Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite |
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| Authors: | Jacqueline Michel |
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| Institution: | Research Planning Institute, Inc., 925 Gervais Street, Columbia, South Carolina, U.S.A. 29201 |
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| Abstract: | Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total 232Th was gradually removed during weathering. Although the whole-rock activity ratios were in equilibrium, there were large excesses of 228Th in the leachable fraction of both parent rock () and partially weathered saprolite (), due to alpha recoil and release of daughter 228Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and , indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and were in equilibrium in both the whole-rock and leachable fractions. 230Th was deficient relative to 234U and 226Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction. |
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