Rare earth elements (REE) and other trace element concentrations as well as 143Nd/144Nd and 87Sr/86Sr in saprolites developed on the Cayce metadiabase dike, South Carolina, document extreme REE mobilization during chemical weathering and enrichment of REE on clays. Saprolites display a bimodal distribution in their total REE concentrations (ΣREE) and light-REE (LREE)/heavy-REE (HREE) ratios. Shallower (0–6 m depth), Group I, saprolites have high ΣREE (up to 2633 ppm) with enrichment of LREE > MREE > HREE. Group II saprolites, at >6 m depth, have lower ΣREE (45–67 ppm) and relatively flat LREE/HREE, similar to the unweathered metadiabase. Group I saprolites are more weathered (chemical index of alteration (CIA) values of 87–95), than the Group II saprolites (CIA = 46–88, with most <55). Mass balance calculations using 143Nd/144Nd rules out significant input of dust to the weathering profile, which is consistent with the lack of depth-dependent variation in εNd. Weathering and REE enrichment occurred through a three-stage process. Stage 1 involved regional weathering during which saprolites developed on both the metadiabase dike and Liberty Hill granite country rock. During this stage, breakdown of LREE-rich accessory minerals (e.g., titanite) in the granite released REE and radiogenic Sr to weathering fluids that penetrated the metadiabase where these elements were adsorbed onto clays, consistent with their Nd and Sr isotopic compositions. The kaolinite/smectite (K/S) ratio in Group II saprolites negatively correlates with Sm/Nd and positively with Y/Ho and Rb/Sr ratios indicating preferential adsorption of lighter REE (e.g., Nd), Y, and Rb by kaolinites; no trends are seen in Group I saprolites, suggesting that these samples were overprinted by later events that did not impact the Group II saprolites. Stage 2 involved replacement of smectite veins by siderite in the entire saprolite at high pH and under reducing conditions such as found in swamps, which did not affect the adsorbed REE in the clays. In stage 3, siderite dissolution under acidic and oxidized conditions at the shallowest depths (upper 2 m) led to the formation of Fe3+-smectite and LREE mobilization; this REE-bearing fluid percolated downwards where the REE were adsorbed onto clays to develop a REE-enriched zone locally in the upper 6 m (within the Group I saprolites). Carbonates may have also acted as depositional ligands and induced REE precipitation. This study shows that REE can be extremely mobile during chemical weathering under specific conditions and may be deposited onto secondary clay minerals like kaolinites, which absorb REE, particularly LREE, leading to local enrichments.

中文翻译：

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南卡罗来纳州腐泥石中记录的风化过程中的极端稀土元素 （REE） 富集


在南卡罗来纳州 Cayce 变辉绿岩岩上开发的腐泥岩中的稀土元素 （REE） 和其他微量元素浓度以及 143Nd/144Nd 和 87Sr/86Sr，记录了化学风化过程中 REE 的极端动员和 REE 在粘土上的富集。腐泥石的总 REE 浓度 （ΣREE） 和轻 REE （LREE）/重 REE （HREE） 比率呈双峰分布。较浅（0-6 m 深）的 I 类腐泥石具有高 ΣREE（高达 2633 ppm），并富集了 LREE > MREE > HREE。II 类腐泥岩，深度 >6 m，具有较低的 ΣREE （45–67 ppm） 和相对平坦的 LREE/HREE，类似于未风化的变辉绿岩。I 组腐生岩比 II 组腐生岩（CIA = 46-88，大多数 <55）风化程度更高（蚀变化学指数 （CIA） 值为 87-95）。使用 143Nd/144Nd 进行质量平衡计算排除了大量尘埃进入风化剖面的可能性，这与 εNd 中没有深度依赖性变化的情况一致。风化和 REE 富集通过三个阶段的过程进行。第一阶段涉及区域风化，在此期间，腐泥岩在变辉绿岩岩脉和自由山花岗岩乡村岩石上发育。在此阶段，花岗岩中富含 LREE 的辅助矿物（例如钛矿）的分解将 REE 和放射性 Sr 释放到风化液中，这些流体渗透到变绿岩中，这些元素被吸附在粘土上，与它们的 Nd 和 Sr 同位素组成一致。II 族腐岩中的高岭石/蒙脱石 （K/S） 比值与 Sm/Nd 呈负相关，与 Y/Ho 和 Rb/Sr 比值呈正相关，表明较轻的稀土元素优先吸附（例如、Nd）、Y 和 Rb 由高岭石组成;在 I 组腐生岩中没有观察到趋势，这表明这些样品被后来的事件所覆盖，这些事件没有影响 II 组腐生岩。第 2 阶段涉及在高 pH 值和还原条件下（例如在沼泽中发现）用菱铁矿取代整个腐泥石中的蒙脱石脉，这不会影响粘土中吸附的 REE。在第 3 阶段，菱铁矿在最浅深度（上部 2 m）的酸性和氧化条件下溶解导致 Fe3+-蒙脱石的形成和 LREE 动员;这种含 REE 的流体向下渗透，其中 REE 被吸附到粘土上，在上部 6 m 局部形成一个富含 REE 的区域（在 I 组腐泥岩内）。碳酸盐也可能充当沉积配体并诱导 REE 沉淀。这项研究表明，在特定条件下的化学风化过程中，稀土可以非常灵活地移动，并可能沉积在高岭石等次生粘土矿物上，这些矿物吸收稀土元素，特别是 LREE，导致局部富集。