The Low Permeability of the Earth’s Precambrian Crust

Abstract

The large volume of deep groundwater in the Precambrian crust has only recently been understood to be relatively hydrogeologically isolated from the rest of the hydrologic cycle. Currently, the paucity of permeability measurements in the Precambrian crust is a barrier to modeling fluid flow and solute transport in these low porosity and permeability deep environments. Estimates of permeability from prograde metamorphic rocks and geothermal systems have been applied to such groundwater systems, but, as data are few, it is unclear how appropriate this is for Precambrian crust on a global scale. To resolve this, we apply a new approach to constrain permeabilities for Precambrian crust to depths of 3.3 km based on fluid residence times estimated from noble gas analyses. The additional data reveals there is no statistically significant relationship at depths below 1 km, challenging the previous assumption of a global correlation between permeability and depth. Additionally, we show that estimated permeabilities at depths >1 km are at least an order of magnitude lower than previous estimates and possibly much lower. As a consequence, water and solute fluxes at these depths will be extremely limited, imposing important controls on elemental cycling and the distribution of subsurface microbial life.

Behavior of Amagmatic Orogenic Geothermal Systems: Insights From the Agua Blanca Fault, Baja California, Mexico

“…Severe grain‐size reduction in the core reduces permeability and prevents fluid flow across the fault, but the high fracture density of the surrounding damage zone greatly increases permeability and channels fluid flow along the fault. The relatively high permeability of the ABF, which is subjected to changes in stress through the earthquake cycle may lead to fast siphon‐like upflow and then discharge of thermal waters at the surface, such as demonstrated for numerous amagmatic orogenic systems worldwide (e.g., Bucher et al., 2012; Diamond et al., 2018; Ferguson et al., 2023; Forster & Smith, 1988; Ge et al., 2008; Goderniaux et al., 2013; Hubbert, 1940; Stober et al., 2022; Tóth, 1962; Wanner et al., 2020). Variations in permeability and hydraulic head gradients lead to variable water penetration depths of at least a few kilometers, and hence to variable water equilibration temperatures, residence times, and water upflow velocities.…”

Burial and Denudation Alter Microbial Life at the Bottom of the Hypo‐Critical Zone

“…New and compiled data used in this study are available in the Tables S1-S4 and cited in the Methods section. Measured permeability values are also available on CUAHSI HydroShare (Ferguson & McIntosh, 2023)…”