Investigations on mineralogy and geochemistry of a black shale profile on the northern Yangtze platform, China: Weathering fate of rare earth elements and yttrium (REY) and its implications

摘要

The mineralogical and geochemical compositions of an 11.5 m black shale weathering profile on the northern Yangtze platform, China, are documented to investigate the fate of rare earth elements and yttrium (REY) and their potential implications during chemical weathering. Our results signify that the profile has experienced an expected weathering trend in initial to moderate weathering stages based on the chemical index of alteration (CIA) values (54.45-75.50). Major elements (Ca, Mg, Na and K) have been leached to various degrees throughout the profile, roughly coupled with the weathering trends of primary minerals. Most REY were released from the black shale matrix, transported downwards and partially precipitated in a fracture. Lanthanum, Ce and Pr exhibited less mobility than other REY, which was connected with the weathering-resistant rhabdophane ((La, Ce)PO4). The REY enrichment in the fracture resulted from the adsorption of Fe-Mn oxyhydroxides and kaolinite. Negligible Ce anomalies (Ce/Ce* values of 0.95-1.12) and prominent negative Eu anomalies (Eu/Eu* values of 0.40-0.77) are observed throughout the profile. The Ce/Ce* values are covariant with the sizes of the first tetrad effect, while Eu anomalies were inherited from the bedrock, and the variation in Eu/Eu* values is related to plagioclase dissolution. LREE/HREE, Sm/Nd and Y/Ho fractionation occurred during black shale weathering. The LREE/HREE fractionation was mainly controlled by primary and secondary minerals (e.g., rhabdophane and kaolinite). The Sm-Nd and Y-Ho fractionation was affected by the tetrad effect. Fracture zones in black shales would be new potential targets for REY prospecting and extraction. Highly REY-enriched zones may occur in soils and underground aquifers, which would pose potential threats to crops and humans. Our findings, combined with previous studies, suggest that LREE/HREE fractionation is indicative of the weathering degree of continental rocks.