摘要：
Weathering of carbonate plays an important role in influencing the budgets of many elements to rivers and oceans. However, the mobilization and re-distribution of major and trace elements during weathering of carbonate and their mechanism, are still a matter of debate. We here report the variation characteristics of major and trace elements, and mineral compositions of saprolites from a weathering profile developed on argillaceous carbonate rocks in northern Guangdong, South China, to investigate this issue. The results indicate that almost all of Sr and Ca were removed from the profile by aqueous solutions during moderate chemical weathering of carbonate, due to the high solubility of calcite. However, other alkaline earth metals, such as Mg, and Ba and alkali metals, are relatively enriched in the upper profile, mainly due to the adsorption of those elements by clay minerals. As for the transition metals, their enrichment was observed in the upper profile, which is likely to have been caused by the adsorption of transition metals by oxides/hydroxyls of Fe and Al. It is worth noting that the UCC-normalized rare-earth element (REE) patterns of saprolites in the profile are characterized by a strong enrichment in HREE (heavy REE) relative to LREE (light REE). Ce anomalies are all negative, and show large variations in the upper profile (0‒1.5 m) withδCe changing from 0.30 to 0.84, but limited variations in the lower profile (> 1.5 m), around 0.8. The mobilization and re-distribution of the REEs in this profile were controlled by the variation of pH values. In the nearly neutral aqueous solution, the REEs tend to form complexes with carbonate ions. As a result, their capacity of transferring is significantly improved. At the sections under basic conditions, the carbonate complexes decompose, and the REEs are deposited and enriched. Given that the HREEs were preferentially combined with carbonate ions compared to the LREEs, the HREEs are likely to be transported more easily. As a result, more HREEs will be enriched in the lower sections than the LREEs, showing a significant HREE-enriched fractionation pattern of this profile.%碳酸盐岩化学风化对于大型水系以及海水的化学组成具有重要的影响。然而,当前对碳酸盐岩化学风化过程中元素的迁移特征及其控制机制的认识尚存一些争议。本文通过报道粤北地区的一个典型碳酸盐岩风化剖面的矿物组成和主元素、微量元素含量的变化特征,试图探讨碳酸盐岩化学风化过程的元素地球化学行为。结果显示,碱土金属Ca、Sr大量迁出剖面,这与碳酸钙的溶解直接相关；而其他碱金属-碱土金属元素则发生了不同程度的富集,可能归因于黏土矿物对这些元素的吸附；过渡金属元素也存在明显的富集现象,可能与Fe、Al的氧化物/氢氧化物对其的吸附有关。平均上地壳(UCC)标准化的稀土元素(REE)图解显示,相对新鲜的母岩,风化产物中的稀土元素发生了显著的富集与分异,同时伴随着Ce的显著负异常(δCe=0.30~0.84)。本剖面上REE的迁移和再分配主要受控于介质pH值的变化：在近中性的条件下,碳酸根离子可以和REE形成稳定的络合物,显著提高了REE迁移能力,并在碱性较强的层位之上富集和再分配。然而,由于重稀土元素(HREE)的原子半径小于轻稀土元素(LREE),造成 HREE 与碳酸根络合物的稳定性显著大于 LREE,最终导致碳酸盐岩风化过程中HREE的富集程度明显超过LREE。
