Rare earth elements (REEs) have gained attention as tracers of pedogenic processes over the last few decades. Clay-sized fractions (CSFs, < 2 μm) may play a crucial role in hosting REEs. To better understand the pedochemical signals of REEs in clay-sized phases, such as iron oxides and phyllosilicates, we analyzed REE speciation in CSFs of carbonate rocks (limestone), clastic rocks (sandstone and shale), and their derived saprolites. Our results quantified the REE content (41.2–144.6 mg kg) and speciation in CSFs of parent materials, revealing that REEs were primarily hosted in amorphous iron oxides (Fe), followed by crystalline iron oxides (Fe) and phyllosilicates. Compared to parent materials, saprolites derived from carbonate rocks exhibited more than a two-fold enrichment of REEs in major clay-sized phases, confirming the role of CSFs as REE sinks during carbonate rock weathering. Furthermore, the initial REE patterns of CSFs of carbonate rocks underwent alteration throughout the weathering process, likely due to water–mineral interactions. Our findings suggest that REEs in CSFs record the weathering fingerprint for soils derived from carbonate rocks, while they are indicative of provenance for soils originating from clastic rocks.

中文翻译：

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粘土大小部分中的稀土元素：对母质到土壤的风化指纹的影响

在过去的几十年里，稀土元素（REE）作为成土过程的示踪剂受到了人们的关注。粘土大小的部分（CSF，< 2 μm）可能在稀土元素的容纳中发挥至关重要的作用。为了更好地了解粘土大小的稀土元素（如氧化铁和层状硅酸盐）的土壤化学信号，我们分析了碳酸盐岩（石灰岩）、碎屑岩（砂岩和页岩）及其衍生腐岩的 CSF 中的稀土元素形态。我们的结果量化了母材 CSF 中的 REE 含量 (41.2–144.6 mg kg) 和形态，表明 REE 主要存在于无定形氧化铁 (Fe) 中，其次是结晶氧化铁 (Fe) 和页硅酸盐。与母质相比，源自碳酸盐岩的腐泥土在主要粘土尺寸相中表现出超过两倍的稀土元素富集，证实了 CSF 在碳酸盐岩风化过程中作为稀土元素汇的作用。此外，碳酸盐岩 CSF 的初始 REE 模式在整个风化过程中发生了变化，这可能是由于水与矿物的相互作用。我们的研究结果表明，CSF 中的稀土元素记录了碳酸盐岩土壤的风化指纹，同时它们也表明了碎屑岩土壤的来源。