母質及化育作用對土壤稀土元素含量及分佈的影響

Abstract

稀土元素 (rare earth elements, REEs) 在土壤中的含量及組成，主要由母質岩性決定。不過，個別REEs和土壤黏粒、有機質及氧化鐵結合程度各異，導致化育過程中輕REEs (light REEs, LREEs) 與重REEs (heavy REEs, HREEs) 於土壤剖面出現分佈不一致的分化現象 (fractionation)。然而，母質與土壤性質對REEs的地球化學行為之影響卻鮮少被研究。本論文以臺灣東部一個涵蓋片岩、安山岩質集塊岩、砂頁岩互層、海階沖積物及基性岩五種岩性之岩性土序中的六個土壤剖面，探討化育過程中母質岩性差異如何影響REEs的分化。另外，以臺灣中部涵蓋海拔範圍49 m至2394 m之頁岩海拔梯度剖面的九個土壤剖面，解釋化育程度範圍較為廣泛下的REEs分化。 結果顯示，個別REE之含量最高的為Ce、La和Nd，且剖面間個別REE含量之多寡順序相同。此外，供試剖面之REEs全量 (170–319 mg/kg) 在全球背景範圍內 (10.9–562 mg/kg)，同時略高於REEs於地殼中的平均含量 (169 mg/kg)。REEs特徵曲線與分化指標 (ΣLREEs/ΣHREEs、LaN/YbN及LaN/SmN) 皆指出，化育自富矽母質的剖面傾向富集LREEs，而母質中含量鐵較高的剖面則相對富集HREEs，表示REEs亞族組成比例主要受到母質岩性所影響。在所有剖面中，REEs、黏粒及連二亞硫酸鹽-檸檬酸鹽-碳酸氫鹽 (dithionite-citrate-bicarbonate, DCB) 可萃取鐵 (Fed) 含量均隨著深度增加而上升；然而，GdN/YbN隨著黏粒與Fed上升而降低。另一方面，DCB萃取液中REEs濃度會隨鐵上升而增加，且雷射剝蝕電漿質譜儀和電子微探儀分析亦顯示，相較於土壤基質REEs在鐵結核與黏粒膜中出現明顯的濃縮現象。另一方面，儘管土壤中LREEs之全量及DCB可萃取量均高於HREEs，但HREEs與黏粒及氧化鐵的結合程度較高。另外，母質岩性會影響土壤中REEs亞族組成，不過在化育過程中，黏粒及氧化鐵逐漸主導REEs的進一步分化，造成GdN/YbN隨著土壤風化程度上升而提高。

The composition and concentration of rare earth elements (REEs) in soils are mainly controlled by the lithology of parent materials. Meanwhile, the distinct affinities of REEs with clay minerals, organic matters, and iron (Fe) oxides lead to the fractionation of light REEs (LREEs) and heavy REEs (HREEs) in soil pedons. Yet, the interaction between parent materials and soil properties affecting the pedochemical behaviors of REEs in soils has barely been investigated. Thus, this study elucidated the influence from lithology on REEs concentration and distribution during the pedogenetic processes with six pedons from a lithosequence including schist, andesitic agglomerate, sandstone interstratified with shale alluvium, terrace deposits, and mafic rocks identified in eastern Taiwan. In addition, nine soil profiles derived from shale along an elevation gradient covering an altitudinal gradient from 49 m to 2394 m above sea level in central Taiwan were analyzed to interpret REEs fractionation under wide-ranging pedogenetic processes. The results indicated that Ce, La, and Nd were the most abundant REEs in this study, and the order of individual REE levels was identical in all pedons. Moreover, the ΣREEs (170–319 mg/kg) was within the total REEs level often found in soils worldwide (10.9–562 mg/kg) and higher than that in Earth’s crust (169 mg/kg). The REEs patterns and fractionation proxies (ΣLREEs/ΣHREEs, LaN/YbN, and LaN/SmN) indicated that pedons derived from silicon-enriched parent materials tended to be enriched in LREEs; conversely, pedons derived from Fe-enriched parent materials tended to be enriched in HREEs. In the studied pedons, REEs, clay, and dithionite-citrate-bicarbonate (DCB) extractable Fe (Fed) contents increased with increased soil depth; yet, GdN/YbN ratio decreased with increasing clay and Fed contents. Furthermore, the DCB-extractable REEs contents increased the increasing Fed and the condensations of REEs in Fe nodules and clay coatings compared with the soil matrix were identified by the laser ablation inductively coupled plasma mass spectrometry and the electron probe microanalyzer. Additionally, clay particles and pedogenic Fe oxides exhibited a stronger affinity for HREEs over LREEs, although the total and DCB-extractable amounts of LREEs were higher than that of HREEs. Moreover, the lithological properties of parent materials majorly affected the composition of REEs in soils; yet, during the pedogenetic processes, clay particles and pedogenic Fe oxides gradually dominated the further fractionation of REEs and led to the increased GdN/YbN ratio with the increasing degree of weathering.