Dynamic model of the genesis of calcretes replacing silicate rocks in semi-arid regions

Abstract

In both pedogenic and groundwater calcretes, calcium carbonate precipitates in voids, or displacing other grains, or replacing underlying parent silicates. Replacement textures are widespread in pedogenic calcrete. Many calcretes also contain magnesium layer silicates and minor chert. We present a reaction-transport model that accounts for the genesis of replacement in calcretes and for their mineralogy. Replacement is difficult to account for geochemically because it requires simultaneous removal of large amounts of silicates and import of also large amounts of CaCO 3. In the model the genesis of replacement is directly related to seasonally alternating dry-wet climates and to appropriate groundwater (or circulating soil water) compositions. In a dry season, water evaporation causes CaCO 3 and sepiolite (or attapulgite) to precipitate. If groundwater contains enough Mg 2+, sepiolite precipitation by the chemical-divide mechanism depletes SiO 2(aq), resulting in the dissolution of parent silicates. In the following wet season, sepiolite dissolves fast, and silica and cations are flushed away by rainwater, making room for CaCO 3 precipitation in the next dry season. As climate cycles repeat, CaCO 3 is accumulated and silicates are removed. The sepiolite (or attapulgite, or Mg-smectite) serves as a temporary storage of silica between seasons. If the groundwater contains too little aqueous Mg then the model predicts growth of calcium carbonate without removing silicates, thus producing void filling and or displacive textures instead of replacement. The model consists of a set of nonlinear partial differential equations taking account of mass conservation, dispersion, advection, rainwater infiltration, evaporation, and the kinetics of mineral reactions. The hydrodynamics of unsaturated media is applied in determining water flow in calcrete profiles. Wet/dry seasonal changes are incorporated by alternating the upper boundary conditions. The model successfully produces the mineral and textural zonation observed in many calcretes (namely, at the bottom of the profile the parent rock is first replaced by sepiolite, only part of which is in turn replaced by calcite, whereas at the top of the calcrete the sepiolite is itself completely replaced by the calcite, which appears to “directly” replace the parent rock). The model can produce calcrete near the Earth's surface well above the water table. Low P CO 2 , intensive evaporation, and long dry seasons all are predicted to produce thicker calcretes. Calcretes constitute an effective geochemical tool that, via replacement, removes parent silicate rocks and shapes the landscape of semi-arid countries. The model herein provides a mechanism that accounts for the efficiency of replacement in removing silicates.