Author name: Hans Keppler

The Subduction of Hydrogen: Deep Water Cycling, Induced Seismicity, and Plate Tectonics

The dynamic equilibrium between mantle degassing and water recycling in subduction zones controls the variation of sea level in deep geologic time, as well as the size of Earth’s interior hydrogen reservoir. While the principles of water transport and water release by common hydrous minerals in the subducted crust are relatively well understood, the importance of deep serpentinization of the slab, the contribution of nominally anhydrous minerals and dense hydrous magnesium silicates to water transport, and the mechanisms of water subduction into the lower mantle are still subjects of active research. A quantitative understanding of these processes is required to constrain the evolution of Earth’s deep water cycle through geologic time and the role of water in stabilizing plate tectonics.

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Water in the Mantle

Subducting slabs transport water stored in hydrous minerals into the transition zone and lower mantle. The water storage capacity of the upper and lower mantles is less than 0.2 wt%. The transition zone has a storage capacity of approximately 0.5–1 wt% due to a water solubility of about 1–3 wt% in wadsleyite and ringwoodite, which are the major con- stituents of the transition zone. Thus, the transition zone may be a major water reservoir in the Earth’s interior. Recent geophysical observations suggest the existence of the hydrated transition zone beneath subduction zones. Water or hydrogen may be transported as far as the bottom of the lower mantle by reacting with metallic iron in the lower mantle to form hydrous phases or iron hydride.

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December 2025 --The Variscan Orogeny in Europe – Understanding Supercontinent Formation

The Variscan orogen formed between 380 and 300 million years ago through several accretionary and collisional cycles, culminating with the construction of the Pangea supercontinent. This process occurred via sequential opening and closure of oceanic basins, synchronous detachment of Gondwana derived continental ribbons, and their outboard amalgamation onto the Laurussia margin. The Variscan orogen is rather unique compared with other orogenic belts on Earth: its overthickened and dominantly magmatic crust in the central belt, surprisingly minor mantle involvement in the magmatic and geodynamic processes, coherent and pulsed magmatism along the collision suture, and its complex accretionary history. Because its final product, Pangea, is the youngest and best-understood supercontinent on Earth, the Variscan orogeny offers clues for understanding the mechanisms of supercontinent formation.