Geochemical mass balances and weathering rates in forested watersheds of the southern Blue Ridge; III, Cation budgets and the weathering rate of amphibole

Abstract

The geochemical mass balance of alkali and alkaline- earth elements in a southern Blue Ridge watershed underlain by amphibolite (Carroll Knob Complex; Watershed 3 of the U.S. Forest Service Coweeta Hydrologic Laboratory, Otto, North Carolina) can be explained by weathering of plagioclase (oligoclase-andesine), horn- blende (ferroan pargasite), biotite, and calcite, and elemental uptake by forest biota. Feldspar and biotite weathering rates (calculated from geochemical mass balance) in the amphibolitic watershed are similar to previously calculated rates for compositionally similar minerals in the surrounding gneissic/schistose watersheds. However, the similarity of feldspar weathering rates despite significant differences in modal abun- dance suggests that feldspar in the Carroll Knob Complex is somewhat less reactive per unit modal abundance than is plagioclase in the surrounding rocks. Textural differences between the Carroll Knob Complex and the surrounding rocks (smaller surface—area—to—volume- ratio of coarser crystals in the Carroll Knob Complex) could be respon- sible. Hornblende and plagioclase weathering rates in Coweeta Water- shed 3 are approximately equal (when normalized for differences in modal abundance of the two minerals), consistent with experimentally determined rates and empirical weathering series.