Dating of chemical weathering processes by in situ measurement of U-series disequilibria in supergene Fe-oxy/hydroxides using LA-MC-ICPMS

Constraints on the timing of weathering processes in the northern Australian regolith have been obtained by in situ measurement of U-series disequilibria in U-rich supergene Fe-oxy/hydroxides using a laser ablation-MC-ICPMS technique. This approach has permitted the measurement of U-234/U-238 and Th-210/U-238 activity ratios in finely crystalline Fe-oxy/hydroxides from the Ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes. Th-230/U-238 systematics in the finely crystalline Fe-oxy/hydroxides yield Th-230-ages ranging from 60 to 350 ka. The most reliable Th-230-ages cluster between 122 and 216 ka, suggesting Fe-oxy/hydroxides formation and associated weathering, peaked during the previous 2 interglacial periods. This is supported by the Th-230-ages and isotope composition of pisolith (Fe-oxy/hydroxide pedogenic nodules) cores which are demonstrated to behave as closed systems. The U isotopic composition of the Fe-oxy/hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite. Secondary overprinting is evident in some samples as a large range in U-238/U-238. Our results suggest that weathering intensity varies with global climate cycles and that, together with weathering events dated by 40Ar/39Ar of Mn-oxides elsewhere in northern Australia ([Feng, Y.X and Vasconcelos, P., 2001. Quaternary continental weathering qeochronology by laser-heating 40Ar/39Ar analysis of supergene cryptomelane. Geology, 29(7): 635-638.]), weathering rates in this region are orbitally forced. (c) 2006 Elsevier B.V. All rights reserved. Constraints on the timing of weathering processes in the northern Australian regolith have been obtained by in situ measurement of U-series disequilibria in U-rich supergene Fe-oxy/hydroxides using a laser ablation-MC-ICPMS technique. This approach has permitted the measurement of U-234/U-238 and Th-210/U-238 activity ratios in finely crystalline Fe-oxy/hydroxides from the Ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes. Th-230/U-238 systematics in the finely crystalline Fe-oxy/hydroxides yield Th-230-ages ranging from 60 to 350 ka. The most reliable Th-230-ages cluster between 122 and 216 ka, suggesting Fe-oxy/hydroxides formation and associated weathering, peaked during the previous 2 interglacial periods. This is supported by the Th-230-ages and isotope composition of pisolith (Fe-oxy/hydroxide pedogenic nodules) cores which are demonstrated to behave as closed systems. The U isotopic composition of the Fe-oxy/hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite. Secondary overprinting is evident in some samples as a large range in U-238/U-238. Our results suggest that weathering intensity varies with global climate cycles and that, together with weathering events dated by 40Ar/39Ar of Mn-oxides elsewhere in northern Australia ([Feng, Y.X and Vasconcelos, P., 2001. Quaternary continental weathering qeochronology by laser-heating 40Ar/39Ar analysis of supergene cryptomelane. Geology, 29(7): 635-638.]), weathering rates in this region are orbitally forced. (c) 2006 Elsevier B.V. All rights reserved.