en.Wedoany.com Reported - Recently, a team led by researcher Wang Yu from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, in collaboration with Professor Chen Chunfei from China University of Geosciences (Wuhan), discovered through high-temperature and high-pressure experiments that phengite, widely distributed in subduction zones, is a key "transporter" carrying halogens into the deep Earth. The related findings were published online in the international academic journal Science Advances in the early hours of April 2, Beijing time.
Halogen elements such as fluorine and chlorine not only control the formation of strategic mineral resources like diamonds but also influence the distribution of volcanic earthquakes and even play a significant role in Earth's long-term climate evolution. According to traditional geochemical views, these elements are largely lost during the early stages of plate subduction as hydrous minerals dehydrate and decompose, making it difficult for them to enter the mantle hundreds of kilometers deep. However, high-density saline fluid inclusions have been discovered in deep diamonds from various locations around the world, and anomalous fluorine enrichment has been recorded in some mantle minerals. How these elements, which were expected to be "lost halfway" in shallow regions, manage to break through barriers and reach the deep Earth has long been a key puzzle in the study of deep volatile cycling.
Based on potassium-rich altered oceanic crust as the starting material, the research team systematically studied the stability of phengite and its capacity to carry fluorine and chlorine under conditions equivalent to pressures (5 to 11 GPa) and temperatures (850 to 1200°C) at depths of 150 to 330 kilometers underground. The experimental results revealed that phengite can remain stable under extreme conditions at depths of about 330 kilometers, meaning it can "escort" fluorine and chlorine to depths of the Earth previously thought to be unreachable.
More notably, phengite does not carry both elements "all the way to the bottom." When it destabilizes and decomposes or undergoes partial melting at greater depths, fluorine and chlorine exhibit significant "geochemical decoupling": chlorine is more likely to enter the released fluid or melt, while fluorine is captured by the newly formed high-pressure mineral—wadsleyite—and continues to migrate deeper into the mantle in solid form.
This discovery also provides new clues for explaining the formation mechanism of deep diamonds. The study shows that under conditions at depths of about 330 kilometers, the supercritical fluid released from the decomposition of phengite is rich in potassium and chlorine, with chlorine content as high as 9.6% to 19.9%, which closely matches the composition of saline inclusions found in natural deep diamonds. The decomposition of phengite in subducted oceanic crust at great depths is likely a significant source of high-salinity fluids in the deep mantle.
Based on the experimental data, the research team further estimated the annual halogen flux transported by phengite to the deep mantle: approximately 1.7 to 2.6 million tons for fluorine and 0.52 to 1.1 million tons for chlorine. Using the water volume of West Lake in Hangzhou (about 14.5 million tons) as a reference, on average, every 4 to 6.5 years, the total mass of halogens transported to the deep Earth through this subduction process is equivalent to "one West Lake."
The first author of the paper is Liu Yingzhuo, a Ph.D. candidate at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, with corresponding author Researcher Wang Yu. Collaborators include Professor Chen Chunfei from China University of Geosciences (Wuhan), Associate Researcher Gao Mingdi, and Researcher Xu Yigang from the Guangzhou Institute of Geochemistry.
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