『Abstract
Major, trace and organic elements of a laterite profile developed
on Neogene basalts in northern Hainan Island, South China were
reported in this paper, the aim of which was to investigate element
mobilization and re-distribution during extreme weathering. The
results indicate that most of the elements have been mobilized
and transferred downwards along the profile by aqueous solution.
Organic matter (OM) can significantly improve the transport of
insoluble elements. Among all the elements, Th is the least mobile.
As for the general conservative elements during incipient chemical
weathering, such as Fe, Ti, Zr, Hf, Nb and Ta, the removals are
up to 20-40% in the upper profile. However, these elements behave
as conservatively as Th in the lower profile. In the middle profile,
oxic environment occurs, accompanied with significant OM decomposition.
The Mn and Ce transferred downward are readily oxidized into insoluble
Mn(IV) and Ce(IV) and precipitate in the oxic front. Important
OM decomposition decreases the capacity of transfer of insoluble
elements in aqueous solution. Consequently, Al significantly precipitates
in the oxic front, and REEs, with the exception of Ce, precipitate
largely in the OM-depleted layers. Co and U are also concentrated
in the oxic front in association with Mn and Ce, respectively.
However, Cr shows a negative correlation with Mn because its response
to redox condition changes is reversed from that of Mn. Mn oxides/hydroxides,
Fe oxides/hydroxides and secondary phosphate minerals other than
clay minerals are potential hosts for REEs except for Ce in the
profile; REEs with high concentrations in the profile seem closely
associated with Mn oxides/hydroxides. Remarkable, highly correlated,
Ce and Gd anomalies are observed in the profile. Ce anomalies
are caused by Ce precipitation in the oxic environment and successive
decomposition of organic matter. Gd anomalies are likely to have
resulted from lower stability constants of Gd-OM complexes compared
to those of neighboring REEs. The overall elemental behaviors
in this profile suggest that organic matter plays a very important
role in the mobilization and re-distribution of the elements during
extreme weathering.』
1. Introduction
2. Geological setting, weathering profile and sampling
3. Analytical methods
4. Results
5. Discussion: Mobilization and re-distribution of elements
5.1. Conservative elements
5.2. Al, Mn, Co, Ce, U and Cr
5.3. Rare earth elements (REEs)
5.3.1. Re-distribution of REEs
5.3.2. Fractionation of REEs
6. Summary
Acknowledgments
References