喀斯特河流溶解态稀土元素组成变化及其控制因素

CONTROLLING FACTORS FOR VARIATION IN DISSOLVED RARE-EARTH ELEMENTS IN KARST DRAINAGE BASIN

以化学萃取-反萃取分离法为手段,结合等离子质谱分析仪测量了喀斯特地区乌江、沅江两大水系的枯水期河流的溶解态稀土元素含量。该两河流的化学组成代表了流经碳酸盐岩地层的河水的化学组成。与世界其它地区低pH的河水不同,喀斯特地区河水溶解态稀土元素含量较低,页岩标准化配分模式并不平坦,轻、重稀土元素分异因河流的不同而不同,乌江水系上游轻稀土元素(LREE)相对重稀土元素(HREE)富集,中下游HREE相对于LREE富集,沅江水系河水HREE相对于LREE富集,页岩标准化配分模式具明显的Ce、Eu负异常。乌江、沅江及其支流有高的溶解盐,含有较高的溶质浓度,河水水化学(高碱度、高离子含量、高pH值)和水/粒相互作用控制了喀斯特河水中溶解态稀土元素的含量和轻、重稀土元素的分异。

Concentrations of dissolved rare-earth elements (REE) in two major rivers (the Wujiang river and the Yuanjiang river) in karst-dominated region are measured by means of solvent extraction and back-extraction and subsequent measurements on ICP-MS during winter. The rivers and their tributaries have very low dissolved REE concentration, which are lower than those in most large rivers in the world. The high pH and high ion concentration in the rivers are the most important factors controlling the concentration of dissolved REE in the river water. The shale (PASS)-normalized REE patterns for the dissolved loads are shaped with from light REE-enriched to heavy REE-enriched features. The light REE is more enriched than the heavy REE in the upper reaches of the Wujiang, while the heavy REE is more enriched than the light REE in the middle and lower reaches. The heavy REE is more enriched than the light REE in the Yuanjiang. A number of river water samples assume the shale-normalized REE patterns with negative Ce and Eu anomaly. The fractionation between heavy and light REE can be ascribed to several factors, such as source, water chemistry and water/particle interaction, among which the water/particle interaction might have played an important role.