『Abstract
The potential of 238U-234U-230Th
chronometry for constraining the formation rate of weathering
rinds developed on fresh rocks is assessed by analyzing a weathering
rind on a basaltic clast from a 125 kyr old Costa Rican alluvial
terrace. Eighteen subsamples were collected from one section of
the clast by drilling cores (4 mm in diameter and 5 mm depth)
along two transects staradding the core-rind boundary. Variations
of major and trace element concentrations along the two transects
point out (a) intense loss of alkaline and alkaline-earth elements,
(b) conservative behaviour of elements such as Zr, Hf and Th,
and (c) external input of U into the rind without any evidence
of U loss during basalt weathering. In addition, variations in
U concentrations along the transects show that the main U-Th fractionation
process associated with the weathering of the basaltic clast is
an external input of U (without addition nor loss of Th) in the
basalt rind transition zone, and that, once deposited U is immobile
in the weathering rind. In the frame of this interpretation scheme,
a weathering rate of 0.5±0.2 mm/kyr can be calculated for the
studied clast, which is consistent with geological and isotopic
evidence constraining the depositional ages of the terraces. In
addition, the variations in the (234U/238U)
ratio along the analysed transects as well as the increase in
Sr isotopic ratios within the weathering rind are best explained
by temporal variation of the U activity ratios and Sr isotope
ratios of the soil solutions brought into the rind. This work
highlights how well detailed U-Th chronological studies of weathering
rinds can (1) constrain the formation rates of weathering systems,
and (2) record the time variation of isotopic composition of weathering
fluids.
Keywords: U-series chronometry; weathering rind; weathering rates;
basalt weathering; isotopic record』
1. Introduction
2. Sample location and description
3. Sampling procedure and analytical method
4. Results and interpretations
4.1. Mobility pattern of major and trace elements
4.2. Weathering rates from 238U-234U-230Th
radioactive disequilibria
4.3. Paleo-isotopic record of weathering fluids
5. Conclusion
Acknowledgments
Appendix A. Supplementary data
References