SIMS depth profiles of weathered plagioclase and processes affecting dissolved Al and Si in some acidic soil solutions

Abstract

Many natural catchments are now under threat of acidification from natural and anthropogenic sources. Acids introduced to the catchments are neutralized by reaction with soil constituents (including clay minerals and feldspars) and a better understanding of the mechanisms of neutralization is required to evaluate effects of acid deposition in sensitive catchments. The response of feldspars to these acids has been generally studied by monitoring the composition of the weathering solutions. The dissolution of albite and perthite is initially incongruent, with Na- and K-depleted residues produced at the expense of the feldspars^1–4. Such residues can be detected by secondary ion mass spectrometry using the specimen isolation technique provided the residues are greater than ˜40 Å thick. Here we report secondary ion mass spectrometric studies of naturally weathered feldspar surfaces. The acidic soil waters of many catchments have high contents of total dissolved aluminium^5–10, including some from the Plastic Lake Catchment in Ontario, and we consider the effects of feldspar dissolution on Al contents of acidic soil waters.