Surface volatiles—chemical substances that easily become gases or fluids at relatively low temperatures and pressures—are transported into the Earth through subduction zones, with some being transported into the deep mantle and others to shallower depths.
According to conventional models, halogens such as fluorine (F) and chlorine (Cl) are largely released at shallow depths when hydrous minerals break down, making it difficult for them to reach the deep mantle. However, a new study has identified high-density saline inclusions in deep diamonds and fluorine enrichment in mantle minerals, indicating that some halogens survive transport to great depths.
At the heart of this study, a team led by Prof. WANG Yu from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences (CAS), together with collaborators including Prof. CHEN Chunfei from China University of Geosciences (Wuhan), has used high-pressure and high-temperature experiments to provide evidence for how these halogens reached the deep mantle.
Their results, published in Science Advances on April 1, show that phengite, a mineral widely distributed in subduction zones, may act as a key carrier that transports halogens into the deep mantle of the Earth. Using altered oceanic crust analogs, the researchers found that phengite remains stable up to 11 gigapascals (GPa) and 1,050 °C (1,922 °F), allowing it to carry fluorine and chlorine to depths of about 330 km (205 mi).
The experiments further reveal that fluorine and chlorine follow different paths once phengite becomes unstable. Chlorine preferentially enters the released fluid, whereas most fluorine is retained in the newly formed high-pressure KMgF 3 phase, which can carry fluorine deeper into the mantle. In other words, chlorine is more readily mobilized by deep fluids, while fluorine can remain in solid phases and continue its downward transport.
These findings also shed light on the origin of saline inclusions in deep diamonds. The breakdown of phengite produces potassium- and chlorine-rich fluids containing 9.6–19.9 wt.% Cl, which closely matches the composition of high-density saline inclusions observed in natural diamond samples. These results suggest that the breakdown of phengite in subduction zones may be an important source of deep saline fluids linked to diamond formation and cratonic metasomatism.
Based on their experimental results, the researchers estimated that phengite can transport 1.7 × 10 12 -2.6 × 10 12 g/yr of F and 0.52 × 10 12 -1.1 × 10 12 g/yr of Cl into the deep mantle, suggesting that phengite is an important mineral pathway in the global deep halogen cycle.
Together, these results provide new experimental evidence to resolve the long-standing contradiction between shallow halogen loss and deep halogen enrichment, and offer new insight into deep fluid activity, diamond formation, and Earth's long-term chemical evolution.
This work was supported by the National Key R&D Program of China, the Strategic Priority Research Program of CAS, and the Natural Science Foundation of Guangdong Province, China.
Science Advances
Experimental study
Not applicable
Phengite-mediated fluorine and chlorine fluxes from subduction zones to the deep mantle
1-Apr-2026