Mobility of rare earth elements in the system soils–plants–groundwaters: a case study of an arid area (Oman)

Groundwater samples from six wells and various species of plants from soils developed on ophiolites were collected from an arid area (AlKhod area, Oman) and analyzed for trace elements including rare earth elements (REEs). The distribution patterns of REEs in plants indicated an enrichment in middle REEs (MREEs?=?Sm to Dy) and heavy REEs (HREEs?=?Ho to Lu), when they are normalized to the REE composition of the Post Archean Australian Shale (PAAS), with a significant negative anomaly in Ce and a positive anomaly in Eu. Compared to Oman ophiolites, the REEs in different species of plants are depleted in Ce and enriched in MREEs and slightly enriched in light REE (LREE?=?from La to Nd). Relative to PAAS, the distribution of REEs in groundwaters revealed similar patterns to the REE distribution in plants. The distribution patterns of REEs in plants relative to those in waters are nearly flat. These patterns suggest that the transfer of REEs from soil solutions to the groundwaters in Oman occurs without any significant fractionation.     

Fractionation characteristics of rare earth elements (REEs) linked with secondary Fe,Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements (REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs (La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs (LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs (HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs. The substantial fractions of REEs in soils extracted by oxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile (10 %–30 %), which were similar to the mass fraction of Fe (10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.     

Fractionation of rare earth elements in refractory inclusions from the Ningqiang meteorite: Origin of positive anomalies in Ce, Eu, and Yb

Ion microprobe analyses of rare earth elements (REEs), Ba, and Hf were performed for various types of refractory inclusions including amoeboid olivine aggregates (AOAs) from the Ningqiang ungrouped carbonaceous chondrite to search for possible relationships between REE abundance patterns and bulk chemical compositions of the inclusions. Four types of CI-normalized REE patterns were recognized: (1) nearly flat (unfractionated) pattern with or without Eu (and Yb) anomalies (Groups I, III, or V), (2) depletions of ultrarefractory heavy REEs (HREEs) relative to light REEs (LREEs), and depletions of Eu and Yb (Group II, but without depletion of Yb in some cases), (3) depletions of ultrarefractory HREEs with positive anomalies in Ce, (Eu), and Yb (Modified Group II), and (4) nearly flat pattern with positive anomalies in Ce, (Eu), and Yb (Modified Group I). No systematic correlation was found between bulk chemical compositions and REE patterns of the inclusions. This suggests that the observed REE fractionations occurred prior to condensation of major elements (e.g., Mg and Si) which defined bulk chemical compositions of the inclusions. It is remarkable that 7 out of 19 inclusions show positive anomalies in Ce, Yb, and in some cases, Eu as well (Modified Group I and Modified Group II), suggesting that such anomalies are rather common among inclusions in the Ningqiang and possibly in other primitive meteorites. Two possible mechanisms are considered for the formation of Modified Group II and Modified Group I patterns. In Model 1, Modified Group II is formed by a process similar to that produced Group II but removal of ultrarefractory dust occurred at slightly lower temperatures, where not only ultrarefractory HREEs but some fraction of LREEs had been condensed and removed from the system. Modified Group I may be explained by addition of an unfractionated component to the Modified Group II component, or alternatively, by partial removal of ultrarefractory dust from the system. In Model 2, Modified Group II is formed by later addition of Ce, (Eu), and Yb onto fine-grained dust or inclusions having HREE-depleted, Group II-like REE patterns. Similarly, Modified Group I is explained by later addition of Ce, (Eu), and Yb onto those with almost unfractionated REE patterns. The observed REE data show that both the degree of HREE-depletion (e.g., Er-depletion) and that of fractionation among HREEs (e.g., depletion in the Er/Gd ratio) for Modified Group II are very similar to those for Group II. Model 1 predicts almost complete removal of ultrarefractory HREEs from the system, resulting in much higher HREE-depletion for Modified Group II, which is not consistent with the present observations. Addition of an unfractionated component may explain moderate depletion of HREEs in Modified Group II, but it will diminish fractionation among HREEs, which is not consistent with the present observations. In contrast, Model 2 predicts no correlations between Ce-(Eu)-Yb-enrichment and HREE-depletion or between Ce-(Eu)-Yb-enrichment and fractionation among HREEs, consistent with the present observations. Hence, Model 2 seems more likely. If this is the case, at least two distinct regions with different REE characteristics are required for the formation of Modified Group II inclusions: one is a high temperature region where Group II-like (HREE-depleted) inclusions or their precursors are formed by condensation from a fractionated gas after removal of ultrarefractory dust, and another is a low temperature region enriched in Ce, Eu, and Yb in the gas phase. Abundant occurrence of positive Ce-(Eu)-Yb anomalies suggests that migration of solid materials from one region to another occurs rather frequently in the solar nebula. The most likely place satisfying such conditions for the formation of these inclusions may be the innermost part of the protoplanetary disk.     

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH₃COO⁻) or strong (i.e., ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).     

大同盆地高砷地下水稀土元素特征及其指示意义

对大同盆地典型高砷地下水开展了稀土元素地球化学研究.研究表明: 高砷地下水具有低∑REE含量及富集重稀土(HREEs)特征.地下水中低含量∑REE与含水层沉积物中Fe-Mn氧化物/氢氧化物对REEs的吸附有关.地下水中重稀土元素相对于轻稀土元素的富集可能是吸附作用和碳酸根离子同REEs发生络合作用的共同结果.采用平均大陆上地壳标准化的地下水稀土元素分布表现出显著的Ce及Eu正异常.地下水Ce/Ce*值及Eu含量与Fe+Mn具有显著相关性, 表明铁锰氧化物还原性溶解是控制Ce/Ce*值及Eu含量特征的主要因素.Ce/Ce*值及Eu含量与As浓度的关系表明, Ce异常及Eu含量特征能对地下水中As的富集进行有效指示.      

Geochemistry and origin of rare earth elements (REEs) in the Shengli River oil shale, northern Tibet, China

The Shengli River-Changshe Mountain oil shale zone, located in the North Qiangtang depression, northern Tibet plateau, represents a potentially large marine oil shale resource in China. Twenty-eight samples including oil shale, micritic limestone and marl were collected from the Shengli River area to determine the contents and distribution patterns of rare earth elements (REEs) in marine oil shale. Oil shale samples from the Shengli River area have high ash yield (61.86–67.48%) and TOC content (8.02–13.67%) with low total sulfur (S_t,d) content (0.76–1.39%) and intermediate shale oil content (3.60–16.30%). The total rare earth element (ΣREE) content in oil shale samples is notably depleted (46.79–67.90 μg/g), approximately one third of the mean value of the North American Shale Composite (NASC), and lower than that of world-wide black shales and Chinese coals, but higher than that of world-wide coals and micritic limestone samples (29.21 μg/g) from the Shengli River area. The oil shale samples from the Shengli River area exhibit shale-like Chondrite or NASC-normalized REE patterns similar to those of micritic limestone and marl samples from this area, indicating that REEs of these different lithological samples may have been derived from a similar terrigenous source.REE contents of oil shale samples are highly positive correlated with ash yield and show a positive correlation with Fe and a weakly positive correlation with organic sulfur, and the vertical variations of REEs mainly follow those of Si, Al, K and Ti. All these facts indicate that the REE contents in oil shale seams are mainly controlled by clay minerals and, to a lesser extent, by pyrite, as well as partly associated with oil shale organic constituents. Rare earth elements in the Shengli River oil shale have originated from two sources: a felsic volcanic rock source and a clastic or/and limestone source.     

重庆龙潭组煤中稀土元素地球化学及地质成因分析

运用电感耦合等离子体质谱(ICP MS)、X射线荧光光谱(XRF)、X射线衍射(XRD)等方法对重庆龙潭组煤中稀土元素的地球化学特征进行研究。结果表明,各煤样稀土元素分布模式相似,总体呈左高右低的宽缓的“V”型曲线,轻稀土曲线段“右倾”,重稀土曲线段较为“平坦”,成煤沼泽受陆源碎屑影响较大。南桐5#煤稀土元素含量高,接近全国平均值的2倍,受汉南古陆、大巴山古陆和龙门山岛屿控制,少量陆源碎屑物来自康滇古陆。东林6#煤中的稀土元素含量低于全国平均值水平,海水作用明显,煤样中稀土元素含量未受碱性火山灰的影响,但其底板受碱性火山灰影响强烈。根据煤层稀土元素含量及其比值分析,研究区龙潭组为海湾/潟湖潮坪及浅海碳酸盐沉积的沉积环境。     

Rare earth element and stable isotope (O, S) geochemistry of barite from the Bijgan deposit, Markazi Province, Iran

The Bijgan barite deposit, which is located northeast of Delijan in Markazi Province of Iran, occurs as a small lenticular body at the uppermost part of an Eocene volcano-sedimentary rock unit. The presence of fossiliferous and carbonaceous strata suggests that the host rocks were deposited in a quiet marine sedimentary environment. Barite, calcite, iron oxides and carbonaceous clay materials are found as massive patches as well as thin layers in the deposit. Barite is marked by very low concentrations of Sr (1–2%) and total amounts of rare earth elements (REEs) (6.25–17.39?ppm). Chondrite-normalized REE patterns of barite indicate a fractionation of light REEs (LREEs) from La to Sm, similar to those for barite of different origins from elsewhere. The La_CN/Lu_CN ratios and chondrite-normalized REE patterns reveal that barite in the Bijgan deposit is enriched in LREE relative to heavy rare earth elements (HREEs). The similarity between the Ce/La ratios in the barite samples and those found in deep-sea barite supports a marine origin for barite. Lanthanum and Gd exhibit positive anomalies, which are common features of marine chemical sediments. Cerium shows a negative anomaly in most samples that was inherited from the negative Ce anomaly of hydrothermal fluid that mixed with seawater at the time of barite precipitation. The δ¹⁸O values of barites show a narrow range of 9.1–11.4‰, which is close to or slightly lower than that of contemporaneous seawater at the end of the Eocene. This suggests a contribution of oxygen from seawater in the barite-forming solution. The δ³⁴S values of barites (9.5–15.3‰) are lower than that of contemporaneous seawater, which suggests a contribution of magmatic sulfur to the ore-forming solution. The oxygen and sulfur isotope ratios indicate that submarine hydrothermal vent fluids are a good analog for solutions that precipitated barite, due to similarities in the isotopic composition of the sulfates. The available data including tectonic setting, host rock characteristics, REE geochemistry, and oxygen and sulfur isotopic compositions support a submarine hydrothermal origin for the Bijgan barite deposit. At the seafloor, barite deposition occurred where ascending Ba-bearing hydrothermal fluids encountered seawater. Sulfate was derived from the sulfate-bearing marine waters, and, to a lesser extent, by oxidized H₂S, which was derived from magmatic hydrothermal fluids.     

淄博煤田煤的稀土元素地球化学特征

应用电感耦合等离子体质谱法（ICP—MS）对淄博煤田煤（35个样品,其中12个顶底板样品及1个夹矸样品）的稀土元素进行测试,并测定了灰分及相关的伴生元素含量。全面分析了稀土元素、地球化学参数、空间分布规律及分布模式,探讨了淄博煤田石炭—二叠纪煤中稀土元素的特征及来源。研究表明:与国内外其他地区相比,淄博地区煤中稀土元素相对富集;太原组煤中的稀土元素高于山西组,在同一煤层中自顶部到底部呈现增高趋势,并在顶底板和夹矸中明显富集;Eu存在明显的负异常,而Ce值基本正常,表明成煤环境可能为浅海或封闭海域,不同煤层稀土元素分布模式基本一致,稀土元素与灰分正相关,与反映陆源的元素相关性较好,而与反映海相的元素相关性较差。     

Environmental biogeochemical characteristics of rare earth elements in soil and soil-grown plants of the Hetai goldfield,Guangdong Province,China

Very few studies deal with the biogeochemical behaviors of rare earth elements (REEs) in goldfields. This paper presents the geochemical and biogeochemical characteristics of REEs within the soil–plant system in the Hetai goldfield, Guangdong, China. The samples from the goldfield region show anomalies in distribution patterns and behavioral characteristics of REES as compared with those from the background areas. The REEs in rocks, soils, and plants prove to be much higher than those in the surrounding regions. The distribution patterns of REEs are characterized by LREE-enrichment and HREE-depletion, with the REE concentrations in Layer A being the highest. Differentiations between LREEs and HREEs may lead to some extent of negative Eu anomaly in the soils. Research results demonstrate that the REEs in a soil profile can be transferred and accumulated during the mineral formation and supergenic geochemical processes, and the anomalies are obviously related to the geological settings for the REE-bearing ore-forming processes and to the geochemical characteristics of the habitats for the REE-bearing plants. For Dicranopteris dichotoma, the total amount of REEs in the tissues shows an order of leaf > root > stem, while for Pinus massoniana the order becomes root > leaf > stem. The distribution patterns of REEs in Pinus massoniana leaves are similar to those in soils where the plants grow up in the mineralization area. However, in the background areas the REE distribution patterns for Pinus massoniana stems are similar to those for soils where the plants grow up. Parameters such as biological absorption coefficients and biological transfer coefficients show the differences in REE absorption features among plants and indicate that REEs can be transferred among plant organs. The two coefficients can reveal the different survival mechanisms for the two plant species, which are subject to long-term REE-affected stress conditions in the gold mineralization zone.     

巢湖的稀土元素地球化学特征

采用液-液萃取法和ICP-MS测试技术对巢湖的溶解态稀土元素进行了分析。结果表明,巢湖的溶解态稀土的含量与世界淡水相当,丰水期的样品含量高于其他季节。pH值和悬浮物、胶体是控制巢湖水体中溶解态稀土含量的主要因素。巢湖的溶解态稀土的分布模式以平坦型为主,少数呈现重稀土富集。丰水期和枯水期的溶解态稀土的(La/Yb)N值从西半湖区到东半湖区呈现有规律性的逐渐增大,并且丰水期的(La/Yb)N值低于枯水期。在富营养化湖泊中,胶体和水生生物可能是造成这一现象的主要原因。     

Comparison of the behaviour of rare earth elements in surface waters,overburden groundwaters and bedrock groundwaters in two granitoidic settings,Eastern Sweden

This work, which was done within the Swedish nuclear waste management program, was carried out in order to increase the understanding of the mobility and fate of rare earth elements (REEs) in natural boreal waters in granitoidic terrain. Two areas were studied, Forsmark and Simpevarp, one of which will be selected as a site for spent nuclear fuel. The highest REE concentrations were found in the overburden groundwaters, in Simpevarp in particular (median ∑REE 52 μg/L), but also in Forsmark (median ∑REE 6.7 μg/L). The fractionation patterns in these waters were characterised by light REE (LREE) enrichment and negative Ce and Eu anomalies. In contrast, the surface waters had relatively low REE concentrations. They were characterised either by an increase in relative concentrations throughout the lanthanide series (Forsmark which has a carbonate-rich till) or flat patterns (Simpevarp with carbonate-poor till), and had negative Ce and Eu anomalies. In the bedrock groundwaters, the concentrations and fractionation patterns of REEs were entirely different from those in the overburden groundwaters. The median La concentrations were low (just above 0.1 μg/L in both areas), only in a few samples were the concentrations of several REEs (and in a couple of rare cases all REEs) above the detection limit, and there was an increase in the relative concentrations throughout the lanthanide series. In contrast to these large spatial variations, the temporal trends were characterised by small (or non existent) variations in REE-fractionation patterns but rather large variations in concentrations. The Visual MINTEQ speciation calculations predicted that all REEs in all waters were closely associated with dissolved organic matter, and not with carbonate. In the hydrochemical data for the overburden groundwater in particular, there was however a strong indication of association with inorganic colloids, which were not included in the speciation model. Overall the results showed that within a typical boreal granitoidic setting, overburden groundwaters are enriched in REEs, organic complexes are much more important than carbonate complexes, there is little evidence of significant mixing of REEs between different water types (surface, overburden, bedrock) and spatial variations are more extensive than temporal ones.     

Geochemical characteristics and provenance implication of rare earth elements in surface sediments from bays along Guangdong Coast, Southeast China

A systematic study of the granulometric properties and the occurrence and distribution of rare earth elements (REE) within surface sediments from ten bays situated along the coast of Southeast China has facilitated a more rigorous understanding of constraints on sediment provenance in the area. The results show that REE concentrations are similar within a single bay, but vary considerably (133.58–251.77 mg/kg) among the bays. The chondrite-normalized distribution patterns show the typical enrichment of light REEs (LREEs: La–Eu) relative to heavy REEs (HREEs: Gd–Lu), and an apparent depletion of Eu, which is diagnostic of a terrigenous sediment source. Obvious enrichments of the middle REEs (MREEs: Sm–Ho) in the PAAS-normalized (Post-Archean Australian Shale) distribution patterns of these bay sediments are similar to results reported from large rivers in China. Comparing the REE composition of the bay sediments with those of adjoining fluvial sediments and with the bedrock of the surrounding drainage basins, the latter are indicated as the dominant sediment source. The uniform REE distribution patterns, and MREE enrichments, prove that the sediments are derived from the material transported by the streams and rivers that discharge into the bays.     

西藏尕尔勤铜矿床花岗闪长斑岩地球化学特征及成矿作用研究

对尕尔勤铜矿床花岗闪长斑岩及其锆石的稀土元素进行了分析,并对其成矿作用进行了研究。结果表明,花岗闪长斑岩稀土元素总含量变化范围不大（ΣREE=48.64×10-6~78.12×10-6）,LREE/HREE=8.67~11.68,所有样品都呈轻稀土元素相对富集、重稀土元素亏损的右倾型分配模式;δEu由弱负异常→弱正异常演化,这是因为地幔底辟作用引发地壳部分重熔形成长英质岩浆的过程中,逐步消弱了结晶分异导致的负Eu异常进而出现弱的正Eu异常。锆石具有典型的振荡环带,稀土总量较高（ΣREE=735.78×10-6~6792.10×10-6）,相对亏损轻稀土,富集重稀土,正Ce异常明显,并呈现弱的负Eu异常,这是因为在地幔流体作用下,重稀土元素及Ce较其它轻稀土元素更容易进入锆石晶格所致,Eu呈弱的负异常则是成岩后期岩体受氧化淋滤所致。综合分析,揭示出地幔流体作用导致花岗闪长斑岩具有壳幔混染甚至成矿特征,同时还能透过岩浆与围岩发生物质和能量的交换,导致变质砂岩成矿的成因机制。      

Rare-earth elements in Luochuan loess section,Shaanxi province

TheΣ REE in loesses of different ages in the Luochuan section shows a narrow range of variation, indicating the homogeneity of loessic materials in chemical composition. The REEs in loess are concentrated mainly in silt-sized detrital minerals. Loesses and paleosols of different ages all are relatively enriched in rare-earth elements of the Ce family, and show similar REE distribution patterns. The fractionation among various REEs in the loess is different from that in morainic, marine and lacustrine sediments, but is similar to that in sand samples from deserts in Northwest China. The ratios of Ce/Ce* and Eu/Eu* reflect that the provenance of loessic materials and their accumulating area are all in an oxidation environment with weakly basic mediums under arid or semi-arid climate.     

The aquatic chemistry of rare earth elements in rivers and estuaries

Laboratory experiments were carried out to determine how pH, colloids and salinity control the fractionation of rare earth elements (REEs) in river and estuarine waters. By using natural waters as the reaction media (river water from the Connecticut, Hudson and Mississippi Rivers) geochemical reactions can be studied in isolation from the large temporal and spatial variability inherent in river and estuarine chemistry. Experiments, field studies and chemical models form a consistent picture whereby REE fractionation is controlled by surface/solution reactions. The concentration and fractionation of REEs dissolved in river waters are highly pH dependent. Higher pH results in lower concentrations and more fractionated composition relative to the crustal abundance. With increasing pH the order of REE adsorption onto river particle surfaces is LREEs > MREEs > HREEs. With decreasing pH, REEs are released from surfaces in the same order. Within the dissolved (<0.22 µm) pool of river waters, Fe-organic colloids are major carriers of REEs. Filtration through filters and ultrafilters with progressively finer pore sizes results in filtrates which are lower in absolute concentrations and more fractionated. The order of fractionation with respect to shale, HREEs > MREEs > LREEs, is most pronounced in the solution pool, defined here as <5K and <50K ultrafiltrates. Colloidal particles have shale-like REE compositions and are highly LREE enriched relative to the REE composition of the dissolved and solution pools. The addition of sea water to river water causes the coagulation of colloidal REEs within the dissolved pool. Fractionation accompanies coagulation with the order of sea water-induced removal being LREEs > MREEs > HREEs. While the large scale removal of dissolved river REEs in estuaries is well established, the release of dissolved REEs off river particles is a less studied process. Laboratory experiments show that there is both release and fractionation of REEs when river particles are leached with seawater. The order of sea water-induced release of dissolved REE(III) (LREEs > MREEs > HREEs) from Connecticut River particles is the same as that associated with lowering the pH and the same as that associated with colloidal particles. River waters, stripped of their colloidal particles by coagulation in estuaries, have highly evolved REE composition. That is, the solution pool of REEs in river waters are strongly HREE-enriched and are fractionated to the same extent as that of Atlantic surface seawater. This strengthens the conclusions of previous studies that the evolved REE composition of sea water is coupled to chemical weathering on the continents and reactions in estuaries. Moreover, the release of dissolved Nd from river particles to sea water may help to reconcile the incompatibility between the long oceanic residence times of Nd (7100 yr) and the inter-ocean variations of the Nd isotopic composition of sea water. Using new data on dissolved and particle phases of the Amazon and Mississippi Rivers, a comparison of field and laboratory experiments highlights key features of REE fractionation in major river systems. The dissolved pool of both rivers is highly fractionated (HREE enriched) with respect to the REE composition of their suspended particles. In addition, the dissolved pool of the Mississippi River has a large negative Ce-anomaly suggesting in-situ oxidation of Ce(III). One intriguing feature is the well developed maximum in the middle REE sector of the shale normalized patterns for the dissolved pool of Amazon River water. This feature might reflect competition between surface adsorption and solution complexation with carbonate and phosphate anions.     

Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases

Settling particles were sampled monthly for 1 year using an automated time-series sediment trap positioned at similar depths at two sites of high diatomaceous productivity in the North Pacific Ocean and Bering Sea. The particles were analyzed for rare earth elements (REEs) by inductively coupled plasma mass spectrometry (ICP-MS) with and without chemical treatment of the bulk samples to isolate siliceous fractions. The REE composition of the bulk samples is explained largely by the contribution of two distinct components: (i) carbonate with a higher REE concentration, a negative Ce anomaly and lighter REE (LREE) enrichment; (ii) opal with a lower REE concentration, a weaker negative Ce anomaly and heavier REE (HREE) enrichment.The siliceous fractions of settling particles are characterized by high Si/Al ratios (30-190), reflecting high diatom productivity at the studied sites. The La/Al ratio of the siliceous fraction is close to that of the upper crust, but the Lu/Al and Lu/La ratios are significantly higher than those of the upper crust or airborne particles, indicating the presence of excess HREEs in the siliceous fraction. Diatoms are believed to be important carriers of HREEs.The Ce anomaly, Eu anomaly, slope of the REE pattern, and ΣREE of the siliceous fraction vary exponentially with decreasing total mass flux. They can be well-reproduced according to the differential dissolution kinetics of elements in the order of Ce < lighter REEs (LREEs) < Eu = heavier REEs (HREEs) < Si from settling particles, where the dissolution rate is critically reduced through particle aggregation. This order is consistent with the vertical distribution of dissolved REEs and Si in oceans. The differential dissolution kinetics leads to HREE enrichment of the original diatoms and REE enrichment of dissolved diatoms. The Lu/Si ratio of the siliceous fraction of settling particles recovered from some of the highest diatom fluxes is identical to that of the two elements dissolved in deep seawater, providing further evidence for the dissolution of siliceous matter in deep water.     

Use of rare earth element patterns to trace the provenance of the atmospheric dust near Beijing, China

Atmospheric dust is one of the important indicators of urban air quality. In this study, atmospheric dust and topsoil samples were collected monthly from four sites in Beijing and surrounding areas, and analyzed for rare earth elements (REEs). Total REE concentrations (∑REE) in the dusts ranged from 43.10 to 164.43 mg kg^?1; concentrations in Beijing and Miyun were higher than those in Zhangbei and Fengning. Most of sampling sites showed the same seasonal depletion in dust ∑REE concentrations in summer, probably caused by the “dilution effect” of added organic matter in summertime samples. Most of the dusts have consistent Ce-anomaly (Ce/Ce*) values (0.91–1.00) and Eu anomaly (Eu/Eu*) values (0.93–1.36); some dusts show slight positive Eu anomalies. The REE patterns of both the dusts and topsoils (normalized to post-Archean shales from Australia, PASS) show different signatures at different sites, whereas the temporal patterns are consistent at each site. These results suggest that local topsoils are likely to be the main provenance of the dusts in the study area.     

Geology,geochemistry, sulfur isotope composition,and fluid inclusion data of Farsesh barite deposit,Lorestan Province,Iran

Farsesh barite in the central part of Iranian Sanandaj-Sirjan zone is a sample of epigenetic hydrothermal mineralization in dolomitized limestone, which provides appropriate chemicophysical conditions making the passage of mineral-bearing fluids possible. Barite veins may range from a few centimeters to 2 m in thickness that increases downward. The microthermometry measurements obtained from more than 30 fluid inclusions show relative homogenization temperatures ranging from 125 to 200 °C with an average of 110 °C for Farsesh barite deposits. The mean salinity measured proves 16 times as much as weight percentage of NaCl for barite. Coexistence of liquid- and vapor-rich fluid inclusions in barite minerals may provide an evidence of boiling in ore veins. Moreover, occurrence of bladed calcite, high-grade ore zones, and presence of hydrothermal breccia are all consistent with boiling. Thermometric studies indicate that homogenization temperatures (Th) for primary and pseudosecondary fluid inclusions in barite range from 125 to 200 °C with an average of 1,100 °C. The δ³⁴S values of barite also lie between 8.88 and 16.6 %. The relatively narrow spread in δ³⁴S values may suggest uniform environmental conditions throughout the mineralization field. Thus, δ³⁴S values are lower than those of contemporaneous seawater, which indicates a contribution of magmatic sulfur to the ore-forming solution. Barite is marked by total amounts of rare Earth elements (REEs) (6.25–17.39 ppm). Moreover, chondrite-normalized REE patterns of barite indicate a fractionation of light REEs (i.e., LREEs) from La to Sm, similar to those for barite from different origins. The La_CN/Lu_CN ratios and chondrite-normalized REE patterns reveal that barite in Farsesh deposit is enriched in LREEs compared with heavy rare Earth elements (HREEs). Similarity between Ce/La ratios in barite samples and those found in deep-sea barite supports its marine origin. Lanthanum and Gd exhibit positive anomalies, which are common features of chemical marine sediments. Cerium shows a negative anomaly in most samples inherited from the negative Ce anomaly of hydrothermal fluid that is mixed with seawater at barite precipitation. The available data including tectonic setting, host rock characteristics, REE geochemistry, and sulfur isotopic compositions may support a hydrothermal submarine origin for Farsesh barite deposit.     

Geochemistry of rare earth elements in the mainstream of the Yangtze River,China

Water, suspended matter, and sediment samples were taken from 8 locations along the Yangtze River in 1992. The concentration and speciation (exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter, and residual forms) of rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) were determined by instrumental neutron activation analysis (INAA).The contents of the soluble fraction of REEs in the river are low, and REEs mainly reside in particulate form. In the particles, the chondrite-normalized distribution patterns show significant LREE enrichment and Eu-depletion. While normalized to shales, both sediments and suspended matter samples show relative LREE enrichment and HREE depletion. REEs are relatively enriched in fine-grained fractions of the sediments.The speciation characteristics of REEs in the sediments and suspended matter are very similar. The amount of the five forms follows the order: residual>>bound to organic matter∼bound to Fe–Mn oxides>bound to carbonates>>exchangeable. About 65 to 85% of REEs in the particles exist in the residual form, and the exchangeable form is very low. High proportions of residual REEs reveal that REEs in sediments and suspended matter are controlled by their abundances in the earth's crust. Carbonate, Fe–Mn oxide and organic fractions of REEs in sediments account for 2.4–6.9%, 5.2–11.1%, and 7.3–14.0% of the total contents respectively. They are similar to those in the suspended matter. This shows that carbonates, Fe–Mn oxides and organic matter play important roles during the particle-water interaction processes. By normalization to shales, the 3 forms of REEs follow convex shapes according to atomic number with middle REE (Sm, Eu, and Tb) enrichment, while light REE and heavy REE are depleted.