Laboratory Study of Lithologic Controls on Solute Fluxes from Granite Weathering

Lithology is a widely recognized first-order control on chemical weathering rates of landscapes. This may help explain why topography often differs from one rock type to the next: If lithologic differences in weathering rates lead to variations in erosion rates, then they could be expressed in the landforms that characterize the topography. However, the linkages between lithology, chemical weathering, and topography have been hard to quantify from measurements in the field because it is difficult to isolate lithology from other factors that influence chemical weathering rates. To overcome this limitation, we conducted a controlled laboratory study of weathering of granitoid rocks sampled from the Laramie Range, Wyoming. The topography surrounding our sampling sites is bimodal, with gently sloped, deeply weathered surfaces juxtaposed by tall, weathering-resistant tors. Although granitoid bedrock underlies both modes of topography, differences in mafic mineral content are big enough in some areas to explain the bimodality in landscape form, allowing us to explore the possibility of lithologic control on weathering and its implications for topography. We conducted through-flow reactor experiments on three granitoids with mafic mineral content ranging from 7 to 36% by volume. Six reactors were loaded with crushed rock (two replicates each) for gravity-fed, passive flow experiments conducted over six months. We found Ca, K, and SO4 dominate the elemental outputs of weathering measured in effluent from all three granitoids. For most elements, flux from the reactors increases rapidly, peaks at ~30 days, then declines gradually. However solute yield varies in magnitude with overall mafic content; mafic-rich rock produced more solutes than either of the mafic-poor rocks, especially for Ca (5-6 times greater) and SO4 (11-17 times greater). Here we discuss our results and the connections between lithology, chemical weathering, and topography within granitoids of the Laramie Range.