『Abstract
　Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9％, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity (Navarre-Sitchler et al., 2009). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9％) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.』

1. Introduction
2. Weathering age
3. Methods
　3.1. Electron microprobe analyses
　3.2. Mineralogical changes
　3.3. Chemistry of reacting fluids
　3.4. Modeling approach
　　3.4.1. Reactive transport modeling
　　3.4.2. Effective diffusion coefficients
　　3.4.3. Mineral reaction rates
4. Results
　4.1. Weathering rind thickness in Qt3
　4.2. Electron microprobe analyses
　4.3. Mineralogical changes
　4.4. Chemistry of reacting fluids
5. Discussion
　5.1. Features of the weathering rinds
　5.2. Model development
　　5.2.1. Initial conditions
　　5.2.2. Secondary minerals
6. Model results
　6.1. Scenario 1 - uncoupled model with no update of porosity
　6.2. Scenario 2 - update of porosity using unmodified Archie's Law
　6.3. Scenario 3 - adjustment of accessible mineral surface area with unmodified Archie's Law
　6.4. Scenario 4 - incorporation of a diffusion threshold model based on critical porosity
7. Model implications
　7.1. Reaction front thickness
　7.2. Rind advance rate
8. Implications for weathering studies across scales
9. Conclusions
Acknowledgments
Appendix A. Supplementary data
References