Evolution of uranogenic and thorogenic lead, 2. Some differences in the variations of the206Pb/204Pb and208Pb/204Pb ratios |
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Authors: | Blagoi G Amov |
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Affiliation: | Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 1184 Sofia Bulgaria |
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Abstract: | On the basis of a dynamic model of a continuous Pb isotope evolution, the variations in the isotopic ratios of uranogenic and thorogenic Pb in a number of young ore deposits, ocean sediments and volcanic rocks from mid-ocean ridges, oceanic islands, island arcs and continents are evaluated. The deviations in the Th/U ratio from its model values are calculated. A model parameterT which indicates a more ancient enrichment in U/Pb (crustal-type Pb sources) or younger enrichment in U/Pb (mantle-type Pb sources) is also calculated. The relative change in the Th/U ratio is close to 1 for mantle-type Pb sources (lowerT values) and decreases for crustal-type Pb sources (higherT values). An interpretation of these changes could be connected with the differences in the U and Th geochemical behaviours in metamorphic processes under the conditions of a deeper (mantle) or crustal source owing to the differences in their valence states. In some cases they have similar (U4+ and Th4+) migration abilities; in others (U6+ and Th4+), considerably different. |
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