Physics
Scientific paper
Jun 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1249m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 48, Issue 6, pp.1249-1255
Physics
4
Scientific paper
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 232 Th was gradually removed during weathering. Although the whole-rock 228 Th / 232 Th activity ratios were in equilibrium, there were large excesses of 228 Th in the leachable fraction of both parent rock ( 228 Th / 232 Th = 2.06) and partially weathered saprolite ( 228 Th / 232 Th = 3-6.5), due to alpha recoil and release of daughter 228 Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and 228 Th / 232 Th = 1, 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 234 U / 238 U were in equilibrium in both the whole-rock and leachable fractions. 230 Th was deficient relative to 234 U and 226 Ra 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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