Weathering assessment through geochemical modeling: A quantitative approach in Eastern Sierras Pampeanas (Northwest Argentina)

Abstract

The Sierra de Velasco (Eastern Sierras Pampeanas, Argentina) offers an ideal setting to investigate chemical weathering in granitic semi-arid catchments characterized by a “weathering-limited” erosional regime. Geochemical proxies and inverse modeling techniques were applied to quantitatively assess weathering processes through both solid materials and dissolved load of rivers. Chemical Index of Alteration (CIA) values ranging from ∼53 to ∼60 reflected incipient surface weathering in the exposed granitic rocks. The composition of dissolved phases indicated silicate alteration mainly controls the hydrochemistry of river waters. Inverse modeling results showed that the incongruent hydrolysis of plagioclase and micas are the dominant weathering reactions, accounting for over 30 % of the total mass transfer between solid and aqueous phases. Kaolinite and illite were identified as the main weathering products. Elevated concentrations of dissolved F− were controlled by the dissolution of F-rich biotite (∼2 % of the mass transfer) with minor contributions from fluorite and fluorapatite (<1 %). CO2(g) consumption represented approximately 35 % of the total transferred phases by weathering reactions. These findings may be used in weathering assessments of other granitic catchments in mid-latitude and extra-tropical regions. Last, these results highlight the role of weathering processes in CO2 sequestration, even where chemical alteration remains at an early stage.