DYNAMIC-MODEL OF THE GENESIS OF CALCRETES REPLACING SILICATE ROCKS INSEMIARID REGIONS

In both pedogenic and groundwater calcretes, calcium carbonate precipi tates in voids, or displacing other grains, or replacing underlying pa rent silicates. Replacement textures are widespread in pedogenic calcr ete. Many calcretes also contain magnesium layer silicates and minor c hert. We present a reaction-transport model that accounts for the gene sis of replacement in calcretes and for their mineralogy. Replacement is difficult to account for geochemically because it requires simultan eous removal of large amounts of silicates and import of also large am ounts of CaCO3. In the model the genesis of replacement is directly re lated to seasonally alternating dry-wet climates and to appropriate gr oundwater (or circulating soil water) compositions. In a dry season, w ater evaporation causes CaCO3 and sepiolite (or attapulgite) to precip itate. If groundwater contains enough Mg2+, sepiolite precipitation by the chemical-divide mechanism depletes SiO2 (aq), resulting in the di ssolution of parent silicates. In the following wet season, sepiolite dissolves fast, and silica and cations are flushed away by rainwater, making room for CaCO3 precipitation in the next dry season. As climate cycles repeat, CaCO3 is accumulated and silicates are removed. The se piolite (or attapulgite, or Mg-smectite) serves as a temporary storage of silica between seasons. If the groundwater contains too little aqu eous Mg then the model predicts growth of calcium carbonate without re moving silicates, thus producing void filling and or displacive textur es instead of replacement. The model consists of a set of nonlinear pa rtial differential equations taking account of mass conservation, disp ersion, advection, rainwater infiltration, evaporation, and the kineti cs of mineral reactions. The hydrodynamics of unsaturated media is app lied 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 obs erved in many calcretes (namely, at the bottom of the profile the pare nt 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 ''direc tly'' replace the parent rock). The model can produce calcrete near th e Earth's surface well above the water table. Low P(CO)2, intensive ev aporation, and long dry seasons all are predicted to produce thicker c alcretes. Calcretes constitute an effective geochemical tool that, via replacement, removes parent silicate rocks and shapes the landscape o f semi-arid countries. The model herein provides a mechanism that acco unts for the efficiency of replacement in removing silicates.