Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Authors

  • Alexis Navarre-Sitchler,

    1. Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA
    2. Center for Environmental Kinetics Analysis, Pennsylvania State University, University Park, Pennsylvania, USA
    Search for more papers by this author
  • Carl I. Steefel,

    1. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
    2. Center for Environmental Kinetics Analysis, Pennsylvania State University, University Park, Pennsylvania, USA
    Search for more papers by this author
  • Li Yang,

    1. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
    2. Center for Environmental Kinetics Analysis, Pennsylvania State University, University Park, Pennsylvania, USA
    Search for more papers by this author
  • Liviu Tomutsa,

    1. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
    Search for more papers by this author
  • Susan L. Brantley

    1. Center for Environmental Kinetics Analysis, Pennsylvania State University, University Park, Pennsylvania, USA
    2. Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania, USA
    Search for more papers by this author

Abstract

[1] 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.

Ancillary