Conversion of low-permeability bedrock to regolith begins with the transport of meteoric water into protolith. Infiltrating meteoric water contains dissolved oxygen and is acidified by CO<sub>2</sub> and organic acids, promoting weathering of primary minerals in the rock. Removal of soluble material during weathering decreases major element concentrations and rock bulk density with a corresponding increase in porosity. Growth of nano- and micro-scale porosity in incipiently weathered crystalline rocks is diagnostic of the very first stages of regolith formation and mode of pore opening may control long-term weathering behavior and regolith thickness.<br/><br/>Here we use neutron scattering, electron microscopy, and detailed chemical analysis to evaluate modes of porosity development in weathered crystalline bedrock of felsic and mafic cmoposition. We use results from these studies to highlight differences 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 in granitic rocks opens pathways to advect meteoric fluid, promoting deeper weathering. In contrast, in mafic rocks that weather without weathering induced fracturing, porosity opens as minerals dissolve connecting pores in tortuous path and 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 where typical felsic minerals dissolve more slowly than typical mafic minerals. 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.

Probing chemical and porosity changes at the bedrock saprolite interface

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.