Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast
Article first published online: 12 MAY 2009
Copyright 2009 by the American Geophysical Union.
Journal of Geophysical Research: Earth Surface (2003–2012)
Volume 114, Issue F2, June 2009
How to Cite
2009), Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast, J. Geophys. Res., 114, F02016, doi:10.1029/2008JF001060., , , , and (
- Issue published online: 12 MAY 2009
- Article first published online: 12 MAY 2009
- Manuscript Accepted: 26 FEB 2009
- Manuscript Revised: 28 JAN 2009
- Manuscript Received: 30 APR 2008
- effective porosity;
- basalt weathering
 Weathering of rocks as a result of exposure to water and the atmosphere can cause significant changes in their chemistry and porosity. In low-porosity rocks, such as basalts, changes in porosity, resulting from chemical weathering, are likely to modify the rock's effective diffusivity and permeability, affecting the rate of solute transport and thus potentially the rate of overall weathering to the extent that transport is the rate limiting step. Changes in total porosity as a result of mineral dissolution and precipitation have typically been used to calculate effective diffusion coefficients through Archie's law for reactive transport simulations of chemical weathering, but this approach fails to account for unconnected porosity that does not contribute to transport. In this study, we combine synchrotron X-ray microcomputed tomography (μCT) and laboratory and numerical diffusion experiments to examine changes in both total and effective porosity and effective diffusion coefficients across a weathering interface in a weathered basalt clast from Costa Rica. The μCT data indicate that below a critical value of ∼9%, the porosity is largely unconnected in the basalt clast. The μCT data were further used to construct a numerical pore network model to determine upscaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. By using effective porosity as the scaling parameter and accounting for critical porosity, a model is developed that accurately predicts continuum-scale effective diffusivities across the weathering interface of the basalt clast.