Serpentinization is a process in which ultrabasic rocks, mainly composed of olivine, undergo hydrothermal metamorphism, forming secondary minerals such as serpentine, magnetite, and brucite, and generating hydrogen gas. Hydrogen can react with CO2 in the environment to generate hydrocarbons, which is of great significance for microbial activity and early life origins on the ocean floor. It is also a clean energy source. Therefore, the mechanism of hydrogen generation has attracted much attention, and its basic principle is that Fe2+in olivine or pyroxene is oxidized to Fe3+, while H ⁺ in water is reduced to H2. The current research controversy on hydrogen generation from serpentinite focuses on three issues: (1) whether the formation of magnetite is synchronous with the generation of H ₂; (2) How does silicon activity affect H2 generation; (3) Is there a difference in the occurrence form of Fe3+in serpentinite at different stages of serpentinite mineralization. Early studies often regarded the appearance of magnetite as a sign of hydrogen production and believed that the two were synchronous, but the serpentine lattice can also accommodate a large amount of Fe3+. So, what is the connection between the generation of Fe3+and its occurrence state in serpentine at different stages and hydrogen generation? Previous studies on oceanic drilling serpentinite have found that there was no significant generation of magnetite in the early stages of serpentinite formation, and only when the degree of serpentinite formation exceeded about 50% did a large amount of magnetite quickly appear. Based on this, the serpentinite process is divided into two stages: the first stage is the hydration of olivine, producing serpentinite and brucite; The second stage is the reaction of iron rich brucite with SiO2 rich fluid to form serpentine and magnetite and release H2. According to this model, the first stage reaction mainly occurs under the condition of low silicon activity to generate serpentine and brucite. However, brucite is not commonly found in natural serpentinite and often interacts with other minerals, making it difficult to analyze. This makes it difficult to preserve information on different stages of serpentinite formation, thereby limiting our understanding of Fe3+distribution behavior and H2 generation mechanisms.

The Heimulin area of the Qinling orogenic belt not only develops large fibrous serpentine asbestos deposits, but also has the world's only super large fibrous brucite deposit, which may record more information about early serpentinite mineralization. This study selected Heimulin serpentinite to investigate the behavior of iron during serpentinite formation and its relationship with H2 generation. The serpentinite rocks in this area can be divided into four types: the first type retains the olivine illusion, with the core being serpentinite and only sporadic magnetite development (Figure 1a, c); The second type also has a pseudomorphic structure, with a mixture of serpentine and brucite in the core, almost free of magnetite, and surrounded by serpentine with a large amount of fine-grained magnetite (Figure 1b, d); The third type is banded serpentinite formed through recrystallization, where magnetite is mostly produced in the form of veins between banded serpentinite (Figure 1e); The fourth type develops serpentinite veins, where serpentinite is distributed between the particles of serpentinite and gradually replaces it (Figure 1f). In addition, the area also develops rocks rich in brucite, consisting of brucite (>80%, including granular and fibrous types), magnetite, and a small amount of serpentinite (Figure 1g, h).

Stage 1: Olivine consumes dissolved silicon in the fluid, generating a small amount of magnetite and Fe3+- containing serpentine (existing in the form of double octahedra and vacancies), while releasing a small amount of H2. Due to the low silicon content of the pre arc fluid itself, the magnetite produced in this stage is very limited, and some primary magnetite may react with subsequent silicon rich fluids to dissolve (corresponding to the process of Fe3+entering the serpentine lattice in the form of double octahedra and vacancies in the first type of serpentinite).

Second stage: As the serpentinite reaction proceeds, the silicon activity of the system decreases, and olivine reacts with water to form serpentine and rich iron magnesium rich ore. This reaction does not involve electron transfer, therefore it does not produce H2 (corresponding to the almost no Fe3+substitution of serpentine in the first and second types of serpentinite).

Stage 3: The serpentinite process releases heat, causing local heating and slow dissolution of pyroxene, resulting in an increase in local silicon activity. Rich iron magnesium stone reacts with SiO2 and AQ to generate a large amount of fine-grained magnetite and serpentine with green cone type substitution characteristics (corresponding to the process of Fe3+substitution into serpentine in the second type of serpentine with green cone type). Global data on the composition of serpentinite shows that most serpentinite exhibits green cone type substitution characteristics (Figure 3), indicating that the third stage may be widespread and may be the main stage of H2 generation. Therefore, the generation of H2 requires appropriate silicon activity: when the silicon activity is too low, the generation of a large amount of brucite will inhibit the production of H2; When it is too high, it is easy to form talc, which is also not conducive to the generation of H2.

In the third type of serpentinite, serpentinite recrystallizes to form a banded structure under heating or stress, and the Fe3+/∑ Fe ratio, iron content, and Fe3+substitution degree of the whole rock are lower than those of the first type of serpentinite (Figure 2c), indicating that Fe3+may be redistributed during serpentinite recrystallization and enriched between serpentinite particles to form magnetite veins. Despite the development of magnetite veins, no new Fe3+was generated during this stage, so no H2 was produced. The fourth type of serpentinite also underwent recrystallization and developed a large amount of fibrous serpentinite. The Fe3+/∑ Fe ratio of the whole rock increased, while the iron content and Fe3+substitution degree of serpentine decreased compared to the first type of serpentine (Figure 2c). This may be related to the dissolution of brucite during recrystallization, causing some Fe2+and Mg to migrate out of the system, ultimately forming a fibrous brucite deposit. Meanwhile, due to the low activity of SiO ₂ in the system, the serpentinization process in the third stage is relatively limited, resulting in a lower Fe3+/∑ Fe ratio in the system; In this Fe3+deficient system, recrystallization is more conducive to the formation of chrysotile, resulting in the formation of large chrysotile asbestos minerals.

This study deepens our understanding of the hydrogen generation mechanism and the genesis of fibrous brucite in serpentinite processes: Fe3+first occupies octahedral positions and then tetrahedral positions when entering the lattice of serpentine;

; The entry of Fe3+into the tetrahedral position of the second type of serpentine marks the main stage of H2 generation; The recrystallization after serpentinite mineralization mainly involves the redistribution of iron between serpentinite and magnetite, without any new oxidation-reduction reactions, so no additional H2 is produced. Low silicon activity is beneficial for the stable existence of brucite in the pre arc environment, and subsequent dissolution and recrystallization processes may form high-grade fibrous brucite deposits. This achievement was published in the internationally renowned geological journal "Geochimica et Cosmochimica Acta" under the title "Iron Redox and Hydrogen Generation During Multistage Serpentinization Under Low Silica Activity: Insights from the Heimulin Serpentinites, Central China". The research has been supported by the Key Research and Development Project of the Ministry of Science and Technology (2023YFF0807100), the National Natural Science Foundation of China (42373022, 92479209), and the Shaanxi Provincial Natural Science Basic Research Program (2023-JC-YB-246). Gong Meijun, a master's student at Northwest University, is the first author of the paper, and Associate Professor Wu Kai is the sole corresponding author.

Article link: https://www.sciencedirect.com/science/article/abs/pii/S0016703726004576