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| Titel |
Experimental investigation of uranium-series isotope mobility in a basaltic weathering profile |
| VerfasserIn |
Anthony Dosseto, Davide Menozzi, Leslie Kinsley |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250110120
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| Publikation (Nr.) |
 EGU/EGU2015-10093.pdf |
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| Zusammenfassung |
| The measurement of uranium (U)-series isotopes in regolith can be used to determine the
formation rate of weathering profiles. This approach aims at following how the U-series
isotope composition of primary minerals (i.e. those derived from the parent material) vary
during the development of the weathering profile. Nevertheless, regolith samples are a
complex mixture of primary minerals, secondary minerals that are the residue of primary
mineral weathering, secondary minerals that precipitate from pore water, minerals derived
from atmospheric deposition and organic matter. In this study, firstly we aim at isolating
primary minerals and the secondary minerals derived from them, by evaluating a
sequential extraction procedure designed to eliminate carbonates, Fe-Mn oxides
and organic matter. Secondly, we investigate the behaviour of U-series isotopes
during primary mineral dissolution by applying a mild HF/HCl etching solution to
the residues of the sequential extraction. These experiments were performed on
bedrock, saprolite and soil derived from a basaltic weathering profile in south-eastern
Australia.
Results show that up to 50% of U is removed during sequential extraction, suggesting that
(i) there is a large pool of labile U in the bedrock and (ii) secondary phases and organic
matter account for a large fraction of the U budget in bulk saprolite and soil. Sequential
extraction has little impact on the (234U/238U) activity ratio of bedrock and saprolite, whilst it
shows a decrease in soil. This suggests that the pool of U removed from bedrock and
saprolite has a (234U/238U) similar to that of primary minerals; but in the soil, the U
removed (mostly from organic matter) is enriched in 234U. This is expected as
organic matter uptakes U from pore solutions, which are generally enriched in 234U.
During HF/HCl etching, the (234U/238U) of bedrock and saprolite is greater than 1.
Sheng and Kuroda [1] previously proposed that (234U/238U) >1 in rocks could
be explained by the preferential dissolution of a soluble phase, depleted in 234U
relative to 238U as a result of alpha recoil of 234Th into a more resistant phase.
In contrast, (234U/238U) ratios show little variation in the soil, suggesting further
hydrolysis has little effect on the U isotope ratio of residual primary minerals in the
soil.
These results illustrate the complexity of U-series isotope behaviour during chemical
weathering. While sequential extraction is a recommended approach to isolate primary
minerals and follow their isotopic evolution during regolith formation, it also uncovers that
U-series isotope mobility is more diverse than previously postulated.
Reference
[1] Sheng, Z., Kuroda, P., 1986a. Isotopic fractionation of uranium: Extremely high
enrichments of 234U in the acid-residues of a Colorado carnotite. Radiochim. Acta 39,
131-138. |
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