CLUES TO CRITICAL ZONE EVOLUTION FROM MINERAL SURFACE CHEMISTRY: AN NMR STUDY OF FELDSPARS FROM TWO SOIL CHRONOSEQUENCES

Variability in mineral dissolution rates through time constitutes a significant control on the linkage between erosion and chemical weathering in the Critical Zone (CZ). Changes to surface chemistry of mineral grains—as a function of duration in the CZ—is a pertinent issue, because the interplay between grain surface evolution and weathering rates is still not well understood. Addressing this problem will require integrating macro-scale field investigation with rigorous laboratory analysis. Combining a recently developed NMR experiment with elemental and surface area analysis of feldspars, this work aims to coax out insights on the evolution of silicate mineral grains as they reside in a weathering profile. Samples were taken from members of glacial chronosequences along Bishop Creek in the Sierra Nevada Mountains, CA and in the Colorado Front Range. Feldspar mineral grains were separated from the aggregate of each sample and treated with (3,3,3 trifluoropropyl) dimethylchlorosilane (TFS), a probe molecule that selectively binds to lone Q³ hydroxyl groups on the mineral surface. One pulse ¹⁹F MAS NMR experiments on each sample indicated fluorine spin quantities associated with TFS. These were correlated with original hydroxyl site concentrations. Treated and analyzed samples were subsequently dissolved and run through an MP-OES instrument for elemental analysis, to determine whether or not time-dependent mineralogical evolution was also at play. Results of the NMR experiments show a positive correlation between Q³ hydroxyl site count and exposure age. Elemental analysis by MP-OES confirms that mineralogical differences among samples were not significant enough to overshadow the trend. Coupled with studies of cation flux in weathering systems, this work suggests a possible connection between Q³ hydroxyl development and non stoichiometric cation release at the mineral surface. This correlation may aid our understanding of observed second order rates of mineral dissolution in nature.