Weathering of plagioclase across variable flow and solute transport regimes

Abstract

The study area is situated in a fault zone with fractured granites and metasediments. In a conceptual model, infiltrating water first passes the bedrock cover of soil and saprolite and then partly enters the fractures. Weathering reactions of minerals occur in small pores and fissures in the bedrock cover zone to continue in the larger fractures. Pumping tests were carried out in a number of boreholes to measure the drawdown as a function of pumping time. From the results, values of transmissivity (T) could be derived. In combination with the storage coefficient (S) for similar fault zones, the hydraulic diffusivity (D = T/S) could be computed.

Water samples, collected from the boreholes, represent fluid packets with a history of weathering reactions in the bedrock cover and in the larger fractures. The major element composition of these samples was used by means of the SiB mass balance algorithm (Pacheco and Van der Weijden, 1996) to calculate the moles L−1 of dissolved plagioclase (oligoclase with An ≈ 0.20) and the moles L−1 of secondary phases (gibbsite, halloysite, smectite) precipitated along the flow paths of the samples. These results were then used to calculate the net dissolved silica concentrations () related to dissolution of plagioclase followed by precipitation of each of the secondary phases. An interpretation of a plot of each of these ’s versusD is that at D < 0.7 m2 s−1, dissolution of plagioclase is followed by precipitation of halloysite in the large fractures of the fault zone (open system), whereas at D ⩾ 0.7 m2 s−1 precipitation of both halloysite and smectite occurs in the rock matrix with small fissures and pores (semi-open system). Before being pumped, the percolating fluids travelled 0.01–13.7 years. During these periods, plagioclase weathered at rates (WPl) of 10−(12.9±1.1) moles m−2 s−1, which are approximately 2.2 orders of magnitude higher than solid-state weathering rates reported in various field studies. In this study, it is suggested that part of the apparent discrepancy between the results is due to changes in hydraulic diffusivity of the weathering environments occurring over the geologic times.