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首页> 外文期刊>Geochemical Journal >Transfer of rare earth elements (REE) from manganese oxides to phosphates during early diagenesis in pelagic sediments inferred from REE patterns, X-ray absorption spectroscopy, and chemical leaching method
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Transfer of rare earth elements (REE) from manganese oxides to phosphates during early diagenesis in pelagic sediments inferred from REE patterns, X-ray absorption spectroscopy, and chemical leaching method

机译：从稀土元素模式，X射线吸收光谱法和化学浸出方法推断，成岩早期成岩过程中稀土元素（REE）从锰氧化物向磷酸盐的转移

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摘要

The migration of REEs in pelagic siliceous sediments were studied, especially (i) accumulation of REEs at sea floor to Mn4+ oxides, (ii) release of REEs from Mn4+ oxides accompanied with the reductive dissolution of Mn4+ oxides during early diagenesis, and (iii) incorporation and fixation of REEs released from Mn4+ oxides to phosphates such as apatite below 0.6 meters below sea floor (mbsf). These processes have been indicated by various geochemical findings: (a) chemical compositions of bulk sediment and pore water, (b) REE patterns of bulk sediment, (c) oxidation states of Ce, Mn, and Fe and host phase of Y by XANES, and (d) chemistry of specific phases such as Mn4+ oxides and apatite by means of chemical leaching, LA-ICP-MS, and XANES. The roles of Mn4+ oxides and apatite as host phases of REEs at sea floor and below 0.6 mbsf, respectively, were discussed using the chemical leaching data. Reductive dissolution of Mn4+ oxides and reduction of Ce4+ to Ce3+ with depth were revealed by direct determination of oxidation states of Mn and Ce by XANES. The transfer of REEs released by the reductive dissolution of Mn4+ oxides is strongly suggested by the presence of positive Ce anomalies in apatite at 0.80 mbsf (LA-ICP-MS) and at 1.80 mbsf (chemical leaching), which must be inherited from Mn4+ oxides which can accumulate Ce by oxidizing Ce3+ to Ce4+. This observation shows that apatite fixes the REEs with positive Ce anomaly once dissolved from Mn4+ oxides during early diagenesis. Consequently, we found that total REEs in the two phases (Mn4+ oxides and apatite) are preserved even after diagenetic alteration, because apatite fixes the most of the REEs released from Mn4+ oxides. The results indicate two geochemical implications: (i) REE abundances in apatite in sediment, which has attracted great interests in terms of REE resources, depend on the amount of REEs fixed in Mn (and Fe) oxides initially formed at the sediment surface, and then apatite finally fixes the REEs during early diagenesis; (ii) the reliability of apatite as a proxy of seawater chemistry is affected seriously by the overprint of REE signature by the diagenetic effect. However, if the contribution of REEs in Mn (and Fe) oxide is small, then the REE pattern of apatite can preserve information of the REE pattern of seawater, including the degree of Ce anomaly.
机译：研究了稀土元素在浮游硅质沉积物中的迁移，特别是（i）海底REE在Mn4 +氧化物中的积累，（ii）早成岩过程中Mn4 +氧化物的还原溶解，Mn4 +氧化物的溶解从Mn4 +氧化物中释放出来，（iii）从Mn4 +氧化物释放的REE掺入并固定到磷酸盐（例如海床（mbsf）以下0.6米以下的磷灰石）上。这些过程已通过各种地球化学发现表明：（a）大量沉积物和孔隙水的化学成分，（b）大量沉积物的REE模式，（c）XANES氧化Ce，Mn和Fe的氧化态和Y的主体相（d）通过化学浸出，LA-ICP-MS和XANES对特定相（例如Mn4 +氧化物和磷灰石）进行化学处理。利用化学浸出数据讨论了Mn4 +氧化物和磷灰石分别作为海底和0.6 mbsf以下REE的主体相的作用。通过XANES直接测定Mn和Ce的氧化态，揭示了Mn4 +氧化物的还原溶解和Ce4 +还原为Ce3 +的深度。 Mn4 +氧化物的还原溶解释放的REE的转移强烈地暗示了磷灰石在0.80 mbsf（LA-ICP-MS）和1.80 mbsf（化学浸出）中存在正Ce异常，必须从Mn4 +氧化物继承可以通过将Ce3 +氧化为Ce4 +来积累Ce。该观察结果表明，在早期成岩过程中，磷灰石一旦从Mn4 +氧化物中溶解出来，就会以正Ce异常固定REE。因此，我们发现即使在成岩作用改变后，两相（Mn4 +氧化物和磷灰石）中的总REE也得以保留，因为磷灰石固定了Mn4 +氧化物释放的大部分REE。结果表明了两个地球化学意义：（i）沉积物中磷灰石中的REE含量已引起人们对REE资源的极大兴趣，取决于沉积物中最初形成的Mn（和Fe）氧化物中固定的REE量，以及然后磷灰石最终在早期成岩过程中固定了稀土元素。 （ii）磷灰石作为海水化学替代物的可靠性受成岩作用的REE标志叠印的影响很大。但是，如果稀土元素在锰（和铁）氧化物中的贡献很小，则磷灰石的稀土元素分布图可以保留有关海水稀土元素分布图的信息，包括铈异常程度。

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