For decades, geologists have grappled with a fundamental contradiction in how the Earth breathes. While conventional models suggested that halogens—highly reactive elements like fluorine and chlorine—were largely stripped away at shallow depths during the subduction of tectonic plates, physical evidence told a different story. Deep-earth diamonds and mantle minerals frequently revealed high concentrations of these elements, suggesting they were somehow bypassing the shallow “filters” of the crust.
A new study has now identified phengite identified as key carrier of halogens into Earth’s deep mantle, providing a missing link in the global deep halogen cycle. By simulating the extreme conditions of the Earth’s interior, researchers have demonstrated that this specific mineral acts as a high-pressure vessel, shielding these volatiles from being released too early and transporting them to depths far greater than previously thought possible.
The research, led by Professor Wang Yu of the Guangzhou Institute of Geochemistry at the Chinese Academy of Sciences (CAS) and published in Science Advances, utilizes high-pressure and high-temperature experiments to map the journey of surface volatiles from the ocean floor to the deep interior.
The team, which included collaborators such as Professor Chen Chunfei from the China University of Geosciences (Wuhan), focused on altered oceanic crust analogs to spot how phengite behaves as it is pushed deeper into the planet. Their findings suggest that phengite remains stable under immense pressure—up to 11 gigapascals (GPa) and temperatures of 1,050 °C (1,922 °F)—effectively acting as a conveyor belt that carries fluorine and chlorine to depths of approximately 330 kilometers (205 miles).
image:
Schematic diagram showing the deep F, Cl influx of global AOC that pass through the sub-arc depth through phengite
Credit: WANG Yu
The Divergent Paths of Fluorine and Chlorine
One of the most significant revelations of the study is that fluorine and chlorine do not behave identically once they reach the limits of phengite’s stability. When the mineral finally breaks down, the two halogens split, following distinct geochemical pathways that determine how deep they penetrate the mantle.
Chlorine is more readily mobilized by deep fluids. As phengite becomes unstable, chlorine preferentially enters the released fluids, creating a potassium- and chlorine-rich brine. In contrast, most of the fluorine is retained in a newly formed high-pressure phase known as KMgF3. Because this phase remains solid, fluorine can continue its downward transport, potentially reaching even deeper regions of the mantle than chlorine.
This distinction explains why different mantle minerals reveal varying levels of halogen enrichment. While chlorine is more likely to be recycled via fluid-driven processes, fluorine’s ability to remain in a solid phase allows it to penetrate the Earth’s interior more effectively, influencing the long-term chemical evolution of the planet.
Solving the Mystery of Deep Diamonds
The implications of this mineral transport extend to the formation of some of the world’s most coveted gemstones. Natural diamonds often contain “saline inclusions”—tiny pockets of salt-rich fluids trapped during the diamond’s growth. The composition of these inclusions has long puzzled scientists, as they are often too dense to have originated from shallow sources.
The researchers found that the breakdown of phengite produces fluids containing between 9.6 and 19.9 wt.% chlorine. This chemical signature closely matches the high-density saline inclusions observed in natural diamond samples. This suggests that the breakdown of phengite in subduction zones is a primary source of the deep saline fluids that contribute to diamond formation and cratonic metasomatism—the process by which fluids alter the composition of the ancient, stable cores of continents.
Estimated Annual Halogen Flux
To quantify the impact of this process on a global scale, the team estimated the volume of material being transported into the deep mantle annually. The figures underscore the importance of phengite as a primary mineral pathway in the global deep halogen cycle:
| Element | Estimated Annual Flux (grams/year) |
|---|---|
| Fluorine (F) | 1.7 × 1012 to 2.6 × 1012 |
| Chlorine (Cl) | 0.52 × 1012 to 1.1 × 1012 |
Broader Implications for Earth’s Evolution
This discovery does more than explain the presence of salt in diamonds; it reshapes the understanding of “surface volatiles”—substances that easily become gases or fluids at low temperatures. By proving that phengite can transport these elements to depths of 330 km, the study resolves the contradiction between the perceived loss of halogens at shallow depths and their observed enrichment in the deep mantle.
Understanding this cycle is critical for planetary scientists. The movement of volatiles into the mantle influences volcanic activity, the composition of the atmosphere over billions of years, and the overall thermal regulation of the Earth’s interior. By identifying the specific mineral “carriers,” scientists can more accurately model how the Earth recycles its chemical building blocks.
The work was supported by the National Key R&D Program of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the Natural Science Foundation of Guangdong Province.
As researchers continue to refine these models, the next phase of study will likely focus on the interaction between KMgF3 and other deep-mantle minerals to determine exactly how deep fluorine can travel before it is finally sequestered or recycled back to the surface. Further experimental data on other subduction zone minerals may either reinforce or modify these current flux estimates.
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