Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.v51c0304b&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #V51C-0304
Mathematics
Logic
1035 Geochronology, 1045 Low-Temperature Geochemistry
Scientific paper
The establishment of a geochronological framework for weathering processes is essential for an understanding of the evolution of the regolith and its dynamics. However, there are few robust answers regarding the absolute age of weathering and its rates. Nowadays, 40Ar/39Ar analysis of Mn-Oxides (cryptomelane) and K-bearing secondary sulphates have provided one of the few generally reliable chronometers (e.g. 1), but is restricted to high-K secondary phases. This work presents a different approach to obtain geochronological information from weathering minerals, namely measurement of 238U-series disequilibria in authigenic Fe oxy/hydroxides. These may be potentially useful recorders of weathering processes as they commonly occur as weathering products and have high affinity towards dissolved uranyl complexes. Furthermore, U-Th fractionation during weathering has been extensively reported [2], effectively resetting the U/230Th geochronometer. LA-MC-ICP-MS facilitates in situ measurement of 238U-series disequilibria in authigenic microcrystalline iron oxy/hydroxides (precipitated between cracks and veins in partially and heavily weathered chlorite-muscovite schist) and pisoliths (ferruginous concretions). Contrary to previous studies [e.g. 3], in situ measurement of 238U-nuclides enables selective analysis or iron oxy/hydroxides phases, minimizes contributions from allogenic phases and, reduces the need of mathematical corrections to obtain the activity ratios for the authigenic phase [4, 5]. The results suggest that supergene iron oxy/hydroxides are good recorders of weathering processes; they precipitate during the early stages of weathering, reflect the U-isotopic composition of the groundwater, appear to act as closed-systems in weathering conservative environments, and behave in a predictable fashion when subjected to intense weathering and leaching conditions. The 230Th-ages of the iron oxy/hydroxides indicate that the timing and intensity of weathering appears to be largely controlled by global climatic changes, suggesting that weathering rates have not been constant during the last 300 ka in Northern Australia. References: 1 P.M. Vasconcelos. Annual Review in Earth and Planetary Sciences 27(1), 183-229, (1999) 2 M. Ivanovich and R.S. Harmon, Uranium-series disequilibrium : applications to earth, marine, and environmental science, xxxiv, 910 pp., Oxford University Press, Oxford, (1992) 3 S.A. Short, R.T. Lowson, J. Ellis and D.M. Price. Geochimica et Cosmochimica Acta 53, 1379-1389, (1989) 4 K.R. Ludwig and D.M. Titterington. Geochimica et Cosmochimica Acta 58(22), 5031-5042, (1994) 5 Luo, S. and T. L. Ku. Geochimica et Cosmochimica Acta 55(2): 555-564. (1991)
Bernal Jose Joaquin
Eggins Stephen
Eggleton R.
Grün Rainer
McCulloch Malcolm
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