Experimental weathering of a volcaniclastic critical zone profile: Key role of colloidal constituents in aqueous geochemical response

Weathering profiles are often complex, extending from more highly transformed materials in the near surface (e.g., mobile soils) to less weathered parent material (e.g., variably porous bedrock) at depth. It is difficult to resolve from field data the impacts of material properties on the short-term rates of mineral weathering when different depths of the profile are reacted with aggressive meteoric waters (i.e., dilute and undersaturated with respect to primary silicates). In the present study, we aimed to measure variation in mineral transformation reactions that occurs under controlled laboratory conditions for samples collected as a function of depth (e.g., spatial distribution of geologic texture, mineral assemblage, and weathering features) across a deep weathering profile in volcaniclastic parent rock. We conducted a series of batch weathering experiments of extracted core materials from two borings to 35 m across a zero-order catchment in the rhyolitic Jemez River Basin Critical Zone Observatory, NM, USA. Upon reaction with aqueous solutions pre-equilibrated with atmospheric CO₂, mineral dissolution was not limited to one phase, but included a combination of reactions including (at decreasing weathering rates) calcite > zeolites > phyllosilicates > amorphous SiO₂ > feldspar. Mineral transformation rates were dependent on the mineral assemblage, texture, and legacy of hydrothermal alteration. Results also indicated an important role of existing and neoformed colloids in Al, Si, and Fe mobilization and redistribution, especially for materials with evidence of previous hydrothermal alteration. Volcanic breccia, which makes up the top 14 m of the western portion of the catchment, was comprised primarily of weathered lithics, where aqueous solution chemistry was controlled by rapid calcite dissolution/precipitation reactions. Hydrothermally altered tuff, which makes up the top 15 m over most of the catchment, exhibited initial dispersion of colloidal zeolites, which subsequently dissolved, giving rise to smectite precipitation (either in-situ and/or along flowpaths). Solute signatures deriving from water/rock interactions in deep, hydrothermally-altered vesicular tuff were comparable to those in shallow altered tuff, but different from those in deep, unaltered, fracture-dominated tuff. We attribute differences to reactive surface area susceptible to chemical attack by aggressive waters (greater in altered rocks) and primary mineral shielding by Fe and Mn oxides on fracture surfaces in unaltered tuff. This study highlights the use of experimental weathering of extracted cores to help interpret field-based, hydrochemistry with an approach that may be employed in other geologically complex terrains.