09:45 〜 10:00
[SIT20-10] Coupling between fluid flux and dynamical permeability evolution in the middle-lower crust, an example from Sør Rondane Mountains (SRM), East Antarctica.
キーワード：Fluid flux , Hydration, Fluids activity, Permeability , Reaction zones, Timescales
Fluid flow causes hydration reactions, which induce mass transport, and changes rheology of rocks. Permeability increases due to fluid pressure rise cause rock fracturing and provide fluid infiltration. Such fluid activity is related to earthquake generation, tremors, and slow slip events. It was suggested that H2O released due to dehydration reactions increases fluid pressure, causes fracturing, and induce tremors (e.g., Abers et al., 2009; Katsumata and Kamaya, 2003; Obara et al., 2004). However, quantitative constraints on fluid fluxes and crustal permeability are limited, particularly with regards to its temporal evolution. Therefore, it is important to constrain amount of fluid fluxes to understand roles of fluids in seismic events, permeability evolution, and water-rock interaction in the crust. Here, based on metamorphic fluid-rock reaction zones, we constrain fluid fluxes through the fractured crust by thermodynamic modeling of fluid chemistry and observed fracture geometry, and discuss the relation of fluid flux, their duration, and size of fractures that possibly related to seismic events.
We investigated fluid-rock reaction zones in hydrated metamorphic rocks samples from the Mefjell and Brattnipene Sør Rondane Mountains (SRM), East Antarctica. Amphibolite-facies planar fractures and millimeter-scale hydration zones around them provide unique information on duration and hydraulic parameters of fluid infiltration. Based on reactive-transport analysis of trace elements, in our previous study (Mindaleva et al., 2020) we have investigated timescales of fluid infiltration and estimated permeability evolution. The timescales are constrained to tens of hours. The permeability of the wall rock and fractures were estimated to be 10−20–10−22 and 10−8–10−9 m2, respectively. We used these estimations to calculate fluid fluxes in the reaction zones and through the fracture.
To understand amount of fluids transported through the single fracture, we estimated fluid fluxes through the fractures and through the reaction zones. Fluid fluxes through the reaction zone were estimated by two ways: total amount of H2O added to reaction zone (QRZH2O) and modelled fluid flux required for chlorine transport (QRZCl). Fluid fluxes through the fracture were estimated by two different approaches. First method utilizes permeability of the fracture, pressure gradient, and timescales of the fluid infiltration, constrained for each sample (QFRHyd). Second method is based on the advective reaction-transport equation applied to fluid speciation phase equilibrium modelling results (QFRSi). We also measured seismic moment and magnitude from the fluid flux volumes and provide insights about possible seismic events.
Fluid fluxes through the reaction zone log QRZH2O range -4.2– -3.3 m, flux required for Cl transport log QRZCl is same order, ranges -4.7– -3.5 m. Fluid flux through the fracture log QFRHyd ranges 1.6– 3.8 m, and fluid flux estimated by thermodynamic modeling of fluid chemistry log QFRSi ranges 2.1– 2.9 m, respectively.
Magnitude of the possible seismic even estimated from the fluid fluxes by two different approaches ranges -1– 2.6. First method based on the McGarr,1976 equation suggesting relationship between seismic moment and fluid flux. Second method is based on the fault geometry, we used cross-section area as 5–30 × 5–30 m, and slip displacement ranges 1–0.1 mm. Magnitude estimated by McGarr’s equation ranges 2.0– 2.6, while second approach provides ranges -1– 0. We compared our estimations to the results of fluid injection experiments (e.g., Baisch et al., 2006; Baisch et al., 2009; Häring et al., 2008; Deichmann et al., 2014 ). Moment magnitude estimated by first approach is the maximum magnitude observed in series of fluid injection experiments in the shallow crust. However, first approach is not tested for deep crustal conditions and at very low fluid fluxes. Second approach possibly provides more realistic results for our conditions, incorporating geological observations, low fluid fluxes and middle-lower crustal conditions.
Our results show that much amount of fluids are transported by fractures rather than to be stored in the reaction zones. Such fluid infiltration can possibly induce fracturing and low magnitude seismic events in the crust.
We investigated fluid-rock reaction zones in hydrated metamorphic rocks samples from the Mefjell and Brattnipene Sør Rondane Mountains (SRM), East Antarctica. Amphibolite-facies planar fractures and millimeter-scale hydration zones around them provide unique information on duration and hydraulic parameters of fluid infiltration. Based on reactive-transport analysis of trace elements, in our previous study (Mindaleva et al., 2020) we have investigated timescales of fluid infiltration and estimated permeability evolution. The timescales are constrained to tens of hours. The permeability of the wall rock and fractures were estimated to be 10−20–10−22 and 10−8–10−9 m2, respectively. We used these estimations to calculate fluid fluxes in the reaction zones and through the fracture.
To understand amount of fluids transported through the single fracture, we estimated fluid fluxes through the fractures and through the reaction zones. Fluid fluxes through the reaction zone were estimated by two ways: total amount of H2O added to reaction zone (QRZH2O) and modelled fluid flux required for chlorine transport (QRZCl). Fluid fluxes through the fracture were estimated by two different approaches. First method utilizes permeability of the fracture, pressure gradient, and timescales of the fluid infiltration, constrained for each sample (QFRHyd). Second method is based on the advective reaction-transport equation applied to fluid speciation phase equilibrium modelling results (QFRSi). We also measured seismic moment and magnitude from the fluid flux volumes and provide insights about possible seismic events.
Fluid fluxes through the reaction zone log QRZH2O range -4.2– -3.3 m, flux required for Cl transport log QRZCl is same order, ranges -4.7– -3.5 m. Fluid flux through the fracture log QFRHyd ranges 1.6– 3.8 m, and fluid flux estimated by thermodynamic modeling of fluid chemistry log QFRSi ranges 2.1– 2.9 m, respectively.
Magnitude of the possible seismic even estimated from the fluid fluxes by two different approaches ranges -1– 2.6. First method based on the McGarr,1976 equation suggesting relationship between seismic moment and fluid flux. Second method is based on the fault geometry, we used cross-section area as 5–30 × 5–30 m, and slip displacement ranges 1–0.1 mm. Magnitude estimated by McGarr’s equation ranges 2.0– 2.6, while second approach provides ranges -1– 0. We compared our estimations to the results of fluid injection experiments (e.g., Baisch et al., 2006; Baisch et al., 2009; Häring et al., 2008; Deichmann et al., 2014 ). Moment magnitude estimated by first approach is the maximum magnitude observed in series of fluid injection experiments in the shallow crust. However, first approach is not tested for deep crustal conditions and at very low fluid fluxes. Second approach possibly provides more realistic results for our conditions, incorporating geological observations, low fluid fluxes and middle-lower crustal conditions.
Our results show that much amount of fluids are transported by fractures rather than to be stored in the reaction zones. Such fluid infiltration can possibly induce fracturing and low magnitude seismic events in the crust.