Negative Ce anomalies in Mn oxides: The role of Ce4+ mobility during water-mineral interaction

We present one of the very rare natural examples of extremely negative Ce anomalies (up to 4 orders of magnitude) in manganese oxides, caused by higher mobility of Ce4+ compared to REE3+ in an aquatic environment. The young secondary Mn oxides formed together with fluorites and goethites during water-mineral interaction in a hydrothermal fluorite vein. Our findings are in contrast to the oxidative scavenging of Ce, which is commonly observed in Mn oxides. Comparison of REE patterns from modern mine waters with primary and secondary minerals demonstrates that this cannot be solely explained as a source-related feature or by immobilization of Ce, but must at least partially be the result of preferential mobilization of Ce4+ compared to REE3+. We propose that this very unusual behavior is due to strong complexation of Ce4+, most likely by siderophores or similar organic molecules, based on published complex formation constants. The presence of dissolved organic carbon (DOC) even in water samples outflowing at the deepest mine level lends support to this model. Recent experimental studies have also demonstrated that negative Ce anomalies develop in biogenic Mn oxides, but this effect has not yet been reported from a natural environment. Our findings emphasize the relevance of experimental results for natural systems and have considerable implications for the assessment of the mobility of tetravalent actinide elements (especially Pu4+) in earth surface environments or potential high-level permanent repositories. (C) 2012 Elsevier Ltd. All rights reserved.