Massive Mg-rich fluid release across the brucite + serpentine reaction in subduction zones

Researchers at ISTerre have investigated the transformations of serpentinites in subduction zones, focusing on dehydration processes and the mobility of magnesium within these mantle rocks. The analyzed samples come from the Zermatt-Saas unit in the Alps, where high-pressure and high-temperature conditions allow the observation of metamorphic mineral formation and water release.


Microstructural and chemical analyses reveal that brucite and antigorite transform into olivine while simultaneously producing magnesium-rich fluids. This process leads to the formation of olivine and brucite veinlets, indicating that magnesium can locally concentrate and migrate through the rock. Two types of brucite were identified : one intimately intergrown with serpentine, and another forming small, independent hydrothermal veins, reflecting crystallization from fluids released during dehydration.

These observations provide new insight into mineral–fluid interactions in the mantle and their potential influence on tectonic processes. The fluids generated during dehydration may help explain certain episodes of shallow seismicity, including the low-frequency earthquakes documented in the Cascadia, Nankai, and Mexican subduction zones.
Fig. 4 — UTG serpentinites. (A–B) Fresh and weathered serpentinites crosscut by olivine/Ti-clinohumite veins (sample ZER_22-11A). (C–F) Schematic and microscopic images showing brucite-B veinlets cutting through metamorphic olivine, associated with magnesium-enriched bands.




The study thus illustrates how microscopic mineral transformations contribute to the circulation of water and chemical elements within the Earth’s mantle, ultimately influencing its deep dynamics.



The full study is published in Earth and Planetary Science Letters

References :

Kempf, E. D., Hermann, J., Reusser, E., Baumgartner, L. P., Lanari, P. (2020). The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland). Swiss Journal of Geosciences.

Huber, K., Vrijmoed, J. C., John, T. (2022). Formation of olivine veins by reactive fluid flow in a dehydrating serpentinite. Geochemistry, Geophysics, Geosystems.

Debret, B., Bolfan-Casanova, N., Padrón-Navarta, J. A., Andreani, M., Garrido, C. J., Muñoz, M., Trcera, N. (2015). Redox state of iron during high-pressure serpentinite dehydration. Springer Nature Link.

Scientific contacts :

  • E. Legros – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • B. Malvoisin – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • F. Brunet – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • Z. El Yousfi – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • V. Batanova – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • A. Sobolev – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre
  • A.-L. Auzende – Researcher, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre