『Abstract
The behaviour of rare earth elements (REEs) and yttrium (Y) during
chemical weathering processes has been investigated on a 4.05
m thick terra rossa profile over dolomite on the Yunnan-Guizhou
Plateau, China. In this profile, ferromanganese concretions and
gibbsite spots coexist in the terra rossa solum. Analyses of REEs,
Y, Zr, F, S, and TOC, as well as mineralogical studies, were conducted
on a suite of ferromanganese concretions, gibbsite spots, terra
rossas, dolomites, and insoluble residues from the underlying
dolomite. These analyses helped us to understand the mobilization,
redistribution, and fractionation of REEs and Y during chemical
weathering. The REEs and Y are mobilized and redistributed during
the terra rossa formation, REEs, except for Ce, are removed from
the upper and middle profiles, transferred downwards, and then
precipitated in the base profile, resulting in remarkable enrichment
in the terra rossa near the weathering front. The significant
increase of pH near the weathering front was responsible for REE
(excluding Ce) and Y enrichment in the base profile. Y is quite
mobile during extreme chemical weathering, and most of it was
carried away from the profile. Because of very efficient oxidative
fractionation of Ce, a significant positive Ce anomaly in terra
rossa was found in the middle of the profile, whereas the terra
rossa near the weathering front exhibited notably negative Ce
anomalies. Moreover, the presence of cerianite and its content
in the ferromanganese concretions can explain the markedly positive
Ce anomaly and the variation of Ce/Ce* values with
depth. In profile, these Ce anomalies are characteristic of the
geochemical environment, especially the redox condition. Studies
of REE distribution in the ferromanganese concretions, gibbsite
spots and surrounding terra rossa revealed that significant REE
fractionation occurred. REEs and Y were preferentially concentrated
in the ferromanganese concretions rather than in the gibbsite
spots. The water-rock interaction resulted in M-type tetrad effects
in some of the ferromanganese concretions, gibbsite spots, and
terra rossa samples. In addition, the tetrad effect in terrestrial
weathering processes likely played an important role in the fractionation
of REEs and Y, such as Y-Ho, Sm-Nd, and the Eu anomaly change.
Keywords: Mass-balance calculation; Tetrad effect; Ce anomaly;
Eu anomaly; Y/Ho ratio; Sm-Nd fractionation』
1. Introduction
2. Materials and methods
2.1. Background information and sample collection
2.2. Analytical methods
2.3. Statistical analysis
3. Results
3.1. mineral compositions of ferromanganese concretions
3.2. X-ray distribution maps and EDS microprobe analysis
3.3. Distribution of REEs and Y in the profile
3.4. PAAS-normalized REE pattern
4. Discussion
4.1. Mobilization and redistribution of REE and Y
4.2. Fractionation among ferromanganese concretions, gibbsite
spots, and surrounding terra rossa
4.3. The lanthanide tetrad effect
4.4. Non-chondritic Y/Ho ratios
4.5. Cause and implications of the Ce anomaly
4.5.1. The positive Ce anomaly in ferromanganese concretions
4.5.2. The negative Ce anomaly in base terra rossa
4.5.3. The Ce anomaly and redox conditions
4.6. Cause of the Eu anomaly
4.7. Sm-Nd fractionation
5. Conclusion
Acknowledgements
Appendix A.
Appendix B.
Appendix C.
References