Distribution of rare earth elements in crystalline bedrock groundwaters: Oslo and Bergen regions,Norway

One hundred and fifty Norwegian bedrock groundwater samples, from Bergen and from Vestfold (Oslofjord), have been analysed by ICP-MS techniques at two laboratories for a large suite of trace elements including rare earth elements (REEs) and Y. The bedrock lithologies include granites (dominated by the Permian Drammen Granite) and Permian latites/rhomb porphyries from Oslofjord, and Caledonian/Precambrian granitic and gneissic lithologies in the Bergen area. The REEs show good correlation with each other, with the exception of Eu. REEs generally show a weak negative correlation with pH. REE concentrations are highest in waters in acidic lithologies and generally decrease with increasing atomic weight. Yttrium, La, Ce and Nd are the most abundant REEs in the waters, with median concentrations exceeding 0.1 μg/l. On crustal (PAAS)-normalised plots, distinct geochemical signatures are observed for the different lithologies. Most groundwaters exhibit negative Eu anomalies on such plots, except for latitic waters from the Oslo area which show a positive Eu anomaly. Aquifer host-rock-normalised plots for groundwaters from Vestfold indicate minor enrichment in heavier REEs and depletion in Ce during water–rock interaction.     

Controls on the geochemistry of rare earth elements along a groundwater flow path in the Carrizo Sand aquifer,Texas, USA

Groundwater samples were collected along a groundwater flow path in the Carrizo Sand aquifer in south Texas, USA. Field measurements that included pH, specific conductivity, temperature, dissolved oxygen (DO), oxidation–reduction potentials (Eh in mV), alkalinity, iron speciation, and H₂S concentrations were also conducted on site. The geochemistry (i.e., concentrations, shale-normalized patterns, and speciation) of dissolved rare element elements (REEs) in the Carrizo groundwaters are described as a function of distance along a flow path. Eh and other redox indicators (i.e., DO, Fe speciation, H₂S, U, and Re) indicate that redox conditions change along the flow path in the Carrizo Sand aquifer. Within the region of the aquifer proximal to the recharge zone, groundwaters exhibit both highly oxidizing and localized mildly reducing conditions. However, from roughly 10 km to the discharge zone, groundwaters are reducing and exhibit a progressive decrease in redox conditions. Dissolved REE geochemical behavior exhibits regular variations along the groundwater flow path in the Carrizo Sand aquifer. The changes in REE concentrations, shale-normalized patterns, and speciation indicate that REEs are not conservative tracers. With flow down-gradient, redox conditions, pH and solution composite, and adsorption modify groundwater REE concentrations, fractionation patterns, and speciation.     

The solubility control of rare earth elements in natural terrestrial waters and the significance of PO 4 3− and CO 3 2− in limiting dissolved rare earth concentrations: A review of recent information

Rare earth element (REE) concentrations in alkaline lakes, circumneutral pH groundwaters, and an acidic freshwater lake were determined along with the free carbonate, free phosphate, and free sulfate ion concentrations. These parameters were used to evaluate the saturation state of these waters with respect to REE phosphate and carbonate precipitates. Our activity product estimates indicate that the alkaline lake waters and groundwaters are approximately saturated with respect to the REE phosphate precipitates but are significantly undersaturated with respect to REE carbonate and sulfate precipitates. On the other hand, the acidic lake waters are undersaturated with respect to REE sulfate, carbonate, and phosphate precipitates. Although carbonate complexes tend to dominate the speciation of the REEs in neutral and alkaline waters, our results indicate that REE phosphate precipitates are also important in controlling REE behavior. More specifically, elevated carbonate ion concentrations in neutral to alkaline natural waters tend to enhance dissolved REE concentrations through the formation of stable REE-carbonate complexes whereas phosphate ions tend to lead to the removal of the REEs from solution in these waters by the formation of REE-phosphate salts. Removal of REEs by precipitation as phosphate phases in the acid lake (pH=3.6) is inconsequential, however, due to extremely low [PO ₄ ^3– ] _F concentrations (i.e., 10^–23 mol/kg).     

Rare-earth elements in groundwaters from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Data are presented on rare-earth elements (REE) in prefiltered (<450 nm) near-surface and deeper groundwaters and in corresponding particulate matter (>450 nm) from the Osamu Utsumi uranium mine and the Morro do Ferro thorium-REE-deposit. Groundwaters from both sites typically contain between 1–50 μg/l of total REE, but can reach values of up to 160 μg/l in the deepest borehole F4 (U-Mine: 150–415 m). Even higher REE concentrations of up to 29 mg/l were measured in acidic, sulfate-rich near-surface waters of the same site. The chondrite-normalized REE patterns in deeper, more reducing groundwaters and in their corresponding suspended particle fractions are similar to those observed in the bedrock (phonolites), indicating that bedrock leaching and secondary mineral sorption occurred without significant fractionation between these elements, in accordance with the only small variations in the stability constants of the expected REE-sulfate complexes in these waters. Groundwaters from the unsaturated zone of both sites show a very characteristic cerium depletion (less pronounced than that observed in the corresponding suspended particulate fractions), which is most probably related to the oxidation of Ce (III) under the prevailing E_h-conditions of these waters (600 to 800 mV), and to sorption/precipitation reactions of the much less soluble Ce(IV) species. Coarse particulate matter (>450 nm), composed mainly of amorphous ferric hydrous oxides, has a strong capacity for sorption of REE. This is shown by its very high REE concentrations, in some boreholes > 8,000 μg/g (total REE), and by the calculated association ratios R_a (ml/g), which are in the order of 10⁵ to 10⁶. The implications of these findings for the migration behavior of REE in both environments are discussed.     

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.     

Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn–Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*]_NASC as low as 0.08), produced by oxidation of Ce³⁺ to Ce⁴⁺ and preferential removal of Ce⁴⁺ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.     

Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater-saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10-100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg⁻¹), with submarine groundwater discharge (SGD) at the freshwater-saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day⁻¹. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.     

Control of organic and iron colloids on arsenic partition and transport in high arsenic groundwaters in the Hetao basin, Inner Mongolia

Due to the importance of colloids in regulating element transport and mobility in aquifers, As distribution in the colloidal fraction needs to be identified in high As groundwaters. Groundwater samples were filtered in the field through a progressively decreasing pore size (0.45 μm, 100, 30, 10, 5 kDa) using a filtration technique under a N₂ atmosphere. Major and trace elements and organic C (OC) were measured in (ultra)filtrates. The studied groundwater samples have typical physio-chemical characteristics of the basin waters. Declines in concentrations of alkali (Na, K), alkaline-earth (Mg, Ca, Sr, Ba) elements, Mo, Si and Se during ultrafiltration are smaller relative to other elements. Arsenic, Cu, Cr, U and V are generally about 30% lower in 5 kDa ultrafiltrates in comparison with 0.45 μm filtrates. Around 50% of Fe, OC and Al are bound to colloids with grain size between 5 kDa and 0.45 μm. Two types of colloids, including large-size Fe colloids and small-size organic colloids, have been identified. Results indicate that As would be more likely to be associated with small-size organic colloids than Fe colloids. SEM images and EDS analysis and synchrotron XRF analyses confirm the association of As with NOM with molecular weights of 5-10 kDa. The better correlation between As(V) and OC in the 5-10 kDa fraction indicates that the small-size organic colloids have a greater affinity for As(V) than As(III). Arsenic associated with organic complexes may not be immobilized by adsorption, and, therefore, easily transported in the aquifer. Thus, the presence of As-containing colloidal complexes in high As groundwaters must be considered in the modeling of As transport in the aquifers.     

Adsorption of rare earth elements onto Carrizo sand: Experimental investigations and modeling with surface complexation

Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002-0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH²⁺, LnSO₄⁺, and LnCO₃⁺, but not that of Ln(CO₃)₂^- or LnPO₄^o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln³⁺) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln³⁺) but also as aqueous complexes (e.g., as Ln(CO₃)₂^- in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.     

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。     

Rare earth elements in pore waters of marine sediments

The rare earth elements (REEs) were measured in pore waters of the upper ∼25 cm of sediment from one site off Peru and three sites on the California margin. The pore water REE concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern. From these analyses and from comparison to other chemical species measured (dissolved Fe, Mn, Ba, oxygen, nitrate, phosphate), it is suggested that pore water REEs can be grouped into three categories: those that are from an Fe-source, those that are from a POC-source, and cerium oxide. REEs from the Fe-source appear where anoxia is reached; they have a distinctive “middle-REE (MREE) enriched” pattern. The concentrations in this source are so elevated that they dominate REE trends in the Fe-oxide reduction zone. The net result of flux from the POC-source is relative enrichment of heavy-REEs (HREEs) over light-REEs (LREEs), reflecting remineralizing POC and complexation with DOC. A common “linear” REE pattern, seen in both oxic and anoxic sediments, is associated with this POC-source, as well as a “HREE enriched” pattern that is seen in surficial sediments at the Peru site. Overall, the pore water results indicate that Mn-oxides are not an important carrier of REEs in the oceans.A REE biogeochemical model is presented which attempts to reconcile REE behavior in the water and sediment columns of the oceans. The model proposes that POC, Fe-oxide and Ce-oxide sources can explain the REE concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water. The model is also consistent with our observation that the “Ce-anomaly” of pore water does not exceed unity under any redox condition.     

Chemical and physical characterization of suspended particles and colloids in waters from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil

Data are presented on suspended particles and colloids in groundwaters from the Osamu Utsumi mine and the Morro do Ferro analogue study sites. Cross-flow ultrafiltration with membranes of different pore sizes (450 nm to 1.5 nm) was used to prepare colloid concentrates and ultrafiltrates for analyses of major and trace elements and U- and Th-isotopic compositions. Additional characterization of colloidal and particulate material was performed by ESCA, SEM and X-ray diffraction. The results indicate the presence of low concentrations of colloids in these waters (typically < 500 μg/l), composed mainly of iron/organic species. Minor portions of uranium and other trace elements, but significant fractions of the total concentrations of Th and REE in prefiltered waters (< 450 nm) were associated with these colloids.Suspended particles (> 450 nm), also composed mainly of hydrous ferric oxides and humic-like compounds, show the same trend as the colloids with respect to U, Th and REE associations, but elemental concentrations were typically higher by a factor of 1,000 or more. In waters of low pH and with high sulphate content, these associations are considerably lower. Due to the low concentrations of suspended particles in groundwaters from the Osamu Utsumi uranium mine (typically <0.5 mg/l), these particles carry only a minor fraction of U and the REE (<10% of the total concentrations in unfiltered groundwaters), but a significant, usually predominant fraction of Th (30–70%). The suspended particle load in groundwaters from the Morro do Ferro environment is typically higher than in those from the mine by a factor of 5 to 10. This suggests that U, Th and the REE could be transported predominantly by particulate matter. However, these particles and colloids seem to have a low capacity for migration.     

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.     

Speciation of rare earth elements in natural terrestrial waters: assessing the role of dissolved organic matter from the modeling approach

Humic Ion-Binding Model V, which focuses on metal complexation with humic and fulvic acids, was modified to assess the role of dissolved natural organic matter in the speciation of rare earth elements (REEs) in natural terrestrial waters. Intrinsic equilibrium constants for cation-proton exchange with humic substances (i.e., pK_MHA for type A sites, consisting mainly of carboxylic acids), required by the model for each REE, were initially estimated using linear free-energy relationships between the first hydrolysis constants and stability constants for REE metal complexation with lactic and acetic acid. pK_MHA values were further refined by comparison of calculated Model V “fits” to published data sets describing complexation of Eu, Tb, and Dy with humic substances. A subroutine that allows for the simultaneous evaluation of REE complexation with inorganic ligands (e.g., Cl⁻, F⁻, OH⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻), incorporating recently determined stability constants for REE complexes with these ligands, was also linked to Model V. Humic Ion-Binding Model V’s ability to predict REE speciation with natural organic matter in natural waters was evaluated by comparing model results to “speciation” data determined previously with ultrafiltration techniques (i.e., organic acid-rich waters of the Nsimi-Zoetele catchment, Cameroon; dilute, circumneutral-pH waters of the Tamagawa River, Japan, and the Kalix River, northern Sweden). The model predictions compare well with the ultrafiltration studies, especially for the heavy REEs in circumneutral-pH river waters. Subsequent application of the model to world average river water predicts that organic matter complexes are the dominant form of dissolved REEs in bulk river waters draining the continents. Holding major solute, minor solute, and REE concentrations of world average river water constant while varying pH, the model suggests that organic matter complexes would dominate La, Eu, and Lu speciation within the pH ranges of 5.4 to 7.9, 4.8 to 7.3, and 4.9 to 6.9, respectively. For acidic waters, the model predicts that the free metal ion (Ln³⁺) and sulfate complexes (LnSO₄⁺) dominate, whereas in alkaline waters, carbonate complexes (LnCO₃⁺ + Ln[CO₃]₂⁻) are predicted to out-compete humic substances for dissolved REEs. Application of the modified Model V to a “model” groundwater suggests that natural organic matter complexes of REEs are insignificant. However, groundwaters with higher dissolved organic carbon concentrations than the “model” groundwater (i.e., >0.7 mg/L) would exhibit greater fractions of each REE complexed with organic matter. Sensitively analysis indicates that increasing ionic strength can weaken humate-REE interactions, and increasing the concentration of competitive cations such as Fe(III) and Al can lead to a decrease in the amount of REEs bound to dissolved organic matter.     

Rare earth element abundances in hydrothermal fluids from the Manus Basin, Papua New Guinea: Indicators of sub-seafloor hydrothermal processes in back-arc basins

Rare earth element (REE) concentrations are reported for a large suite of seafloor vent fluids from four hydrothermal systems in the Manus back-arc basin (Vienna Woods, PACMANUS, DESMOS and SuSu Knolls vent areas). Sampled vent fluids show a wide range of absolute REE concentrations and chondrite-normalized (REE_N) distribution patterns (La_N/Sm_N ∼ 0.6-11; La_N/Yb_N ∼ 0.6 - 71; ). REE_N distribution patterns in different vent fluids range from light-REE enriched, to mid- and heavy-REE enriched, to flat, and have a range of positive Eu-anomalies. This heterogeneity contrasts markedly with relatively uniform REE_N distribution patterns of mid-ocean ridge hydrothermal fluids. In Manus Basin fluids, aqueous REE compositions do not inherit directly or show a clear relationship with the REE compositions of primary crustal rocks with which hydrothermal fluids interact. These results suggest that the REEs are less sensitive indicators of primary crustal rock composition despite crustal rocks being the dominant source of REEs in submarine hydrothermal fluids. In contrast, differences in aqueous REE compositions are consistently correlated with differences in fluid pH and ligand (chloride, fluoride and sulfate) concentrations. Our results suggest that the REEs can be used as an indicator of the type of magmatic acid volatile (i.e., presence of HF, SO₂) degassing in submarine hydrothermal systems. Additional fluid data suggest that near-seafloor mixing between high-temperature hydrothermal fluid and locally entrained seawater at many vent areas in the Manus Basin causes anhydrite precipitation. Anhydrite effectively incorporates REE and likely affects measured fluid REE concentrations, but does not affect their relative distributions.     

Organic complexation of rare earth elements in natural waters: Evaluating model calculations from ultrafiltration data

The Stockholm Humic Model (SHM) and Humic Ion-Binding Models V and VI were compared for their ability to predict the role of dissolved organic matter (DOM) in the speciation of rare earth elements (REE) in natural waters. Unlike Models V and VI, SHM is part of a speciation code that also allows us to consider dissolution/precipitation, sorption/desorption and oxidation/reduction reactions. In this context, it is particularly interesting to test the performance of SHM. The REE specific equilibrium constants required by the speciation models were estimated using linear free-energy relationships (LFER) between the first hydrolysis constants and the stability constants for REE complexation with lactic and acetic acid. Three datasets were used for the purpose of comparison: (i) World Average River Water (Dissolved Organic Carbon (DOC) = 5 mg L⁻¹), previously investigated using Model V, was reinvestigated using SHM and Model VI; (ii) two natural organic-rich waters (DOC = 18-24 mg L⁻¹), whose REE speciation has already been determined with both Model V and ultrafiltration studies, were also reinvestigated using SHM and Model VI; finally, (iii) new ultrafiltration experiments were carried out on samples of circumneutral-pH (pH 6.2-7.1), organic-rich (DOC = 7-20 mg L⁻¹) groundwaters from the Kervidy-Naizin and Petit-Hermitage catchments, western France. The results were then compared with speciation predictions provided by Model VI and SHM, successively. When applied to World Average River Water, both Model VI and SHM yield comparable results, confirming the earlier finding that a large fraction of the dissolved REE in rivers occurs as organic complexes This implies that the two models are equally valid for calculating REE speciation in low-DOC waters at circumneutral-pH. The two models also successfully reproduced ultrafiltration results obtained for DOC-rich acidic groundwaters and river waters. By contrast, the two models yielded different results when compared to newly obtained ultrafiltration results for DOC-rich (DOC > 7 mg L⁻¹) groundwaters at circumneutral-pH, with Model VI predictions being closer to the ultrafiltration data than SHM. Sensitivity analysis indicates that the “active DOM parameter” (i.e., the proportion of DOC that can effectively complex with REE) is a key parameter for both Model VI and SHM. However, a survey of ultrafiltration results allows the “active DOM parameter” to be precisely determined for the newly ultrafiltered waters studied here. Thus, the observed discrepancy between SHM predictions and ultrafiltration results cannot be explained by the use of inappropriate “active DOM parameter” values in this model. Save this unexplained discrepancy, the results presented in this study demonstrate that both Model VI and SHM can provide reliable estimates of REE speciation in organic-rich waters. However, it is essential to know the proportion of DOM that can actively complex REE before running these two speciation models.     

The aqueous geochemistry of the rare earth elements and yttrium. Part 7. REE,Th and U contents in thermal springs associated with the Idaho batholith

Concentrations of the rare earth elements (REE), Th and U have been determined in thermal waters emerging from a number of locations in and around the Idaho Batholith. Previous investigators have suggested that the source of heat for the geothermal systems studied is the radioactive decay of K, Th and U which are enriched in the rocks through which the fluids flow. Thus, knowledge of the behavior of REE, Th and U in these systems may contribute to a better understanding of the potential consequences of the interaction of hydrothermal fluids with deeply buried nuclear waste. Such studies may also lead to the possible use of REE as an exploration tool for geothermal resources. The thermal waters investigated may be characterized as near-neutral to slightly alkaline, dilute, NaHCO₃-dominated waters with relatively low temperatures of last equilibration with their reservoir rocks (<200°C). REE, Th and U concentrations were measured using Fe(OH)₃ coprecipitation, followed by ICP-MS, which yielded detection limits of 0.01–0.003 μg/l for each element, depending on the volume of fluid sample taken. The concentrations of REE, Th and U measured (from <0.1 up to a few μg/l) are 3–5 orders of magnitude less than chondritic, in agreement with concentrations of these elements measured in other similar continental geothermal systems. The REE exhibit light REE-enriched patterns when normalized to chondrite, but when normalized to NASC or local granites, they exhibit flat or slightly heavy REE-enriched trends. These findings indicate that the REE are either taken up in proportion to their relative concentrations in the source rocks, or that the heavy REE are preferentially mobilized. Concentrations of REE and Th are often higher in unfiltered, compared to filtered samples, indicating an important contribution of suspended particulates, whereas U is apparently truly dissolved. In some of the hot springs the REE concentrations exhibit marked temporal variations, which are greater than the variations observed in major element concentrations, alkalinity and temperature. There are also variations in the fluid concentrations of REE, Th and U related to general location within the study area which may be reflective of variations in the concentrations of these elements in the reservoir rocks at depth. Thermal waters in the southern and central parts of the field area all contain ∑REE concentrations exceeding 0.1 μg/l (up to as high as 3 μg/l), Th exceeding 0.2 μg/l and U generally <0.4 μg/l. In contrast, thermal waters from the northern area contain lower ∑REE (<0.6 μg/l) and Th (<0.1 μg/l), but higher U (>3.0 μg/l). Using experimentally measured and theoretically estimated thermodynamic data, the distribution of species for La, Ce and Nd have been calculated and also the solubility of pure, endmember (La, Ce, Nd) phosphate phases of the monazite structure in selected hot spring fluids. These calculations indicate that, at the emergence temperatures, CO²⁻₃ and OH⁻ complexes of the REE are the predominant species in the thermal waters, whereas at the deep-aquifer temperatures, OH complexes predominate. In these thermal waters, monazite solubility is strongly prograde with respect to temperature, with solubility often decreasing several orders of magnitude upon cooling from the deep-aquifer to the emergence temperature. At the surface temperature, calculated monazite solubilities are, within the uncertainty of the thermodynamic data, comparable to the REE concentrations measured in the filtered samples, whereas at the deep-aquifer temperature, monazite solubilities are generally several orders of magnitude higher than the REE concentrations measured in the filtered or unfiltered samples. Therefore, a tentative model is suggested in which the thermal fluids become saturated with respect to a monazite-like phase (or perhaps an amorphous or hydrated phosphate) upon ascent and cooling, followed by subsequent precipitation of that phase. The temporal variations in REE content can then be explained as a result of sampling variable mixtures of particulate matter and fluid and/or variable degrees of attainment of equilibrium between fluid and solid phosphate.     

Geochemical behavior and speciation modeling of rare earth elements in acid drainages at Sarcheshmeh porphyry copper deposit,Kerman Province,Iran

Acid mine drainage is a major source of water pollution in the Sarcheshmeh porphyry copper mine area. The concentrations of heavy metals and rare earth elements (REEs) in the host rocks, natural waters and acid mine drainage (AMD) associated with mining and tailing impoundments are determined. Contrary to the solid samples, AMDs and impacted stream waters are enriched in middle rare earth elements (MREEs) and heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs). This behavior suggests that REE probably fractionate during sulfide oxidation and acid generation and subsequent transport, so that MREE and HREE are preferentially enriched. Speciation modeling predict that the dominant dissolved REE inorganic species are Ln³⁺, Ln(SO₄)₂⁻, LnSO₄⁺, LnHCO₃²⁺, Ln(CO₃)₂⁻ and LnCO₃⁺. Compared to natural waters, Sarcheshmeh AMD is enriched in REEs and SO₄²⁻. High concentrations of SO₄²⁻ lead to the formation of stable LnSO₄⁺, thereby resulting in higher concentrations of REEs in AMD samples. The model indicates that LnSO₄⁺ is the dissolved form of REE in acid waters, while carbonate and dicarbonate complexes are the most abundant dissolved REE species in alkaline waters. The speciation calculations indicate that other factors besides complexation of the REE's, such as release of MREE from dissolution and/or desorption processes in soluble salts and poorly crystalline iron oxyhydroxy sulfates as well as dissolution of host rock MREE-bearing minerals control the dissolved REE concentrations and, hence, the MREE-enriched patterns of acid mine waters.     

Fractionation of rare-earth elements in plants during experimental growth in varied clay substrates

The distribution and content of rare-earth elements (REEs) were determined in two radish species, the cultivated Raphanus sativus and the wild Raphanus raphanistrum, that were grown under laboratory-controlled conditions, in three substrates consisting of illite for one and two smectite substrates for the others, with the two smectite substrates being characterised by different porosities. The plants were split into leaves and stems + roots for analysis. The results indicate that both species take up systematically higher amounts of REEs when grown in the illite substrate, even considering that the smectite equivalent contains about three times more REEs. The REE uptake is also more plant species than mineral composition dependent: R. raphanistrum takes up 3.5–6.7 times more REEs than R. sativus, depending on the substrate, its porosity and the considered plant segments. Increased substrate porosity favours the take up of the REEs, but no specific uptake is observed in leaves relative to that in the combined stems and roots. The transfer of the REEs from minerals to plant organs does not appear to induce systematically identical patterns: (1) in the case of R. sativus, a positive Eu anomaly is visible in all patterns from both segment groups grown in both substrates. When grown in illite, the heavy REEs are also enriched in the stems and roots, which has not been observed in any other organ or in the other substrate and (2) in the case of R. raphanistrum, a very significant positive Gd anomaly, which is not expected to fractionate relative to the other REEs as do Ce and Eu, is observed in all segments of the plants grown in both substrates. A slight negative Ce anomaly is also visible in some of the REE patterns, suggesting some changes in the oxidation–reduction conditions in the substrates near the roots during plant growth. The comparison of the REE patterns from leaves relative to those of the roots + stems shows that those of R. raphanistrum grown in illite provide a spectrum that is very specific with significant deficits in La, Ce, Gd, Tm, Yb and Lu in the leaves. In the other cases, the patterns do not outline significant differences except for R. sativus grown in illite, in which the leaves are enriched in light and medium REEs from La to Gd relative to the stems + roots.