Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment

We have collected ∼500 stream waters and associated bed-load sediments over an ∼400 km² region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into “dissolved” (<0.45 μm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]_NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce^∗]_NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu^∗]_NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]_NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/Yb]_NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce^∗]_NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu^∗]_NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]_NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]_NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced.The partial extraction recovered, on average ∼20% of the Fe in the total sediment, ∼80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly δ-MnO₂/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments.     

A specific Ce oxidation process during sorption of rare earth elements on biogenic Mn oxide produced by Acremonium sp. strain KR21-2

Sorption of rare earth elements (REEs) and Ce oxidation on natural and synthetic Mn oxides have been investigated by many researchers. Although Mn(II)-oxidizing microorganisms are thought to play an important role in the formation of Mn oxides in most natural environments, Ce oxidation by biogenic Mn oxide and the relevance of microorganisms to the Ce oxidation process have not been well understood. Therefore, in this study, we conducted sorption experiments of REEs on biogenic Mn oxide produced by Acremonium sp. strain KR21-2. The distribution coefficients, K_d(REE), between biogenic Mn oxide (plus hyphae) and 10 mmol/L NaCl solution showed a large positive Ce anomaly and convex tetrad effect variations at pH 3.8, which was consistent with previous works using synthetic Mn oxide. The positive Ce anomaly was caused by oxidation of Ce(III) to Ce(IV) by the biogenic Mn oxide, which was confirmed by analysis of the Ce L_III-edge XANES spectra. With increasing pH, the positive Ce anomaly and convex tetrad effects became less pronounced. Furthermore, negative Ce anomalies were observed at a pH of more than 6.5, suggesting that Ce(IV) was stabilized in the solution (<0.2 μm) phase, although Ce(III) oxidation to Ce(IV) on the biogenic Mn oxide was confirmed by XANES analysis. It was demonstrated that no Ce(III) oxidation occurred during sorption on the hyphae of strain KR21-2 by the K_d(REE) patterns and XANES analysis. The analysis of size exclusion HPLC-ICP-MS showed that some fractions of REEs in the filtrates (<0.2 μm) after sorption experiments were bound to organic molecules (40 and <670 kDa fractions), which were possibly released from hyphae. A line of our data indicates that the negative Ce anomalies under circumneutral pH conditions arose from Ce(III) oxidation on the biogenic Mn oxide and subsequent complexation of Ce(IV) with organic ligands. The suppression of tetrad effects is also explained by the complexation of REEs with organic ligands. The results of this study demonstrate that the coexistence of the biogenic Mn oxide and hyphae of strain KR21-2 produces a specific redox chemistry which cannot be explained by inorganic species.     

Rare earth element fractionation in authigenic illite–smectite from Late Permian clastic rocks,Bowen Basin,Australia: implications for physico-chemical environments of fluids during illitization

REE analyses were performed on authigenic illitic clay minerals from Late Permian mudrocks, sandstones and bentonites from the Bowen Basin (Australia). The mixed-layer illite–smectite exhibit REE patterns with an obvious fractionation of the HREE from the LREE and MREE, which is an apparent function of degree of illitization reaction. The highly illitic (R≥3) illite–smectite from the northern Bowen Basin show a depletion of LREE relative to the less illitic (R=0 and 1) clays. In contrast, an enrichment of HREE for the illite-rich clays relative to less illitic clays is evident for the southern Bowen Basin samples. The North American Shale Composite-normalized (La/Lu)_sn ratios show negative correlations with the illite content in illite–smectite and positive correlations with the δ¹⁸O values of the clays for both the northern and southern Bowen Basin samples. These correlations indicate that the increasing depletion of LREE in hydrothermal fluids is a function of increasing water/rock ratios in the northern Bowen Basin. Good negative correlations between (La/Lu)_sn ratios and illite content in illite–smectite from the southern Bowen Basin suggest the involvement of fluids with higher alkalinity and higher pH in low water/rock ratio conditions. Increasing HREE enrichment with δ¹⁸O decrease indicates the effect of increasing temperature at low water/rock ratios in the southern Bowen Basin.Results of the present study confirm the conclusions of some earlier studies suggesting that REE in illitic clay minerals are mobile and fractionated during illitization and that this fact should be considered in studies of sedimentary processes and in identifying provenance. Moreover, our results show that REE systematic of illitic clay minerals can be applied as an useful technique to gain information about physico-chemical conditions during thermal and fluid flow events in certain sedimentary basins.     

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.     

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.     

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.     

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.     

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.     

Rare earth element sorption by basaltic rock: Experimental data and modeling results using the “Generalised Composite approach”

Sorption of the 14 rare earth elements (REE) by basaltic rock is investigated as a function of pH, ionic strength and aqueous REE concentrations. The rock sample, originating from a terrestrial basalt flow (Rio Grande do Sul State, Brazil), is composed of plagioclase, pyroxene and cryptocrystalline phases. Small amounts of clay minerals are present, due to rock weathering. Batch sorption experiments are carried out under controlled temperature conditions of 20 °C with the <125 μm fraction of the ground rock in solutions of 0.025 M and 0.5 M NaCl and at pH ranging from 2.7 to 8. All 14 REEs are investigated simultaneously with initial concentrations varying from 10⁻⁷ to 10⁻⁴ mol/L. Some experiments are repeated with only europium present to evaluate possible competitive effects between REE. Experimental results show the preferential retention of the heavy REEs at high ionic strength and circumneutral pH conditions. Moreover, results show that REE sorption increases strongly with decreasing ionic strength, indicating two types of sorption sites: exchange and specific sites. Sorption data are described by a Generalised Composite (GC) non-electrostatic model: two kinds of surface reactions are treated, i.e. cation exchange at >XNa sites, and surface complexation at >SOH sites. Total site density (>XNa + >SOH) is determined by measuring the cation exchange capacity (CEC = 52 μmol/m²). Specific concentrations of exchange sites and complexation sites are determined by fitting the Langmuir equation to sorption isotherms of REE and phosphate ions. Site densities of 22 ± 5 and 30 ± 5 μmol/m² are obtained for [>XNa] and [>SOH], respectively. The entire set of REE experimental data is modeled using a single exchange constant (log K_ex = 9.7) and a surface complexation constant that progressively increases from log K = −1.15 for La(III) to −0.4 for Lu(III).The model proves to be fairly robust in describing other aluminosilicate systems. Maintaining the same set of sorption constants and only adjusting the site densities, we obtain good agreement with the literature data on REE/kaolinite and REE/smectite sorption. The Generalised Composite non-electrostatic model appears as an easy and efficient tool for describing sorption by complex aluminosilicate mineral assemblages.     

Chemical effect of pesticide application on soils: evidence from rare earth elements

The main objective of this study is to investigate the distribution of rare earth elements (REEs) in Lolium perenne L. plant species which has been grown on vineyard soils treated with pesticide commonly used in the study area. These plants have been grown on two types of soils: (1) brown calcareous soils developed on loess and (2) brown to calcic brown soils developed on conglomerates. The significant correlation observed between the concentrations of phosphorus and the total amount of REEs, in addition to the enrichment in middle REEs (MREEs), suggests the complexation of REEs with phosphates and organic matter. The soils were enriched in REEs due to pesticide application but the plants were depleted. The ratio of REEs in plant over REEs in soil before application of pesticides is higher than that after application of pesticides. Application of pesticides to crops did not affect the fractionation of REEs neither in leaves nor in roots. No selectivity in uptake of REEs occurred because of pesticides except for Ce and Eu which show a negative anomaly relative to the other REEs.     

Non-Henry’s Law behavior of REE partitioning between fluorapatite and CaF2-rich melts: Controls of intrinsic vacancies and implications for natural apatites

The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF₂-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 ± 0.1 to 25,000 ± 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 °C and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from ∼2 to ∼0.5 with increasing Gd in fluorapatite, hence a marked non-Henry’s Law behavior, but becomes independent of composition at and above ∼5000 and ∼1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry’s Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry’s Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry’s Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca²⁺ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry’s Law in their respective compositional ranges investigated in this study.Nonstoichiometry, such as Ca²⁺ and F⁻ vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry’s Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REEs by natural apatites. Particularly, the non-Henry’s Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites.     

Experimental investigation of main controls to methane adsorption in clay-rich rocks

In this study a series of CH₄ adsorption experiments on clay-rich rocks were conducted at 35 °C, 50 °C and 65 °C and at CH₄ pressure up to 15 MPa under dry conditions. The clay-dominated rock samples used are fresh samples from quarries and mines. Samples are individually dominated by montmorillonite, kaolinite, illite, chlorite, and interstratified illite/smectite. The experimental results show that clay mineral type greatly affects CH₄ sorption capacity under the experimental conditions. In terms of relative CH₄ sorption capacity: montmorillonite ? illite/smectite mixed layer > kaolinite > chlorite > illite. Physisorption is the dominant process for CH₄ absorption on clay minerals, as a result, there is a linear correlation between CH₄ sorption capacity and BET surface area in these clay-mineral dominated rocks. The abundance of micro-mesopores in the size range of a few to a few 10 s of nanometers in montmorillonite clay and illite–smectite interstratified clay results in large BET surface area values for these mineral species.     

Rare earth element studies of surficial sediments from the southwestern Carlsberg Ridge,Indian Ocean

The rare earth element (REE) contents of sixteen surficial calcareous sediments from the southwestern Carlsberg Ridge, Indian Ocean, have been determined. The total REE vary from 35 ppm to 126 ppm and are inversely related to the calcium carbonate content. REEs show a strong positive correlation with Al + Fe + K + Mg + Na (r ²= 0.98) and Mn + Fe + Cu + Ni (r ²= 0.86) suggesting that the REE is associated with a combined phase of clays (mainly illite) and Mn-Fe oxyhydroxides. The aeolian input into these sediments is suggested from the weak positive Eu/Eu* anomaly. Shale-normalized (NASC) pattern along with La_(n)/Yb_(n) ratio suggest enrichment of heavy REE (HREE) relative to the light REE (LREE) with a negative Ce/Ce* anomaly implying retention of a bottom water REE pattern. An erratum to this article is available at .     

Post-depositional redistribution processes and their effects on middle rare earth element precipitation and the cerium anomaly in sediments in the South Korea Plateau,East Sea

We sampled two box-core sediments from the slope of the eastern South Korea Plateau (SKP) in the East Sea (Sea of Japan) at water depths of 1400 and 1700 m. Two chemical fractions of extractable (hydroxylamine/acetic acid) and residual rare earth elements (REEs) together with Al, Ca, Fe, Mg, Mn, P, S, As, Mo, and U were analyzed to assess the post-depositional redistribution of REEs. Extractable Fe and Mn are noticeably abundant in the oxic topmost sediment layer (<3 cm). However, some trace elements (e.g., S, As, Mo, U) are more abundant at depth, where redox conditions are different. Analysis of upper continental crust (UCC)-normalized (La/Gd)_UCC, (La/Yb)_UCC, and (Ce/Ce^*)_UCC revealed that the extractable REE is characterized by middle REE (MREE) enrichment and a positive cerium (Ce) anomaly, different from the case of the residual fraction which shows slight enrichment in light REEs (LREEs) with no Ce anomaly. The extractable MREEs seem to have been incorporated into high-Mg calcite during reductive dissolution of Fe oxyhydroxides. In the top sediment layer, the positive Ce anomaly is attributed to Ce oxide, which can be mobilized in deeper oxygen-poor environments and redistributed in the sediment column. In addition, differential concentrations of Ce and other LREEs in pore water appear to result in variable (Ce/Ce^*)_UCC ratios in the extractable fraction at depth.     

Trace element partitioning between apatite and silicate melts

We present new experimental apatite/melt trace element partition coefficients for a large number of trace elements (Cs, Rb, Ba, La, Ce, Pr, Sm, Gd, Lu, Y, Sr, Zr, Hf, Nb, Ta, U, Pb, and Th). The experiments were conducted at pressures of 1.0 GPa and temperatures of 1250 °C. The rare earth elements (La, Ce, Pr, Sm, Gd, and Lu), Y, and Sr are compatible in apatite, whereas the larger lithophile elements (Cs, Rb, and Ba) are strongly incompatible. Other trace elements such as U, Th, and Pb have partition coefficients close to unity. In all experiments we found D_Hf > D_Zr, D_Ta ≈ D_Nb, and D_Ba > D_Rb > D_Cs. The experiments reveal a strong influence of melt composition on REE partition coefficients. With increasing polymerisation of the melt, apatite/melt partition coefficients for the rare earth elements increase for about an order of magnitude. We also present some results in fluorine-rich and water-rich systems, respectively, but no significant influence of either H₂O or F on the partitioning was found. Furthermore, we also present experimentally determined partition coefficients in close-to natural compositions which should be directly applicable to magmatic processes.     

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.     

REE and actinide microdistribution in Sahara 97072 and ALHA77295 EH3 chondrites: A combined cosmochemical and petrologic investigation

We report the results of rare earth elements (REEs) and U-Th inventory of individual minerals (oldhamite, enstatite and niningerite) in two of the most unequilibrated and primitive EH3 known so far, ALHA77295 and Sahara 97072. Under the highly reducing condition that prevailed during the formation of enstatite chondrites, REEs are mainly chalcophile and concentrated in oldhamite. The study is guided by detailed petrographic investigations of the individual minerals in chondrules, complex sulfide-metal clasts and enstatite-dominated matrices.We developed two textural parameters in order to resolve the evolution of oldhamite condensates and their residence in the solar gas prior to their accretion in the individual objects or in matrices and relate these textural features to the measured REE patterns of the individual oldhamite crystals. These textural parameters are the crystal habit of oldhamite grains (idiomorphic or anhedral) and their host assemblages. REE concentrations were measured by SIMS and LA-ICPMS.Oldhamite grains display REE enrichments (10-100 × CI). Four types of REE patterns are encountered in oldhamite in ALHA77295. In general the REE distributions cannot be assigned to a specific oldhamite-bearing assemblage. The most represented REE pattern is characterized by both slight to large positive Eu and Yb anomalies and is enriched in light REEs relative to heavy REEs. This pattern is present in 97% of oldhamite in Sahara 97072, suggesting a different source region in the reduced part of the nebula or different parental EH asteroids for the two EH3 chondrites. Different parental asteroids are also supported by MgS-FeS zoning profiles in niningerite grains adjacent to troilite revealing both normal and reverse zoning trends and different MnS contents. The observed homogeneity of REE distribution in oldhamite grains in Sahara 97072 is not related to the mild metamorphic event identified in this meteorite that caused breakdown of the major K- and Rb-bearing sulfide (djerfisherite).REE concentrations in enstatite range between 0.2 and 8 × CI. Hence, enstatite is an important REE host next to oldhamite. Most patterns are characterized by negative Eu and Yb anomalies. Niningerites are negligible contributors to bulk EH3 REE inventory. Average positive Eu and Yb anomalies observed in most oldhamite are complimentary to the negative ones in enstatite thus explaining the flat patterns of the bulk meteorites. The condensation calculations based on cosmic abundances predict that the first oldhamite condensates should have flat REE patterns with Eu and Yb depletions since Eu and Yb condense at lower temperature than other REE. However, this pattern is seen in enstatite. Our findings are at odds with the predicted negative Eu and Yb anomalies in oldhamite earliest condensates from a closed system in a reduced solar source. Our petrographic, mineral chemistry and REE abundances of oldhamite, enstatite and niningerite discards an origin of oldhamite by impact melting (Rubin et al., 2009).Our results do not support in first order the scenario of the incorporation of REE in the Earth’s core to explain ¹⁴²Nd excess in terrestrial samples relative to chondrites because oldhamite is the major REE carrier phase and has super-chondritic Sm/Nd ratios.     

EXAFS study on the cause of enrichment of heavy REEs on bacterial cell surfaces

Rare earth element (REE) pattern is a unique geochemical tracer and has been measured for various natural materials. Among these, the REE distribution pattern between bacteria and water exhibits anomalous enrichment in the heavy REE (HREE) part, which can act as a signature of bacteria-related materials in natural samples. In this study, the REE binding site on the cell surface of a Gram-positive bacterium (Bacillus subtilis) responsible for HREE enrichment has been identified using extended X-ray absorption fine structure (EXAFS) coupled with a study of the variation in REE distribution patterns. The EXAFS data showed that the HREEs form complexes with multiple phosphate site (including phosphoester site) with a larger coordination number (CN) at lower REE-bacteria ratios ([REE]/[bac]), while light and middle REEs form complexes to the phosphate site with a lower CN. The fraction coordinated to carboxylate increased for all REEs with increasing [REE]/[bac] ratio. On the other hand, the enrichment of HREE in the REE distribution patterns of the bacteria was less marked with increasing [REE]/[bac] ratio. This result is consistent with the EXAFS data, because the REE pattern of surface complex with multiple phosphate in a reference material exhibits a monotonous increase for heavier REE, while phosphate surface complex with a low CN and a carboxylate site reach a maximum around Sm and Eu. Based on these results, it is clear that the REE are primarily bound to the phosphate site and subsequently to the carboxylate site on the bacterial cell surface.Regarding the pH dependence in the range (3 < pH < 7), both the EXAFS and REE pattern data indicate that the fraction of REE-carboxylate increased as the pH increases. The results above obtained for B. subtilis were also valid for Escherichia coli, a Gram-negative bacterium, showing that similar phosphate and carboxylate sites are also available in the cell walls of E. coli, or other Gram negative bacteria. In all our results, the variation in REE patterns correlated with the binding site indicated by EXAFS, showing that the REE pattern itself reflects the binding site of the REE at the bacterial surface for various parameters (pH and [REE]/[bac] ratio). Thus, the REE patterns can be used to estimate the binding sites for lower [REE]/[bac] ratios where spectroscopic techniques cannot be applied.The average bond length between the REE and oxygen was compared for various REE sorbed on bacteria, showing that the bond length for HREE (Er to Lu) was much shorter than those extrapolated from the trend between La and Dy, because of the selective binding of the HREE as the multiple phosphate surface complexes. Our results are consistent with the selective enrichment of the HREE at the bacterial cell surfaces, considering that chemical species with a shorter bond length are more stable. Thus, it is clear that the HREE enrichment at the bacterial cell surfaces is caused by the formation of the multiple phosphate surface complexes. Based on these results, it is suggested that materials having such phosphate sites such as bacteria and bacteria-related materials can induce anomalous HREE enrichment in natural systems.     

Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing,Southwestern China: Their mobilization,redistribution, and fractionation

In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO₂) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)_N and (La/Sm)_N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.     

Do Archean chemical sediments record ancient seawater rare earth element patterns?

Rare earth elements (REE) concentrations of Archean and Proterozoic chemical sediments are commonly used as proxies to study secular trends in the geochemistry of Precambrian seawater. In addition, similarities in the REE signatures of Archean chemical sediments and modern seawater have led researchers to argue that some Archean rocks originated as biochemical precipitates (i.e., microbial carbonates) in shallow marine (e.g., peritidal) environments. However, terrestrial waters, including river water and groundwater, also commonly exhibit REE fractionation patterns that resemble modern seawater. Here, we present the seawater-like REE data for groundwaters from central México as additional evidence that these patterns are not unique to the marine environment. The shale-normalized REE patterns of the groundwaters are compared to those of modern seawater (open ocean and nearshore), Holocene reefal microbial carbonates and corals, and Archean chemical sediments using statistical means (i.e., ANOVA and Wilcoxon analyses) in order to quantify the similarities and/or differences in the REE patterns. Shale-normalized (SN) Ce anomalies and measures of REE fractionation [i.e., (La/Yb)_SN, (Pr/Yb)_SN, (Nd/Yb)_SN, and (Gd/Yb)_SN] of the central México groundwater samples are statistically indistinguishable from those of modern seawater. Moreover, except for differences in the Ce anomalies, which are lacking in Archean chemical sediments, the REE patterns of the central México groundwaters are also statistically similar to REE patterns of Archean chemical sediments, especially those of the 3.45 Ga Strelley Pool Chert. Consequently, we suggest that without additional information, it may be premature to unequivocally conclude that Archean chemical sediments record REE signatures of an Archean ocean.