Chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks

Abstract

Chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample. Generally, on homogeneous parent rocks, weathering indices change systematically with depth. However, the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth. In this paper, we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic, heterogeneous, metasedimentary bedrock from Coweeta Hydrologic Laboratory, North Carolina, USA. Among the weathering indices evaluated here, the Weathering Index of Parker (WIP) is the most appropriate for application to weathering profiles on heterogeneous (and homogeneous) parent rock. Because the WIP includes only the highly mobile alkali and alkaline earth elements in its formulation, it yields values that differ greatly from those of the parent rock. In addition, the WIP allows for aluminum mobility, unlike other weathering indices. These characteristics combine to make the Weathering Index of Parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks. However, the WIP should be applied judiciously, as alkali and alkaline earth metals may be readily depleted during weathering. In addition to reflecting weathering, the Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA) and Vogt's Residual Index (V) may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and/or alteration at the water table. Weathering indices that include iron in their calculations were not included in this study, because they do not distinguish between ferric and ferrous iron. The oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre-weathered metamorphic rock and interaction with groundwater during weathering.