『Abstract
The Sr-Nd-Hf isotopic compositions of both saprolites and parent
rocks of a profile of intensively weathered Neogene basalts in
Hainan, South China are reported in this paper to investigate
changes of isotopic systematics with high masses. The results
indicate that all these isotopic systematics show significant
changes in saprolites compared to those in corresponding parent
rocks. The 87Sr/86Sr system was more seriously
affected by weathering processes than other isotope systems, with
εSr drifts 30 to 70 away from those of the
parent rocks. In the upper profile (>2.2 m), the Sr isotopes of
the saprolites show an upward increasing of Sr concentrations,
from 〜10μg/g to 〜25μg/g. As nearly all the Sr of the parent rock
has been removed during intensive weathering in this profile,
the upward increasing of Sr concentrations in the upper profile
suggests import of extraneous Sr. Rainwater in this region, which
enriches in Sr (up to 139μg/L) from seawater, may be the important
extraneous source. Thus, the Sr isotopes of the saprolites in
the upper profile may be mainly influenced by import of extraneous
materials, and the Sr isotopic characteristics may not be retained.
In contrast, the εNd and εHf
of the saprolites drift only 0-2.6 and 0-3.7 away from the parent
rocks, respectively. The negative drifts of the εNd
and εHf are coupled with Nd and Hf losses
in the saprolites; i.e., larger proportions of Nd and Hf loss
correspond to lower εNd and εHf.
Compared with the relative high Nd and Hf concentrations of the
saprolites, the contributions of extraneous Nd and Hf both from
wet and dry deposits of aeolian input are negligible. Thus, the
εNd and εHf changes
in the profile are mainly resulted from consecutive removal of
the Nd and Hf. Calculation indicates that the 143Nd/144Nd
and 176Hf/177Hf ratios in saprolites are
all significantly lower than their initial values in the parent
rock. Simply removing part of the Nd and Hf by incongruent decomposing
some of the minerals may not account for this. Fractionation should
be happen, which 143Nd and 176Hf may be
preferentially removed from the profile relative to 144Nd
and 177Hf during intensive chemical weathering, resulting
in lower 143Nd/144Nd and 176Hf/177Hf
ratios in saprolites relative to the parent rock, even though
details for this process is not known. A positive correlation
is observed between the εNd and εHf
of the saprolites. Interestingly, the saprolites with a net loss
of Nd and Hf in the upper profile show good positive correlation,
and the regression line parallels the terrestrial array. By contrast,
saprolites with a net gain of Nd and Hf in the lower profile generally
show higher εHf values at a given εNd value, and the regression line between these
εNd and εHf appears
to parallel the seawater array. This supports the hypothesis that
the contribution of continental Hf from chemical weathering release
is the key to the obliquity of the seawater array away from the
terrestrial array of the global εNd and εHf correlation. Our results also indicate that
caution is needed when using εSr,εNd,
and εHf to trace provenances for sediments
and soils.
Keywords: saprolites; Sr-Nd-Hf isotope; Intensive weathering;
South China』
1. Introduction
2. Materials and analytical methods
3. Results
4. Discussion
4.1. Sr-Nd-Hf isotope variations during chemical weathering
4.1.1. Variations of Sr and Sr isotopic systematics
4.1.2. Variations of the Nd and Hf isotopes
4.2. Correlations between Nd and Hf isotopes
4.3. Implications for Sr, Nd, and Hf isotopes as sediment provenance
proxies
5. Summary
Acknowledgements
References