High resolution LA-ICP-MS analysis of 238U-234U-230Th

High resolution LA-ICP-MS analysis of 238U-234U-230Th in ferruginous concretions: In search for a geochronological proxy for weathering processes


J.P.Bernal, S.E. Eggins, M.T. McCulloch, R.A. Eggleton1
1CRC-LEME. Department of Geology, ANU

Understanding rates of weathering and pedogenic processes requires a geochronological framework to establish the origin and environmental conditions under which these processes take place. The lack of reliable geochronological information has proved to be the limiting factor in linking weathering and pedogenic processes with changing climatic regimes. Difficulties arise due to the texture and small size of weathering minerals, which makes them difficult to separate by standard physical methods for further analysis.

Pisoliths are concentric concretions of approximately 1 cm in diameter formed during the latter stages of weathering. Ferruginous pisoliths show layering of hematite and goethite (Figure 5) suggesting that these materials have been subjected to different cycles of aridity/humidity, controlled by the local climatic conditions.

Figure 5: Ferruginous pisolith from Ranger Uranium Mine, Northern Territory, Australia. Dark bands are hematite rich layers, light bands are goethite rich layers

To obtain geochronological information from these minerals we have developed a methodology to measure 238U-decay series isotopes (238U-234U-230Th) in-situ using Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). We have developed two differentstandards with constrained U-Th composition which allows to assess the precision and accuracy of the methodology. Furthermore, the use of matrix-matched standards have allowed us to account for matrix effects. Our method enable us to measure 238U decay series with a spatial resolution of 80µm and a precision of ( 6 kyr for samples 100 kyr old and ~300 ppm of U.

Analysis of the pisoliths show (see Figure 6) that the outermost layers are younger (60-140 kyr) than the interior layers (160-200 kyr). The outer layers are systematically lower in 234U/238U activity ratios and two scenarios for this are contemplated:
1) Co-precipitation of the goethite and uranium with different 234U/238U and subsequent closed system behaviour.
2) Leaching of U (with preferential dissolution of 234U) at the margins of the pisolith which would give older apparent ages.
Nevertheless, constrains on the rates of grow can still be calculated using appropriate open-system considerations. Our results indicate that a growth rate of 0.010 to 0.020 µm /y for each analysed pisolith.

 

Figure 6: Activity ratios for core, inner and outer layers for the different pisoliths analysed