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

Abstract

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 234U/ 238U and 230Th/ 238U 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. 230Th/ 238U systematics in the finely crystalline Fe-oxy/hydroxides yield 230Th-ages ranging from 60 to 350 ka. The most reliable 230Th-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 230Th-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 234U/ 238U. 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.