POROSITY AND DEEP WEATHERING OF CRYSTALLINE ROCKS

The conversion of low-permeability bedrock to regolith generally begins due to the transport of meteoric water into protolith. Infiltrating meteoric water contains dissolved oxygen and is acidified by CO₂ and organic acids, promoting weathering of primary minerals in the rock. Removal of soluble material during weathering decreases the concentrations of major elements such as Ca, Na, and Mg and the overall mass of the solid, decreasing the bulk density and increasing porosity. Growth of nano- and micro-scale porosity in incipiently weathered crystalline rocks is diagnostic of the very first stages of regolith formation. As weathering increases porosity, more water can infiltrate the rock, leading to more weathering in a positive feedback loop that drives long-term regolith production, and in some cases very deep regolith.

Here we present results from a compilation of studies that quantify porosity in weathering profiles. With this data we evaluate modes of porosity development in weathered crystalline bedrock. These data point to an important difference in systems where weathering induced fracturing opens fracture porosity and allows for deeper transport of water compared to systems where porosity development occurs at the grain scale through mineral dissolution only. Weathering induced fracturing observed in granitic rocks opens pathway to advect meteoric fluid, promoting deeper weathering. In contrast, in studied mafic rocks that weather without weathering induced fracturing, solute transport is dominantly by diffusion leading to less overall weathering and thinner regolith. Thus, the presence or absence of fracturing during weathering may explain thicker regolith on granitic rocks compared to mafic rocks under similar weathering conditions: an observation that is opposite of what would be predicted from mineral dissolution kinetics. A full understanding of regolith formation will only be possible when we can quantitatively describe the changes in porosity and surface area at the pore scale that occur during weathering, especially deep in the Critical Zone.