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Hydrogen and oxygen isotope systematics in weathering profiles

Lawrence, J. R and Taylor, H. P., Jr. (1972) Hydrogen and oxygen isotope systematics in weathering profiles. Geochimica et Cosmochimica Acta, 36 (12). pp. 1377-1393. ISSN 0016-7037. doi:10.1016/0016-7037(72)90068-3.

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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.

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Additional Information:We are grateful to S. EPSTEIN, L. T. SILVER, H. TOURTELOT and J. W. HOSTERMAN for helpful discussions and for supplying somo of the samples analyzed in this study. This research was supported by the National Science Foundation, Grant GA-12945.
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Caltech Division of Geological and Planetary Sciences2086
Issue or Number:12
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ID Code:116591
Deposited By: Tony Diaz
Deposited On:30 Aug 2022 20:27
Last Modified:31 Aug 2022 17:10

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