『Abstract
The daughter to parent (234U/238U) activity
ratio in natural waters is often out of secular radioactive equilibrium.
The major reason for this disequilibrium is related to the energetic
α-decay of 238U and differential release of 234U
relative to 238U. This disequilibrium originates from
(1) preferential release of more loosely bound 234U
from damaged mineral lattice sites or; (2) direct recoil of 234Th
into surrounding media from near mineral surface boundaries, however,
it is unclear which of the two mechanism is most important in
nature. To better quantify the effects of preferential release
of 234U, two continuous laboratory granite leaching
experiments conducted over 1100 h were performed. The leachates
were characterized by declining U concentrations with time and
(234U/238U) initially greater than unity
(up to 1.15), which changed to below unity during leaching (〜0.95).
The early elevated (234U/238U) suggests
that additional 234U id released into solution by referential
release of 234U from mineral phases. However, the excess
234U constitutes a finite pool of easy leachable 234U
and the (234U/238U) values become lower
than unity when this pool is used up. A model based on first-order
kinetics, dissolution rates and preferential release of 234U
from damaged lattice sites was developed and is able to quantitatively
predict the observed pattern of (234U/238U)
values and U concentrations for the two granite leaching experiments.
Extending the modeling to longer time scales more comparable to
natural systems shows that the production of waters with high
(234U/238U) ratios can be achieved in two
distinct regimes (1) slow weathering where the rate of directly
recoiled 234U near mineral surfaces into waters is
high; (2) fast weathering where the role of incipient chemical
weathering and preferential release of loosely bound 234U
are important. The model is able to explain apparent opposite
correlations between physical erosion rates and (234U/238U)
in waters and it provides a new framework that will be useful
for examining weathering regimes, their timescales and their coupling
with physical erosion.』
1. Introduction
2. Methods
3. Results
4. Discussion
4.1. Mineral dissolution rates
4.2. U concentration in the leachates
4.3. (234U/238U) in the leachates
4.4. Modeling of (234U/238U) ratios and
U concentration during granitic leaching experiments
4.5. 234U-238U disequilibrium in natural
waters
5. Conclusive remarks
Acknowledgments
References