09 Feb 2023
 | 09 Feb 2023
Status: this preprint is open for discussion.

Rapid hydration and weakening of anhydrite under stress: Implications for natural hydration in the Earth’s crust and mantle

Johanna Heeb, David Healy, Nicholas E. Timms, and Enrique Gomez-Rivas

Abstract. Mineral hydration is an important geological process that influences the rheology and geochemistry of rocks, and the fluid budget of the Earth’s crust and mantle. Steady-state differential compaction (SSDC), dry and ‘wet’ tests under confining pressure, and axial stress were conducted, for the first time, to investigate the influence of triaxial stress on hydration in anhydrite-gypsum aggregates. Characterization of the samples before and after triaxial experiments were performed with optical and scanning electron microscopy, including energy dispersive spectroscopy and electron backscatter diffraction mapping. Stress-strain data reveal that samples that underwent steady state differential compaction in the presence of fluids are ~14 to ~41 % weaker than samples deformed under ‘wet’ conditions. The microstructural analysis shows that there is a strong temporal and spatial connection between the geometry, distribution, and evolution of fractures and hydration products. The increasing reaction surface area in combination with pre-existing gypsum in a gypsum-bearing anhydrite rock led to rapid gypsification. The crystallographic orientations of newly formed vein-gypsum have a systematic preferred orientation for long distances along veins, beyond the grain boundaries of wall-rock anhydrite. Gypsum crystallographic orientations in {100} and {010} are systematically and preferentially aligned parallel to the direction of maximum shear stress (45° to σ1). Gypsum is also not always topotactically linked to the wall-rock anhydrite in the immediate vicinity. This study proposes that the selective inheritance of crystal orientations from favourably oriented wall-rock anhydrite grains for the minimization of free energy for nucleation under stress leads to the systematic preferred orientation of large new gypsum grains. A sequence is suggested for hydration under stress that requires the development of fractures accompanied by localised hydration. Hydration along fractures with a range of apertures up to 120 µm occurred in under 6 hours. Once formed, gypsum-filled veins represent weak surfaces and are the locations of further shear fracturing, brecciation, and eventual brittle failure. These findings imply that non-hydrostatic stress has a significant influence on hydration rates and subsequent mechanical strength of rocks. This phenomenon is applicable across a wide range of geological environments in Earth’s crust and upper mantle. Please find a graphical abstract in the PDF manuscript document and as PNG with the supplements.

Johanna Heeb et al.

Status: open (until 11 Apr 2023)

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  • RC1: 'Comment on egusphere-2023-161', Sergio Llana-Funez, 15 Mar 2023 reply

Johanna Heeb et al.


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Short summary
Hydration of rocks is a key process in the Earth’s crust and mantle that is accompanied by changes in physical traits and mechanical behaviour of rocks. This study assesses the influence of stress on hydration reaction kinetics and mechanics in experiments on anhydrite. We show that hydration occurs readily under stress and results in localized hydration along fractures and mechanic weakening. New gypsum growth is selective, and depends on the stress field and host anhydrite crystal orientation.