Variations of Sr-Nd-Hf isotopic systematics in basalt during intensive weathering

The Sr-Nd-Hf isotopic compositions of both saprolites and parent rocks of a profile of intensively weathered Neogene basalt 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 Sr-87/Sr-86 system was more seriously affected by weathering processes than other isotope systems, with epsilon(Sr) 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 trend with epsilon(Sr) changing from similar to 50 at 2.2 m to similar to 70 at 0.5 m, accompanying a upward increasing of Sr concentrations, from similar to 10 mu g/g to similar to 25 mu g/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 139 mu g/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 epsilon(Nd) and epsilon(Hf) of the saprolites drift only 0-2.6 and 0-3.7 away from the parent rocks, respectively. The negative drifts of the epsilon(Nd) and epsilon(Hf) are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower epsilon(Nd) and epsilon(Hf). 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 epsilon(Nd) and epsilon(Hf) changes in the profile are mainly resulted from consecutive removal of the Nd and HE Calculation indicates that the Nd-143/Nd-144 and Hf-176/Hf-177 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 Nd-143 and Hf-176 may be preferentially removed from the profile relative to Nd-144 and Hf-177 during intensive chemical weathering, resulting in lower Nd-143/Nd-144 and Hf-176/Hf-177 ratios in saprolites relative to the parent rock, even though details for this process is not known. A positive correlation is observed between the epsilon(Nd) and epsilon(Hf) 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 epsilon(Hf) values at a given epsilon(Nd) value, and the regression line between these epsilon(Nd) and epsilon(Hf) 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 epsilon(Nd) nd epsilon(Hf) correlation. Our results also indicate that caution is needed when using epsilon(Sr), epsilon(Nd), and epsilon(Hf) to trace provenances for sediments and soils. (C) 2009 Elsevier B.V. All rights reserved.