| 25 Sep 2025
Impact of differential stress on fracture due to volume increasing hydration
Abstract. The volume increase that accompanies many hydration reactions can stress and fracture the surrounding rock, a process commonly called reaction-induced fracture. Reaction-induced fracture accelerates the rate of hydration by creating new pathways for fluids to migrate into reactive rock and by generating new reactive surface areas. The evolution of reaction-induced fracture also depends on the background stress state, which varies among different tectonic environments. We investigate the impact of tectonic stresses on reaction-induced fracture, using 2-D hydraulic-chemical-mechanical distinct element models. The results indicate that the general pattern of reaction-induced fracture depends on the orientation of background tectonic stresses relative to fluid-supplying channels. A spalling fracture pattern characterized by short cracks parallel to and along fluid-supplying channels occurs when the maximum principal tectonic stress is parallel to the channels whereas a branching fracture pattern characterized by long tensile cracks propagate in a hierarchical manner into unreacted part of the rock is expected when the tectonic stress is hydrostatic or when the maximum principal tectonic stress is normal to fluid-supplying channels. Spalling localizes hydration and fluid flow along the channels whereas branching promotes spatially extensive hydration and fluid flow away from the fluid supply. The results indicate tectonic stresses may guide the hydration distribution in the oceanic lithosphere at mid-ocean ridges and outer rises and in the cold mantle wedge corner in subduction zones.
Through observations in natural rocks, experiments and previous models it is well known that volume increasing hydration reactions, such as serpentinization, lead to fracture nucleation, i.e., reaction-induced fracturing. In their manuscript McElwee et al. bring this process a step forward by investigating how tectonic stresses in various settings influence fracture propagation. Through numerical models they test different stress configurations and find that large fracture networks branching into the surrounding rock form in tensile regimes. To the contrary, in compressional regimes such networks do not form, or only when the reaction is already well advanced, with sever implications on the hydration stage of mid ocean ridges and bending faults. These results are significant and certainly of interest for the community. I only have a few minor comments.
The manuscript is well written and I really enjoyed reading it. Specifically, I acknowledge the detailed discussion on model limitations. All models were run at 1 MPa confining pressure while it is known from experiments that high confining pressures inhibit fracture nucleation. However, I miss a similar discussion on the effect of temperature. We know that the serpentinization rate is sensitive to temperature and maximum reaction rates are reached at 270 – 300 °C. Within the mantle wedge we expect strong temperature gradients, such that reaction rate varies in space as do elastic parameters. In other words, when the reaction is fastest the mechanical behavior may favour visco-elasto-plastic rather than brittle responses to the reaction. At higher temperature, the reaction rate slows down, further supporting non-brittle behavior due to decreased strain rates.
To me it was not clear how the model deals with volume expansion on the scale of individual disks. The mechanical approach explains in detail how the elastic properties change continously from non-reacted to fully reacted disks. The chemical approach explains how fast this transition occurs. However, the serpentinization reaction is strongly volume increasing and hence, the disks are expected to expand. While certain bonds will break and form new fluid pathways, others will ultimately close, which is the often discussed processes of clogging. How exactly is this treated in the model?
Furthermore, the volume change may be slightly dependent on pressure and temperature. Possibly this goes too far for this manuscript, but it might be interesting to test how temperature and pressure will affect the volume change and thus the fracture propagation in various tectonic settings.
Minor comments
Line 10 (and throughout the manuscript): to refer to the process, change “reaction-induced fracture” to “reaction-induced fraturing”.
Line 40: It could be helpful for the reader to have a reference to figure 6 here.
Figure 6: In this figure, the compressional and extensional regimes within the mantle wedge could be labelled/highlighted in order to help the reader.
Line 110: How are the values of Pmin and Pmax determined?