Deep in Earth’s crust, intense heat and pressure can drive chemical changes in rocks known as metamorphic reactions. These reactions can release fluids that flow through tiny pores between the rock grains. Described mathematically, this flow occurs as a sphere-shaped wave that propagates through the hot, viscous rock.
In a new paper, Omlin et al. used 3-D computer modeling to simulate the behavior of reactive porosity waves and compared it with that of viscous porosity waves, another type of fluid transport wave that is relatively well understood. The findings revealed a previously unknown mechanism by which fluids might flow through rock beneath Earth’s surface.
This revelation builds on the researchers’ previous work of developing mathematical equations that describe a system in which metamorphic reactions release volatile substances (like water or carbon dioxide) from porous, viscous rock. The released volatile substances become fluid that fills pores between rock grains. Changes in pressure resulting from the reactions cause the fluid to flow with respect to the solid rock.
The research team used these equations to simulate reactive porosity waves via graphic processing unit (GPU) parallel processing. The 3-D simulations revealed how the waves would behave over time.
A model of reactive porosity wave formation from a large ellipsoid anomaly, displayed for a constant amount of fluid (1.4%) in the system. The color of the top surface is the degree of rock hydration (red for a hydrated rock, blue for a dehydrated rock). The initial ellipsoid distribution of the fluid evolves to form several spherical anomalies (waves). The fluid flow is promoted by rock dehydration at the top of the waves and by rock hydration at the bottom. Credit: Omlin et al. [2017]The scientists found that reactive porosity waves can travel long distances at constant velocity. They also behave similarly to viscous porosity waves (which are triggered by rock deformation, not chemical reactions) in that they increase the porosity—the amount of fluid-filled space between grains—of a rock as they pass through.
However, the research team found, reactive and viscous porosity waves use different mechanisms to travel through rock. A viscous porosity wave propagates as changes in fluid pressure compress and decompress the rock. For a reactive porosity wave, chemical reactions that release volatile substances—not rock compaction—drive pressure and porosity changes.
The researchers also simulated what happens when two reactive porosity waves collide. They found that the spherical waves pass through each other before each recovers its own initial shape. Such behavior is characteristic of a soliton, a type of wave that keeps its shape while traveling at constant velocity. This is the first study to show that chemical reactions, and not just rock deformation, can produce soliton-like porosity waves.
Their findings could provide new insights into the formation of rock veins formed by metamorphic reactions, as well as fluid release from rocks in subduction zones. (Geophysical Research Letters, https://doi.org/10.1002/2017GL074293, 2017)
—Sarah Stanley, Freelance Writer
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