『Abstract
The content of radiation-induced defects (RIDs) in kaolinite
samples originating from lateritic soils and continental detritic
sediments of the middle Amazon Basin (Brazil) is investigated
using electron paramagnetic resonance. The paleodose registered
by kaolinites ranges from 80 to 900 kGy. Present-day dose rates
of radiation, determined from the whole-rock U and Th content,
range between 4000 and 40,000 mGy/ka. In most samples, U and Th
concentrations are correlated, suggesting that U has not been
remobilized by lateritization. This observation is consistent
with the fact that 〜80％ of the total U content is incorporated
in resistant minerals, such as zircon and Ti oxides. The heterogeneous
distribution of U, observed by induced fission tracks mapping,
makes it possible to neglect the α-radiation contribution of the
U decay chains in the dose-rate calculation. The interpretation
of the measured content of RIDs in kaolinite is then performed
using the calculated present-day dose rate and assuming equilibrium
in the radioactive decay chains. For the sedimentary samples,
the amount of RIDs is broadly correlated to the dose rate ad provides
apparent absolute ages older than 20 Ma. The RID contents in kaolinites
from the lateritic soils provide apparent ages ranging from 10
to 6 Ma. The high RID content of these lateritic kaolinites shows
that their chemical, isotopic, and crystallographic properties
are not representative of present-day weathering conditions. Models
assuming the “dynamical equilibrium” of kaolinites with local
physical-chemical conditions prevailing in lateritic soils are
thus questionable. Alternatively, our findings bring strong support
for the use of the isotopic composition of kaolinites to decipher
continental paleo-climates.』
1. Introduction
2. Geological setting and sampling
2.1. Site I
2.2. Site II
2.3. Site III
3. Experimental
4. Results
4.1. EPR signal of structural Fe3+ and crystallographic
order of kaolinite
4.2. EPR signal of radiation-induced defects and paleodose
4.3. Assessment of dose rate
4.4. Paleodose vs. dose rate relations and age of kaolinites
5. Discussion
5.1. Evidence of past formation of kaolinite in sedimentary
layers
5.2. Inheritance, new formation, and translocation of kaolinites
in soft latosol
5.3. Preservation of kaolinites within iron-rich nodules
5.4. Absolute dating of lateritization processes
6. Conclusions
Acknowledgments
References