Formation mechanism of the superhigh concentration of REE and the strong negative Ce anomalies in the carbonate rock weathering profiles in Guizhou Province, China

As well known, the REE concentration is very low in carbonate rocks. However, superhigh concentration beds of REE, where the REE concentration can be up to about 31 000 μg/g, are found at the weathering front of carbonate rock weathering profiles in Guizhou, China. The strong negative Ce anomalies are also found in those REE enrichment beds, whose δCe can be low to 0.007. The phenomena of low background, strong enrichment and strong fractionation of REE have some certain universality in the carbonate rock weathering profiles in Guizhou. According to the results of mass balance calculation, it is considered that carbonate rocks can supply enough REE sources. The soluble existing states of REE in the parent rocks and enrichment beds, which are found by chemical leaching experiments, are in favor of activation and transportation. A sudden change and narrow alkali barrier, which is considered that it is the result of rapid solution of carbonate minerals, is also found according to the variation of the pH values of samples along the weathering profiles. The alkali barrier may be the most important factor that causes the REE enrichment. During the early stage of carbonate rocks weathering, the alkali barrier can preserve the REE effectively, which is released by weathering. And the barrier can also cause the complexes and/or ions of REE that were carried from the upper part of the profile to weathering front in infiltration water to precipitate and adsorb on clay. The strong negative anomalies of Ce in the studied profiles suggest that significant fractionation has been occurred between cerium and other rare earth elements during weathering processes. The soluble Ce3+ is oxidized to Ce4+ and precipitates from solution as very insoluble CeO2. Consequently, the solution shows a negative Ce anomaly. However, there is not a weathering profile or a groundwater sample in weathering profile having such strong negative Ce anomaly as our studied profiles. And this imply that the fractionation between cerium and other rare earth elements by oxidization from Ce3+ to Ce4+ and precipitation from solution can not explain the formation of the strong negative Ce anomaly in the studied profiles entirely. In alkali solution, Ce3+ can be oxidized to Ce4+ easily, and it has great mobilization (compared with other light REE) results from preferential formation of complexes with carbonate ligands, which are released by dissolution of carbonate minerals at weathering front. Therefore, the fractionation between cerium and other REE is not only occurred in the upper part of the profile during the later stage of weathering, but also occurred at weathering front during the early stage of weathering.