WEATHERING, POROSITY AND EFFECTIVE DIFFUSION COEFFICIENTS: IS ARCHIE'S LAW THE BEST MODEL?

Weathering rinds can be defined as small-scale saprolites that develop on clasts. Developed in a controlled environment and protected from physical weathering by surrounding material, these clasts provide a unique setting to examine chemical weathering processes without complications due to the removal of weathered material by physical erosion.  Weathered basalt clasts have been collected from a chronosequence of alluvial terraces along the Pacific coast of Costa Rica to investigate the processes that control weathering rind advance.  The parent basalt has very low porosity (1-3%) therefore, transport is diffusion controlled as are weathering reactions in the reaction front (~1 mm wide).  The dissolution of basalt at the interface results in the creation of secondary porosity (35-50% in the rind) thereby changing the effective diffusion coefficient.  According to Archie's Law the effective diffusion coefficient (D_e) is related to porosity (φ) by D_e = D_oφ², where D_o is the diffusion coefficient in pure water.  Values of the exponent on porosity can range from 0.33 to 5 for diffusive gas in soils to aqueous diffusion in clays.  Reactive transport models of this system using Archie's Law are unable to predict the thin (~1 mm) reaction front.  In order to better understand how effective diffusion coefficients are changing across the weathering front we use micro-computed tomography data with 4 μm resolution to measure both total and effective porosity.  We define effective porosity as porosity contained in through going pathways across a sample interval.  To address scaling effects we use sample intervals ranging from 100 μm to 250 μm.  Results show that at low total porosity (<15%), effective porosity is 0% but over a range of 18-25% total porosity, the effective porosity increases from 0 to 100% where effective porosity equals total porosity.  An effective diffusion coefficient of 1.8x10^-11 cm² was measured in parent basalt where porosity was 3%.  Numerical modeling to match the diffusion profiles indicates a tortuosity of 1.2x10^-5 in the core.  Additional diffusion experiments are to be performed on altered samples, those results will be presented.  To our knowledge, this study represents the first attempt to determine how diffusion coefficients are affected by changes in porosity due to weathering processes.