Weathering Advance Rates in Basalt: Prediction and Comparison Across Scales

Open Access
Author:
Sitchler, Alexis K
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 25, 2008
Committee Members:
  • Susan Louise Brantley, Committee Chair
  • Eric Kirby, Committee Member
  • Jennifer Macalady, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • chemical weathering
  • basalt
  • porosity
  • diffusion
  • scaling
  • reactive transport modeling
  • weathering rinds
Abstract:
ABSTRACT Saprolite formation is an important contributor of unconsolidated minerals and dissolved ions that impact important environmental and geological processes. However, the rates and mechanisms of saprolite formation are poorly understood. Basalt weathering advance rates at scales from laboratory to watershed were compiled. Four of the 7 orders of magnitude variability in these weathering advance rates can be attributed to the fractal nature of surface area. Fractal dimensions were derived and used to predict weathering advance rates at any spatial scale from weathering advance rates in laboratory experiments. Chemical weathering is studied in weathering rinds formed on basalt clasts from Costa Rica. Conceptually these weathering rinds are small scale saprolites, developed in the absence of erosion, that provide information about the mechanisms and rates of saprolite formation. Chemical and physical characterization of changes associated weathering rind formation were combined with weathering advance rate and methods from chapter 2 to calculate one of the first field basalt weathering rates not influenced by erosion. Porosity changes associated with weathering can induce changes in fluid transport rates and thus potentially the overall chemical reaction rates. Computed tomography data demonstrate increases in total and effective porosity with weathering. A critical porosity of ~12% was observed that must be achieved before increases in effective porosity are measured. Archie’s Law, commonly used to estimate effective diffusion coefficients from total porosity, was modified to incorporate the critical porosity. This modified Archie’s Law predicts measured effective diffusion coefficients better than the original Archie’s Law. The simplest reactive transport model that predicts 4 important features of the weathering system includes the modified Archie’s Law and variable surface area of primary minerals. We hypothesize that the reaction front thickness is related to heterogeneity size in any given system. Reaction front thickness in the Costa Rica basalts is apparently controlled by plagioclase grain size indicating that the largest heterogeneity that influences chemical weathering at the clast scale is grain size. In watersheds, heterogeneities such as fracture spacing, or variations in erosional regime and topography should have greater influence over reaction front thickness.