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