach the denser rock region just below it, the upper mantle, to depths of up to 660 kilometers. The lower mantle begins there and consists of rocks with a similar chemical composition to the upper mantle but with different crystalline structures due to increased temperature and pressure at greater depths, extending to 2,900 km below the surface.
The study was published in May in the journal Scientific Reports. It was conducted at Sirius, the particle accelerator at the Brazilian Synchrotron Light Laboratory (LNLS), which is part of the Brazilian Center for Research in Energy and Materials (CNPEM) in Campinas in the state of São Paulo. The study is based on geologist Carolina Camarda's master's thesis and was supervised by geologist Tiago Jalowitzki of the University of Brasília (UnB) and physicist Hélio Tolentino of the LNLS. It was funded by the Coordination for the Improvement of Higher Education Personnel (CAPES), which is affiliated with the Brazilian Ministry of Education. Camarda conducted the research in collaboration with Fernanda Gervasoni , a geologist at the Federal University of Pelotas (UFPel) in Rio Grande do Sul state. Gervasoni is currently completing a postdoctoral fellowship at the LNLS with funding from FAPESP under the supervision of Tolentino.
In July 2018, Gervasoni and Jalowitzki visited a gold miners' cooperative in the Chapadão region of the municipality of Juína in the state of Mato Grosso and received a donation of ultra-deep diamonds. These diamonds are found in only a few places in the world and form at depths greater than 300 km. Most diamonds originate about 150 km below the surface. In both cases, they are brought to the surface by magma that fuels volcanic eruptions.
Many super-deep diamonds have little value as jewelry because of their irregular appearance and the presence of small dark fragments of other materials trapped within the stone during its formation, known as mineral inclusions. For geologists, these inclusions are more valuable than the diamond itself, because they act as time capsules, preserving evidence of the conditions inside the Earth where these stones formed hundreds of millions of years ago.
Made in Brazil
The super-deep diamonds from Juína were discovered in the late 1980s, and since then, they have been the source of numerous scientific discoveries. Until now, these studies had been carried out only by groups led by foreign researchers. "Ours is the first study conducted entirely by a Brazilian team using Brazilian instruments," Gervasoni points out.
The study is also the first in the world to analyze a super-deep diamond using synchrotron light techniques from start to finish. One of the Juína stones was chosen at random and was among the first materials analyzed at the Mogno and Carnaúba beamlines at Sirius, even during the commissioning phase of the new LNLS equipment, prior to their official inauguration in 2023 and 2021, respectively.
Synchrotron light is extremely bright electromagnetic radiation emitted by electrons traveling around a particle accelerator at speeds close to the speed of light. The beamlines direct specific bands of this radiation to research stations for material analysis.
At the Mogno beamline, high-resolution X-ray microtomography mapped approximately one hundred mineral inclusions within the diamond. Next, the Carnaúba beamline used X-ray spectroscopy to determine the chemical composition of the inclusions.
One of them caught the team's attention because it appeared to contain iron hydroxide, which is a very rare occurrence in the deep mantle because there are few hydrated minerals and most rocks do not support oxidation reactions. "When researchers find iron hydroxides, they usually dismiss the inclusion, assuming that some microscopic fracture in the diamond caused oxidation through contact with air," explains 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."
The team then used the Ema beamline at the Sirius facility to characterize all the minerals in the inclusion. Surprisingly, the X-ray diffraction technique identified the iron oxyhydroxide goethite (FeOOH) and the iron oxides hematite (Fe₂O₃) and magnetite (Fe₃O₄). While these minerals are common in soil and on the ocean floor, their coexistence within a microscopic inclusion is considered impossible under the temperature and pressure conditions on the planet's surface.
Journey to the Earth's interior
Until a few years ago, researchers generally agreed that goethite could not withstand the subduction process, in which an oceanic plate sinks beneath another tectonic plate of the Earth's crust toward the mantle. This process occurs today off the west coast of South America, where the Pacific Ocean crust subducts beneath the continent. The result is the volcanic and tectonic activity that formed (and continues to form) the Andes mountain range. Early in the process, temperatures above 200 °C would rapidly transform goethite into hematite and water.
However, Camarda and his colleagues noted that the diffraction results from Sirius were very similar to those obtained in a shock wave compression experiment conducted at Sichuan University in China in 2021. This experiment showed that goethite can withstand pressures between 35 and 57 gigapascals (GPa) – equivalent to about 500 times the pressure at the deepest point in the ocean – and temperatures between 877 and 1,827 °C. These conditions occur in the lower mantle, at depths between 900 and 1,250 kilometers.
Many researchers are skeptical of these results, as shock experiments do not exactly replicate the physical conditions of the lower and upper mantle. In 2021, however, a team led by researchers at the University of Bayreuth in Germany demonstrated that goethite withstood similar pressure and temperature conditions in a compression experiment using diamond anvil cells. These cells are capable of more accurately simulating the mantle environment. Other diamond anvil experiments have tested the resistance of this mineral but suggested its dehydration at shallower depths.
Therefore, Camarda's team proposes that goethite can survive subduction when sheltered within deep fissures in relatively cool oceanic plates reaching depths greater than 400 km. There, it would begin to transform into hematite and water, or into magnetite, oxygen, and water. This process could extend all the way to the lower mantle.
The diamond that encapsulated these minerals may have formed from the crystallization of carbon found in mantle minerals or the oceanic plate itself. The Earth's mantle is relatively low in carbon and consists mainly of magnesium-, iron-, and silicon-rich minerals. In another inclusion within the same diamond, the team identified one of these minerals: ferropericlase ((Mg, Fe)O), which is abundant in the lower mantle. This is further evidence of the depth at which this diamond formed.
"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 explains. "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."
In 2014, an international team published a study in the journal Nature reporting the discovery of ringwoodite (Mg₂SiO₄), a mineral abundant in the transition zone that can absorb water, in a Juína diamond. The upper mantle appears to harbor large quantities of water originating from the surface. In contrast, most minerals in the lower mantle have little capacity to store water.
In addition to Camarda, Gervasoni, Jalowitzki, and Tolentino, 11 other researchers affiliated with the CNPEM, UnB, and UFRGS participated in the study. Funding for the research was provided by the National Council for Scientific and Technological Development (CNPq), which is affiliated with the Brazilian 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.
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe
