Characterization of the reaction and transport properties of porous rhyolite and its application to the quantitative understanding of the chemical weathering rate

The chemical weathering of porous rhyolite from Kozushima, Japan was studied, and the factors controlling the weathering rate, including advection, diffusion and dissolution, were characterized using laboratory experiments. The results were used to analyze the reaction and transport rates, and the relationship between the weathering rates obtained in the laboratory and field was evaluated. A flow-through dissolution experiment using a rhyolite core demonstrated that the dissolution rate R (mol cm−2 s−1) in deionized water depends on time t (days) according to the relationship R = 2.9 × 10−16 t−0.65. The dissolution rate was also dependent on the concentration of dissolved Si according to the relationship R ∝ (1 − Ωn), where Ω = c/ceq (ceq = 230 μmol L−1) and n = 1.0 or 0.33. The hydraulic conductivity and diffusion coefficients were measured under water-saturated and unsaturated conditions. The saturated hydraulic conductivity was 10 times greater than the unsaturated conductivity, and the saturated effective diffusion coefficient of dissolved Si was 5 times greater than the unsaturated diffusion coefficient. Previous studies on the weathering of the rhyolites from Kozushima have shown that the field dissolution rate over 52,000 years was 3 × 10−19-6 × 10−19 mol Si cm−2 s−1. These values are 12-1000 times lower than the dissolution rates obtained in the laboratory over a reaction time of 277 days. Reactive transport analysis demonstrated that the effect of Ω on the dissolution rate is enhanced as the water saturation decreases. The rate-decreasing effect of Ω was large in the early stages of weathering and then gradually decreased with time. The 52,000-year weathering period is inferred to have occurred following a decrease in the dissolution rate to a value close to the “field rate” during an early weathering stage and subsequent further decrease in the rate owing to the effect of Ω.