Formation and Diagenesis of Weathering Profiles

Abstract

Weathering reactions mainly involve the transformation of feldspars, phyllosilicates, amphiboles, pyroxenes, and volcanic glass to the secondary mineral groups, kandites, illites, smectites, vermiculites, and/or chlorites. Although mineralogical changes are complex, bulk compositional changes to weathering profiles, resulting from chemical weathering, are simple and predictable from kinetic, thermodynamic, and mass balance considerations. Predicted bulk compositional changes are corroborated by studies of Recent weathering profiles developed on a variety of plutonic and volcanic rocks under different climatic regimes. Unlike the mineralogical compositions of profiles, the bulk compositional trends are not noticeably modified by climate; consequently, the simple, predictable bulk compositional trends observed in recent profiles provide a "norm" to which ancient weathering profiles can be compared. Early diagenetic reactions may occur prior to burial of the profile by reaction of groundwaters with secondary weathering products. These often result in abnormally high accumulations of Si, , Ca, and Mg to form clay minerals (smectites) and carbonates. The accumulations may be used as indicators of (paleo-)water tables. Late diagenetic reactions occur during and following burial through reaction of basin waters, trapped seawater, or brines with minerals of profiles. Metasomatism is common and includes production of illites, smectites, and chlorites at the expense of kaolinite and reconstitution of partially degraded feldspars to form potash feldspar and albite. Reaction with seawater (high Na/K and Mg/K) results in Na- and Mg-metasomatism, yielding albite and chlorite at the expense of partially degraded feldspars and clay minerals. In contrast, K-metasomatism of buried weathering profiles is favored around the periphery of subsiding continental sedimentary basins where dilute continental ground waters display low Na/K values. NaCl-rich brines and high temperatures in the deep central parts of basins favor the formation of albite at the expense of K-feldspar.