Climatic and lithological controls on the structure and thickness of granitic weathering zones

Abstract

Weathering of granitic rock near Earth's surface is frequently initiated by oxidative dissolution of FeO in biotite through reaction with atmospheric gases and meteoric water. This oxidation is accompanied by a volumetric expansion that increases the elastic strain energy density in the rock, leading to matrix cracking that may increase water infiltration into the rock that further drives mineral dissolution. In this contribution, we use a coupled non-dimensional, 1-D reactive-transport and fracture-mechanics model to predict how varying biotite abundance and water velocity influence granitic weathering-zone structure and thickness. Weathering-zone thickness and the volume of rock within the weathering zone that undergoes oxidative dissolution increase with water velocity but decrease with biotite abundance. The extent of matrix cracking within the weathering zone mirrors the interplay between water velocity and biotite abundance but is also influenced by crystal size. Matrix cracking potentially extends to greater depths and further into the altered rock with increasing water velocity and, in coarse-crystal granitoids, with decreasing biotite abundance. Fine-crystal granitoids require higher initial biotite abundances to undergo matrix cracking but resulting weathering zones are predicted to be thinner and display a lower volume of matrix-cracked granite. Our model predicts that thicknesses and structures of granitic weathering zones, which are observed to vary from the local to global scale, are influenced by the interplay between biotite abundance, crystal size, and water velocity. The sensitivity of weathering to local variations in these factors implies that predicting details of critical zone structure may be limited by the availability of state information about the weathering rock mass, even if integrated weathering fluxes can be simply estimated.