Volatile-consuming reactions fracture rocks and self-accelerate fluid flow in the lithosphere

Importance

Hydration and carbonation are the primary reactions that drive Earth’s volatile cycles. These reactions are characterized by a large increase in solid volume, up to several tens of percent, and can induce fracturing, fluid flow and other reactions. However, no experimental study has succeeded in a net increase in fluid flow during reactions, and the mechanisms that control acceleration or deceleration remain largely unknown. Here we present clear experimental evidence that hydration reactions can fracture rocks and accelerate fluid flow, under confining pressure (i.e. at a simulated depth). We conclude that a high reaction rate, relative to fluid flow, is essential for fracturing and accelerating fluid flow during these reactions in the Earth.

Summary

The hydration and carbonation reactions within the Earth cause an increase in the solid volume of up to several tens of % by volume, which can induce stresses and fracture of the rock. Observations of naturally hydrated and carbonated peridotite suggest that permeability and fluid flow are enhanced by reaction-induced fracturing. However, permeability enhancement during solid-volume-increasing reactions has not been achieved in the laboratory, and the mechanisms of reaction-accelerated fluid flow remain largely unknown. Here, we present experimental evidence for significant enhancement of permeability by volume-increasing reactions under confining pressure. The hydromechanical behavior of sintered periclase hydration [MgO + H2O → Mg(OH)2] depends mainly on the initial pore-fluid connectivity. Permeability increased by three orders of magnitude for samples with low connectivity, while it decreased by two orders of magnitude for samples with high connectivity. Permeability enhancement was caused by hierarchical fracturing of reactive materials, while decrease was associated with homogeneous pore clogging by reaction products. These behaviors suggest that fluid flow, relative to reaction rate, is the primary control of hydromechanical evolution during bulking reactions. We suggest that extremely high reaction rate and low pore-fluid connectivity lead to local stress perturbations and are essential for reaction-induced fracturing and accelerated fluid flow during hydration/carbonation.

Footnotes

    • Accepted November 16, 2021.
  • Author contributions: research designed by MU; MU, KK and HK conducted research; MU and AO provided new analytical reagents/tools; MU analyzed the data; MU, AO and NT discussed the experimental results and their implications; and MU, AO and NT wrote the article.

  • The authors declare no competing interests.

  • This article is a direct PNAS submission.

  • This article contains additional information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2110776118/-/DCSupplemental.

Data availability

All study data is included in the article and/or IS Annex.

About Lucille Thompson

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