An understanding of the rates and mechanisms of feldspar dissolution is critical to accurately model the role of silicate weathering in the biogeochemical cycling of H, Ca, Mg, Na, K, Si, and Al. We are proposing an interagency, interdisciplinary project to study the mechanisms of plagioclase dissolution reactions occurring under natural and laboratory conditions. The objectives are to (i) determine the composition of surface layers of naturally and laboratory weathered plagioclase grains from the same parent material using a combination of surface sensitive techniques including X-ray photoelectron and Auger electron spectroscopy (XPS, AES), and (ii) compare dissolution rates of plagioclase determined under laboratory conditions to rates calculated from chemical denudation in watersheds of the Silver Creek study area in the southwestern Idaho batholith. We will examine the spatial variability of cation depletion and preferential release of cations from calcic or sodic rich zones, and relate these surface processes to proposed hypotheses for the stoichiometry of plagioclase weathering in well characterized watersheds of the Silver Creek study area. Results from this study will contribute to our understanding of the extent and lateral distribution of altered surface layers and their role in controlling rates of plagioclase dissolution. Information obtained in this project will provide a framework for relating surface processes to measurements of mass balance and chemical denudation made at the watershed scale.
