Mixed‐Mode Strain Localization Generated by Hydration Reaction at Crustal Conditions

Abstract

Hydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X-ray synchrotron microtomography experiments to investigate reaction-induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190°C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time-resolved three-dimensional imaging, we quantify the spatial and temporal distribution of the reaction-induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed-mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction-induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency-magnitude domain, indicative of long-range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization.

Reaction‐Induced Fracturing: When Chemistry Breaks Rocks

“…They also reproduce a complex three-dimensional fracturing pattern often referred to as a mesh structure in serpentines. However, Zheng et al (2019) showed that when the effective pressure applied to the sample exceeds about 30 MPa, the periclase (MgO) to brucite (Mg(OH) 2 ) reaction-induced fracturing process slows dramatically. Such slowing with increasing the confining pressure has also been observed in the hydration of CaO into Ca(OH 2 ) (Wolterbeek et al, 2018).…”

“…This reaction created a force of crystallization that fractured the peridotite. This image was acquired under in situ conditions using synchrotron X‐ray microtomography and the Hades triaxial rig (Zheng et al., 2019). (d and e) Numerical simulations of the hydration reaction of an elliptic grain that leads to an increase of volume and the formation of radial fractures in the surrounding rock (reproduced from Yoshida et al., 2020).…”

“…The authors reproduce and quantify the fracture patterns observed in serpentinized oceanic lithosphere rocks collected during the Oman Drilling Project. Their study is complementary to recent studies where laboratory experiments (Zheng et al, 2019;Zhu et al, 2016) and numerical modeling (Guren et al, 2021;Zhang et al, 2019) have identified key parameters that control the rate of reaction-induced fracturing and identified under which conditions this rate may increase or decrease. These parameters include the temperature, the amount of available water, the effective pressure, and the strength and initial porosity of the rock.…”

Structural and Geochemical Interactions Between Magma and Sedimentary Host Rock: The Hovedøya Case, Oslo Rift, Norway

“…The co‐occurrence of mechanical features predicted by both end‐member modes is called “mixed‐mode” fracturing (Rubin, 1995) and was found both in natural outcrops (e.g., Dering, Micklethwaite, Cruden, et al, 2019; Martinez‐Poza & Druguet, 2016) and laboratory experiments of fluid intrusion in natural rock samples (e.g., Zheng et al, 2019). Mechanical indicators of LEFM, such as tapered intrusion fronts and cavities, were found in some cases in association with dike opening profiles that depart from the parabolic shape expected in elastic materials (Daniels et al, 2012; Vachon & Hieronymus, 2017) and blunt intrusion tips and shear‐mode fractures.…”

Opportunities for Characterizing Geological Flows Using Magnetic Resonance Imaging

“…However, experiments are limited in size to a few centimeters, owing to scanner dimensions, and in the speed of flow they can capture. For instance, Zheng et al. (2019) performed a reactive flow experiment in which water percolated through a porous rock sample (a cylinder 5 mm in height an 2.3 mm in diameter) and the reaction induced fracturing of the sample.…”