Hydrogen and oxygen isotope systematics in weathering profiles

Abstract

δD and δO18 studies undertaken in detail on seven different soil profiles developed on both igneous and sedimentary parent rocks indicate that the parent rock minerals in these soils do not undergo appreciable oxygen or hydrogen isotopic exchange with meteoric waters in the weathering environment. Profiles formed on igneous rocks include: Elberton—Georgia (kaolinite-gibbsite), Big Sur—California (montmorillonite), and Spokane—Washington (halloysite-nontronite). The soils developed on sedimentary parent rocks are represented by 3 profiles formed on an Upper Cretaceous shale in Montana, South Dakota, and Colorado, and a profile on a Precambrian Belt Series shale from Missoula, Montana. The of quartz, biotite and feldspar exhibit no change from fresh rock to the A-horizons in any of these soils. The bulk and of soils formed on shales containing montmorillonite and illite exhibit little or no change relative to the parent rock shale so long as no significant mineralogical changes have occurred. Clay minerals and hydroxides produced during weathering may attain drastically different isotopic compositions than those of the parent rocks, and these weathering products form essentially in isotopic equilibrium with their local meteoric waters. The pairs kaolinite-H2O, montmorillomte-H2O and gibbsite-H2O display the following approximate isotopic fractionation factors: αkaolinite-H2OOX = 1.026; αkaolinite-H2Ohy= 0.968; αmantmorillonite-H2OOX = 1.025 to 1.028 depending on the Fe-content of the montmorillonite; αgibbsite-H2OOX=1.018 and αgibbsite-H2Ohy = 0.984. Complications arise in the case of halloysite because of isotopic exchange between the interlayer water and hydroxyl. The ratios of halloysite are thus not preserved and give no information concerning the conditions of formation of the halloysite soil.