[SCG63-08] Fluid pressure gradients and permeability evolution in the crust: insights from metamorphic fluid-rock reaction zones and hydrothermal experiments
Keywords:Fluid-rock reactions, Permeability, Fluid pressure, Reaction-induced stress
(a) Fluid pressure and permeability evolution constrained from natural metamorphic fluid-rock reaction zones
Here we show (1) various geologic evidences of fracturing and permeability enhancement in the supercritical, deep crustal conditions where plastic deformation is dominant (upper-greenschist to lower-granulite facies conditions of 200–500 MPa, 450–700°C; Tsuchiya et al., 2016; Uno et al., 2017; Nohara et al., 2019; Mindaleva, et al., under revision). (2) A new, quantitative estimates of crustal fluid pressure gradients and permeability recorded in metamorphic fluid-rock reaction zones, associated with crustal fracturing (Uno et al., 2017; Mindaleva et al., under revision).
These results show that the permeability of intact crust is ~10−20–22 m2 for the granulite– and amphibolite–hosted reaction zones, and are several orders smaller than the widely accepted crustal permeability model (~10−18 m2; e.g., Ingebritsen and Manning, 2010). On the other hand, permeability along the fractures are estimated as high as >10−14–15 m2 for the granulite and amphibolite-hosted fractures, which is analogous to the permeability estimated for the hypocenter migrations in the crust (~10−14–15 m2; e.g., Okada et al., 2014; Nakajima and Uchida, 2018). These results show the importance of low permeability of intact amphibolite/granulite-facies metamorphic rocks in conjunction with episodic high permeability of brittle fractures even at high temperature conditions of 450–700°C, both of which affect the regional scale permeability in the deep crust.
(b) Reaction-enhanced permeability, diffusivity and stress during fluid-rock reactions revealed by hydrothermal experiments
We further show the recent advances in reaction-enhanced transport properties of rocks revealed by hydrothermal experiments including (3) more than 2-orders of permeability enhancement by reaction-induced fracturing during hydration reactions (Uno et al., in prep.), (4) reaction-enhanced diffusion that are ~10 times faster than the static diffusion rate during replacement reactions and (5) role of reaction-induced stress or swelling of clays on the weakening of fault (Kameda et al., 2019).
These experimental results show the importance of fluid-rock reactions controlling the hydro-mechanical properties of the crust. We discuss non-dimensional parameters that controls the permeable/impermeable nature of crust during hydration reactions.
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