Geochemistry and dynamics of calcrete genesis in semi-arid regions

Abstract

Calcretes are the calcite crusts widely developed at the Earth's surface in semi-arid regions (e.g., Goudie, 1983; Nahon, 1991; Wright and Tucker, 1991 ). Thickness of calcretes is tens of centimeters to a few meters, and increases downslope. Calcretes pseudomorphically replace all underlying silicate rocks. Parent rock structure and bulk volume are preserved. In the lower part of calcrete profiles, the parent rock is first replaced by a hydrous Mg-silicate (palygorskite, sepiolite, etc.), which is in turn partly replaced by calcite; in the upper part, no palygorskite occurs. Authigenic opal or chalcedony also occurs. The genesis of calcretes presents difficult questions: (1) During calcretization all silicate components must be removed and all CaCO3 needs to be imported, implying huge mass transfer. (2) How is a seasonal wet/dry climate essential in calcrete formation? (3) How can opal precipitate while quartz and other silicates disappear? This is puzzling, because quartz is less soluble than amorphous silica. (4) Why does the palygorskite occur only at the bottom of the calcrete? What role does it play in the process? ( 5 ) Why is the replacement pseudomorphic? Here we propose a dynamic model which solves all those problems. The model explicitly takes account of seasonally alternating climate changes. It allows us to specify any desired climate. In the dry season, evaporation concentrates C a 2+, Mg z+ and SiO2 in the groundwater, resulting in precipitation of calcite and palygorskite. Provided that the initial water contains more Mg 2+ than SiO2, continuing evaporation and palygorskite precipitation deplete SiOz(aq ) through the chemical divide mechanism (Hardie and Eugster, 1970). This depletion causes quartz and feldspar to dissolve; the released Si and AI contribute to form more palygorskite. In the wet season, rainwater comes in from the top, palygorskite becomes unstable and dissolves fast. The components released are washed away by rain, though a small portion of the SiO2 (which is probably highly concentrated because of the high palygorskite dissolution rate) may precipitate as opal or chalcedony. Calcite dissolves also, but at the end of each yearly drywet cycle one small increment of CaCO3 is accumulated while another increment of parent silicates is removed. The replacement is pseudomorphic because those increments are small and are practically simultaneous. After many such cycles, a calcrete begins to form, and continuously evolves over a few hundred thousand years. The model consists of a set of nonlinear partial differential equations taking account of mass conservation, diffusion, advection, evaporation, and the kinetics of mineral reactions. The hydrodynamics of unsaturated media is