Low Rare Earth Element Contents Research Articles

Effect of Pr concentration on the magnetic properties and phase evolution of Ti-substituted Pr yFe 88− yTi 2B 10 ( y = 9.5–7) nanocomposite ribbons

Magnetic properties, phase evolution, and microstructure of melt spun Pr y Fe 88− y Ti 2B 10 nanocomposite ribbons with different Pr concentration ( y = 9.5–7) have been investigated. For Pr 9.5Fe 78.5Ti 2B 10, only the 2:14:1 phase and the α-Fe phase existed, while an additional Fe 3B phase appears for the ribbons with y = 9–7.5. On further decrease of the Pr content ( y = 7), one extra phase, 2:23:3, is found. The exchange coupling effect, as evidenced by a Henkel plot is found, to decrease with the decrease of the rare earth content, which is attributed to the increasing of grain size with decreasing of Pr content. The magnetic properties of the ribbons first improve with the decrease of y, reaching a maximum at y = 8.5 ( B r = 9.6 kG, i H c = 8.5 kOe, and (BH) max = 17.8 MGOe), then decrease with further decrease of the Pr content ( B r = 9.8 kG, i H c = 5.6 kOe, and (BH) max = 14.5 MGOe for y = 7). In comparison with the Ti-free Pr x Fe 90− x B 10 ( x = 9.5–7) ribbons, improved magnetic properties with higher intrinsic coercivity can be obtained in Pr y Fe 88− y Ti 2B 10 nanocomposites. The maximum magnetic properties of Pr y Fe 88− y Ti 2B 10 nanocomposites can be found in the ribbons with lower rare earth content, i.e. x = 9.5 for the former and y = 8.5 for the latter.

Rare earth element concentrations and speciation in organic-rich blackwaters of the Great Dismal Swamp, Virginia, USA

Concentrations of rare earth elements (REE), major inorganic solutes, and dissolved organic carbon (DOC) were measured in waters from the Great Dismal Swamp in southeastern Virginia, including Lake Drummond (pH=4.3), and its principal surface inflows and outflow. Concentrations of REE are high in the lake (average Nd=8 nmol/kg), and the acidic DOC-rich inflow and out flow waters (i.e., Nd=12.5 nmol/kg in Jericho–Washington Ditch and ∼8 nmol/kg in Feeder Ditch, respectively). The slightly acidic (pH 6–6.4) inflow waters from west of the Suffolk Scarp that comprise the chief source of surface water flow into Lake Drummond exhibit lower REE concentrations (Nd=3.4–7 nmol/kg). Great Dismal Swamp waters are all enriched in the middle REEs compared to the light and heavy REEs (LREE and HREE) when normalized to upper continental crustal (UCC) values. Rare earths are positively correlated to DOC concentrations in Great Dismal Swamp waters and inversely correlated with pH. Speciation calculations using a modified version of the well established Humic Ion-Binding Model V predict that organic matter complexes are the predominant form of dissolved REEs in Great Dismal Swamp waters despite significant competition from aqueous Fe and Al for organic matter binding sites. Adsorptive cathodic stripping voltammetry (ACSV) was employed, using a previously developed competitive ligand equilibration approach that involved adding the o-cresolphthalexon (OCP) ligand, to directly investigate REE complexation with natural organic ligands in Great Dismal Swamp waters. Although preliminary, the results of the ACS voltammetric titrations are consistent with dominance of organic matter complexes of REEs in Great Dismal Swamp waters. The results of the speciation modeling and the voltammetric titrations all point towards control of dissolved REEs in Great Dismal Swamp waters by complexation of these heavy metals with natural organic matter.

Controls on the distribution of rare earth elements in shallow groundwaters

Shallow groundwater samples (<0.22μm) collected from a small catchment in Western France (Petit Hermitage catchment) were analyzed for their rare earth elements (REE), dissolved organic carbon (DOC) and trace-element (Fe, Mn, Th and U) contents, with the aim to investigate the controlling factors of REE signatures. Two spatially distinct water types are recognized in this catchment based on changes of REE concentrations and variations of Ce anomalies. These include (i) DOC-poor groundwater flowing below the hillslope domains; this type has low REE contents and records conspicuous negative Ce anomalies; (ii) DOC-rich groundwater from the wetland domains, close to the river network; this type displays much higher REE concentrations, and typically lack negative Ce anomalies. Temporal REE concentration variations were assessed in wetland waters. Results show a marked increase of the REE content at the winter–spring transition, along with variations of DOC, Fe and Mn contents and redox potential changes. Using the above data set in conjunction with previously published results on comparable shallow groundwaters, we demonstrate that factors such as aquifer-rock composition or anthropogenic input probably play a minor role in determining the REE signatures of shallow groundwaters. Most likely, the two dominant factors involved are the organic matter content and the redox status of the waters. We suggest that topography might be the ultimate key parameter by its ability to control the DOC content of groundwater at a catchment scale.

Light Rare Earth Elements enrichment in an acidic mine lake (Lusatia, Germany)

The distribution of Rare Earth Elements (REE) was investigated in the acidic waters (lake and groundwater) of a lignite mining district (Germany). The Fe- and SO4-rich lake water (pH 2.7) displays high REE contents (e.g. La∼70 μg/l, Ce∼160 μg/l) and an enrichment of light REE (LREE) in the NASC normalised pattern. Considering the hydrodynamic model and geochemical data, the lake water composition may be calculated as a mixture of inflowing Quaternary and mining dump groundwaters. The groundwater of the dump aquifer is LREE enriched. Nevertheless, the leachates of dump sediments generally have low REE contents and display flat NASC normalised patterns. However, geochemical differences and REE pattern in undisturbed lignite (LREE enriched pattern and low water soluble REE contents) and the weathered lignite of the dumps (flat REE pattern and high water soluble REE contents) suggest that lignite is probably the main REE source rock for the lake water.

Trace element mobility during spheroidal weathering of basalts and andesites in Hawaii and Guatemala

Trace element mobility has been studied in spheroidally weathered corestone–shell complexes developed on Late Tertiary and Quaternary volcanic rocks. The parent materials are basalts and andesites related to (1) a subduction zone (Tecuamburro and Moyuta Volcanoes, Guatemala) and (2) a hot spot (Hawaiian Islands of Oahu, Maui and Hawai'i). The initial major element compositions are largely preserved in the corestones; each corestone sample shows major element abundances characteristic of the different magmatic processes through which the lavas were generated. However, the changes in trace element compositions due to weathering show similar patterns in the corestone–shell complexes independent of the tectonic setting and age. Element mobility within the different corestone–shell complex was determined using an isovolumetric reference frame. In this method, bulk density is chosen as an independent measure of the extent of weathering. Based on volumetric concentrations, the notion of element immobility cannot be applied to all the corestone–shell complexes. Elements commonly assumed to be immobile (e.g., Zr, Ti, and Al) vary in volumetric concentration for some corestone–shell complex. Despite petrographic evidence of only minor weathering, two corestones (Oahu and Tecuamburro) have become enriched in REE 3+ relative to other lava flows from the same volcanoes, and display negative Ce anomalies. The exfoliated shells from these complexes tend to have lower concentrations of REE 3+, and have positive Ce anomalies. The fractionation of Ce from the other rare earth elements (REE) is inferred to be a manifestation of changes in the valence state of Ce from trivalent to tetravalent under oxidizing conditions. In settings where the development of corestone–shell complex is advanced, an acidic solution can mobilize trivalent REE, leach them out of more extensively weathered materials, and transport them to less acidic portions of the regolith undergoing incipient weathering. The precipitation of the REE 3+ in the rinds and centers of the corestones are likely due to increases of the pH of the weathering solution. Systematic weathering-induced REE behavior (enrichment of most REE, with lesser Ce enrichment resulting in a negative Ce anomaly) can be discerned even in corestone samples that exhibit major element abundances and petrographic features otherwise similar to unweathered basalts and andesites.

Rare earth element behavior in soils and salt pan sediments of a semi-arid granitic terrain in the Western Cape, South Africa

The rare earth element (REE) contents of soils and pan sediments of the Darling Hills granitic terrain of southwestern South Africa were determined to study the behavior of REE during weathering in a semi-arid, Mediterranean climate. The P-rich, S-type bedrock granite has a middle REE (MREE) enrichment relative to post-Archean Australian sedimentary rocks (PAAS) that primarily reflects the REE content of feldspar and accessory apatite. Initial chemical weathering of the granite results in the loss of REE, particularly Ce, Eu and the heavy REE (HREE). The enrichment of REE in soils relative to bedrock granite corresponds to clay mineral and amorphous Fe oxide content in the case of the light REE (LREE) and precipitation of pedogenic calcite in the case of the HREE. Quartz sand dilutes the REE content, and concentration of K-feldspar produces positive Eu anomalies. Cerium is rapidly precipitated during weathering and retained in clayey, upper soil horizons as insoluble CeO 2. Most pan sediments have a bowl-shaped REE pattern that is derived from the accumulation of eroded catchment soils modified by precipitation of evaporative carbonates. REE enrichment, particularly LREE, and the large negative Ce anomaly of oxidized basal pan sediment relative to the bedrock granite suggest that REE entering the pan by groundwater flow are sorbed onto amorphous Fe oxides. In contrast, goethite-rich nodules from the pan have a low REE content and show HREE enrichment relative to the granite. Precipitation of pedogenic calcite and evaporative pan carbonates greatly increases the retention of the HREE in these semi-arid soils. Diagenetic pan dolomite has a REE pattern similar to pedogenic and evaporative calcite, but with a significantly lower REE content. The REE patterns indicate that, although most abundant in the bedrock granite, the MREE are least retained by soils during weathering. The LREE are generally retained by clayey and amorphous Fe-oxide-rich soils and sediments, whereas HREE are retained by precipitation of soil and pan carbonates.

Microstructures and magnetic properties of hydrogenation disproportionation desorption recombination-processed Nd–Fe–B materials with different Nd content of 11.0 and 12.6 at.%

The hydrogenation disproportionation desorption recombination (HDDR) process was performed on the generally used alloy composition of Nd 12.6Fe 63.1Co 17.4Zr 0.1Ga 0.3B 6.5 and a low rare earth content alloy composition of Nd 11.0Fe 65.0Co 17.8Zr 0.1Ga 0.3B 5.8. A detailed evaluation was made of the relationship between the microstructure and magnetic properties of these HDDR-processed magnetic powders with respect to their different rare earth element concentrations. The HDDR-processed powders of both alloy compositions were transformed to the Nd 2Fe 14B phase consisting of fine recombined crystal grains of around 400–500 nm in size and maintained the anisotropic magnetic characteristic that was present before HDDR processing. However, reduction of the rare earth content drastically reduced coercivity, and the alloy composition of Nd 11.0Fe 65.0Co 17.8Zr 0.1Ga 0.3B 5.8 did not manifest magnetic properties. From the results of an examination of their microstructures, it was inferred that the coercivity decreased due to a decline in the concentration of the rare earth element at the grain boundaries of the fine Nd 2Fe 14B grains with the reduction of the rare earth content of the alloys. Accordingly, in magnetic powders obtained by the HDDR process, the nucleation type of coercivity mechanism predominates, in which rare earth-rich regions present at the grain boundaries of fine Nd 2Fe 14B grains play a large role in the manifestation of coercivity.

Crystallization behavior and magnetic properties of the low rare-earth content (Nd=6 at. %) Nd–Fe–Co–V–B nanocomposite magnet ribbons prepared by rapid quenching method

The low rare-earth content α-Fe/Nd2Fe14B nanocomposite magnets having a high soft phase ratio were fabricated by the rapid quenching method and evaluations were made of the crystallization behavior, microstructure, and magnetic properties in the annealing process. An investigation of the relationship between the roller surface velocity, Vs, and magnetic properties showed that changes in the crystal grain size of the α-Fe phase following annealing could be attributed to differences in the amorphous content of the as-melt-spun ribbons. As the amorphous content was increased, their crystal grain size tended to become finer and their magnetic properties tended to improve. However, it was observed that the optimum rapid quenching rate exists. The slight presence of clusters in as-melt-spun ribbons is favorable for obtaining good magnetic properties because the growth of α-Fe crystals is inhibited. The presumed reason for this inhibiting effect is that α-Fe growth occurs in a process in which clusters containing Nd expel this element. In the case of a completely amorphous state, large crystals grew easily, because they consisted of only Fe and Co in the initial stage.

Investigation of high saturation magnetization of low rare-earth content α-Fe/Nd2Fe14B bulk exchange-spring magnets

An investigation was made of the magnetic properties of Nd–Fe–Co–V bulk exchange-spring magnets fabricated by spark plasma sintering and consisting of a hard Nd2Fe14B phase and a soft α-Fe phase and having a low rare-earth content of Nd=2–7 at. %. The high saturation magnetization of the bulk exchange-spring magnets was confirmed experimentally, and the measured data agreed well with the saturation magnetization values estimated from their respective rare-earth content. The saturation magnetization values were nearly uniform and were not dependent on changes in the grain size. Additionally, the specific resistance of the samples nearly coincided with the values interpolated from the specific resistances of an ordinary sintered pure Fe bulk compact and a sintered Nd–Fe–B magnet. These results suggest that the α-Fe/Nd2Fe14B bulk exchange-spring magnets did not contain any appreciable grain boundary phase between the fine nanoscale-size grains or tertiary phase.

Dating of zircon from Ti-clinohumite–bearing garnet peridotite: Implication for timing of mantle metasomatism

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554–494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite–bearing garnet peridotites.

Geochemistry and petrography of gold-quartz-tourmaline veins of the Okote area, southern Ethiopia: implications for gold exploration

Quartz-tourmaline vein-hosting rocks of the Okote area belong to the Neoproterozoic Adola Belt. Metasomatic auriferous quartz-tourmaline veins occur in ductile N–S trending, sinistral shear zones. These veins commonly contain quartz, carbonates, and tourmaline, with minor pyrite, and accessory chalcopyrite, pyrrhotite, and gold. Tourmaline forms isolated euhedral crystals in the fracture surfaces of quartz carbonate veins. Many of the tourmaline crystals are optically zoned with a bluish core and a bluish to brown rim. Electron microprobe analyses show that the tourmalines comprise an intermediate dravite-schorl solid solution with a mean FeO/(FeO + MgO) = 0.47. Abrupt transitions between the colour zones within single tourmaline crystals are accompanied by relative variations in the FeO/(FeO + MgO) ratios. The tourmaline separates indicate that the tourmalines contain highly variable average contents of trace elements. Chondrite-normalized rare earth element (REE) abundances of tourmaline separates from auriferous veins show LREE-enriched to LREE-depleted patterns with negative to positive Eu anomalies and a flat, near-chondritic HREE pattern. The auriferous quartz-tourmaline veins have LREE-enriched patterns without a Eu anomaly and a flat HREE pattern, but tourmaline-free gold-quartz veins have very low REE contents and LREE-depleted patterns also without Eu anomalies. The FeO/(FeO + MgO) ratios, major and trace element compositions, and the types of wall-rock alteration are used to suggest that the sources of boron are dominantly metamorphic (dehydration and devolatilization processes), but do not totally exclude the possibility of a magmatic source. The occurrences of high-grade gold associated with tourmaline make tourmaline a valuable prospecting guide for hydrothermal gold mineralization in the Adola Belt, southern Ethiopia.

磁性材料 低希土類元素含有（Ｎｄ＝２〜７ａｔ．％）α‐Ｆｅ／Ｎｄ２Ｆｅ１４Ｂｔｙｐｅバルク交換スプリング磁石の磁気特性

Magnetic properties and structures of low rare-earth content (Nd=2-7at.%) α-Fe/Nd2e14B type Nd-Fe-Co-V-B system bulk exchange-spring magnets prepared by the Spark Plasma Sintering (SPS) method were investigated. In this range of Nd content, the bulk magnets showed the same magnetic properties as those of the annealed ribbons used as the raw materials. The saturation magnetization and soft/hard phase ratio corresponded with the estimated values. However, the exchange coupling strength decreased conspicuously with a lower Nd content presumably as a result of an increase in the grain size.

Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy

With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth element (REE) in the natural water systems, the ground and surface waters of the Ottana–Orani area (Central Sardinia, Italy) were sampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of 6.0–8.6, total dissolved solid (TDS) of between 0.1 and 0.6 g/l, and major cation composition dominated by Ca and Na, whereas predominant anions are Cl and/or HCO 3. The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution. The concentrations of ∑REE+Y in the samples filtered at 0.4 μm vary between 140 and 1600 ng/l, with La of between 14 and 314 ng/l, and Yb of between <6 and 12 ng/l. A negative Ce anomaly, especially marked at high pH, is observed in the groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ce anomaly. Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO 3 2− ligand are the dominant REE species at pH in the range of 6.7–8.6. The REE 3+ ions dominate the speciation at pH <6.7 and only in the light REE (LREE). The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, when concentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REE (HREE) into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution.

Magnetic characterization of melt spun nanocomposite Nd 4Fe 77.5B 18.5 alloys

Magnetic properties of nanoparticles in melt spun Nd 4Fe 77.5B 18.5 alloys with low rare earth content and effect of thermal annealing have been investigated. The unannealed alloy samples show a high magnetization of 1.7 T with practically no coercivity which is indicative of the formation of metastable Fe 23B 6 phase and the absence of any hard magnetic phases in the unannealed sample. On annealing at 600°C, a symmetric rectangular type hysteresis loop with a saturation magnetization value of 1.44 T was obtained indicating the remanence enhancement due to exchange coupling between the nanocrystalline phases. The coercivity is relatively low at 0.17 T which is attributed to the nanocrystalline precipitation of soft magnetic grains having a dimension larger than twice the wall width of the hard phase and to the inhomogeneous grain structure. Weakening of exchange interaction due to the decomposition of Fe 23B 6 phase and coarsening of crystallite grains due to longer annealing (30 min) have been shown to be responsible for the subsequent deterioration in magnetic properties.

The nature of gold-bearing palaeochannel sediments in the Gidji area north of Kalgoorlie, Western Australia

Remnants of an extensive palaeodrainage network are preserved in the Kalgoorlie region of Western Australia as buried palaeochannels. At Gidji a major palaeochannel up to 60 m deep, is infilled with sand- and clay-rich facies and overlain by younger alluvial/colluvial sands and gravels, aeolian sands and silts, playa deposits, red soils and lag materials. The palaeochannel sediments consist predominantly of low-crystallinity kaolinite, quartz and minor smectite. They contain very little muscovite, illite or chlorite and can be readily distinguished from underlying in situ saprolite by their chemical characteristics (particularly K/Al and Mg/Al ratios, low rare earth element contents and low K/Rb ratios). These sediments were largely deposited during the Mid-Late Eocene as an homogenised mixture derived from a low relief and highly weathered pre-Eocene landsurface. They have undergone post-depositional weathering resulting in mobilisation and redeposition of iron as oxides/oxyhydroxides. The stratigraphy of the palaeochannel sediments indicates a change from dominantly fluvial to fluvio-lacustrine conditions during deposition. In areas of gold mineralisation, the presence of palaeochannels complicates geochemical exploration. Areas of thick palaeochannel clays mask bedrock anomalies. Placer and supergene concentrations of gold associated with the palaeochannels can be confused with bedrock anomalies unless a multi-element approach is taken. The Gidji palaeochannel environment contains a combination of bedrock and placer/supergene gold anomalies. Primary bedrock anomalies generally have As, Sb and W as associated pathfinder elements. Some pathfinder elements (e.g. As) have been concentrated in ferruginous pisoliths and gravels or mechanically dispersed in near surface alluvial/colluvial materials giving rise to false anomalies.

Acid‐sulphate Type Alteration and Mineralization in the Desmos Caldera, Manus Back‐arc Basin, Papua New Guinea

Abstract: The Onsen acid‐sulphate type of mineralization is located in the Desmos caldera, Manus back‐arc basin. Hydrothermal precipitates, fresh and altered basaltic andesite collected from the Desmos caldera were studied to determine mineralization and mobility of elements under seawater dominated condition of hydrothermal alteration.The mineralization is characterized by three stages of advanced argillic alteration. Alteration stage I is characterized by coarse subhedral pyrophyllite with disseminated anhedral pyrite and enargite which were formed in the temperature range of 260–340°C. Alteration stage II which overprinted alteration stage I was formed in the temperature range of 270–310°C and is characterized by euhedral pyrite, quartz, natroalunite, cristobalite and mixed layer minerals of smectite and mica with 14–15 Å XRD peak. Alteration stage III is characterized by amorphous silica, native sulphur, covellite, marcasite and euhedral pyrite, which has overprinted alteration stages I and II.Relative to the fresh basaltic andesite samples, the rims and cores of the partly altered basaltic andesite samples have very low major, minor and rare earth elements content except for SiO2 which is much higher (58–78 wt%) than SiO2 content of the fresh basaltic andesite (55 wt%). REE patterns of the partly altered basaltic andesite specimens are variably depleted in LREE and have pronounced negative Eu anomalies. Normalization of major, minor and REE content of the partly altered basaltic andesites to the fresh basaltic andesite indicates that all the elements except for SiO2 in the partly altered basaltic andesite are strongly lost (e.g. Al2O3 = ‐8.3 to ‐10.9 g/100cm3, Ba = ‐2.2 to ‐5.6 mg/100cm3, La = ‐130 to ‐200 μg/100cm3) during the alteration process. Abnormal depletion of MgO, total Fe as Fe2O3, LREE especially Eu and enrichment of SiO2 in the altered basaltic andesites from the Desmos caldera seafloor is caused by interaction of hot acidic hydrothermal fluid, which originates from a mixing of magmatic fluid and seawater.

Effects of apatite and calcium oxyphosphate on speciation and bioavailability of exogenous rare earth elements in the soil-plant system

The effects of applying apatite and calcium oxyphosphate on the bioavailability and distribution of exogenous rare earth elements (REEs) among different soil phases were investigated through pot experiments, speciation analysis and single extraction. The results showed that calcium oxyphosphate could reduce soil pH and that apatite could increase soil alkalinity. Both additives could transform exchangeable REEs into other phases and reduce their bioavailability, with Ca(H2PO4)2 being more effective in this respect. Mechanisms involved were also discussed. Among many processes, H2PO4− played a dominant role in affecting REE speciation in soil by influencing the Gibbs free energy changes. Results from single extractions and pot experiments showed that even though 0.1M HCl and 0.05M EDTA (pH7.0) had a strong extracting ability, they did not reflect the REEs’ bioavailability in soil. However, NH4OAc (1M), a weak extractant, could be used as a method for reducing REEs bioavailability. Its extracting ability could be further improved by increasing its acidity, particularly useful in soil with low REE content. It was suggested that phosphate should be taken into consideration when studying the environmental behavior of REEs or establishing a model for predicting the environmental risk of REEs in the soil-plant system.

The distribution of rare earth elements in groundwaters: assessing the role of source-rock composition, redox changes and colloidal particles

Rare earth element (REE), dissolved organic carbon (DOC) and trace-element (Al, Mn, Fe, Sr, Ba, U and Th) concentrations were measured in fourteen well water samples (<0.22 μm) and one spring located along two transects set up in a catchment from Western Europe (Kervidy/Coët-Dan catchment, France). Previous hydrological and hydrochemical (NO 3 −, SO 4 2−) investigations demonstrated that three chemically and spatially distinct groundwater types are present in this catchment, which is fully confirmed by the REE and DOC results. These include: (i) a shallow, organic-rich groundwater (4.4 < DOC < 34.6 mg/l) from the wetland areas, close to the river network. This first groundwater type, characterized by the development of temporary reducing conditions, records high and variable REE contents (2 < ΣREE < 16 ppb) and displays slight or no negative Ce anomaly (Ce/Ce∗ = 0.8–1.05); (ii) a shallow, organic-poor (DOC < 3 mg/l), NO 3 −-rich groundwater type (86.8 < NO 3 − < 155 mg/l) located in the weathered schists, below the hillslope domains. This second type corresponds to recently recharged, oxidized water and displays also high and variable REE concentrations (2 ppb < ΣREE < 15 ppb), but distinguish from the former by the occurrence of very strong negative Ce anomalies (Ce/Ce∗ = 0.05–0.10); finally (iii) a deep, organic-poor (DOC < 1 mg/l), nitrate-poor (NO 3 − close to 0.2 mg/l; detection limit) groundwater. This third type corresponds to reduced water flowing into the deep fresh schists and yields low to very low REE contents (ΣREE < 0.15 ppb) as well as slight negative Ce anomaly (Ce/Ce∗ = 0.8 to 0.9). Temporal REE concentration variations were assessed using samples regularly collected over a six month period. Results show that the spatially distributed Ce anomaly and REE pattern signatures are preserved throughout the studied period. By contrast, REE concentrations are quite variable through time, especially in the wetland waters where the REE concentrations are seen to vary in phase with both redox changes and DOC, Fe, U and Th content variations. Three REE-rich water samples (one DOC-rich and two DOC-poor) were also filtered through membranes of decreasing pore size (100,000 D, 30,000 D, 5,000 D). The results show that between about 40% to 65% of the REE present in the shallow, DOC-poor groundwater samples are controlled by the colloidal fraction, which is likely to consist in these inorganic waters of a mixture of mineral phases. In the wetland groundwaters, the fraction of REE controlled by microparticles is higher than 65%, which confirms the predominant role of organic colloids as major REE carriers in wetland waters. Using the above data set in conjunction with analyses of soil samples, we show that the deep Ce anomalies found in the upper non organic part of the aquifer are probably not source-rock inherited features: most likely, these anomalies arise from the oxidative precipitation of Ce. The very low REE content displayed by waters flooding the deep fresh schists is interpreted as due to the combined effects of (i) pH variation, (ii) secondary sulfate mineral precipitation and (iii) the trapping of colloids-borne REE by the aquifer-rock pores. Data from wetland groundwaters show that the REE, Fe, U and Th budgets of these waters are mainly controlled by seasonal changes in redox conditions and organic matter content. However, unlike organic-poor waters, it appears difficult to relate the Ce behaviour in these organic-rich waters solely to redox conditions. It is likely that the complexation of Ce by organic colloids in the organic-rich waters may mask redox changes by inhibiting the development of negative Ce anomalies.

Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositions can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescence imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations in the relative growth velocities of crystallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does fluorapatite, which is locally abundant and has REE contents similar to that of scheelite. In this context, the systematic increase of the Eu-anomaly between type I and type II patterns is produced by the difference between the partition coefficients of Eu2+ and Eu3+, and not by fluid mixing or redox reactions. Consequently, the high positive Eu-anomaly typical of scheelite from gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concentrations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be highly sensitive to the dynamics of the hydrothermal system.

A new martian meteorite from the Sahara: The shergottite Dar al Gani 489

Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo79–59) set in a fine‐grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En75–57 Wo5–15) crystals and interstitial feldspathic glass (An67–56 Or0–1). Minor phases include enstatite (En82–71 Wo2–4), augite (En48–52 Wo29–32), chromite, Ti‐chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O‐isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn(bulk) = 42.1, Ni/Mg(bulk) = 0.002; δ17O = 2.89, δ18O = 4.98, and Δ17O = 0.305), belonging to the basaltic shergottite subgroup.The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites.Noble gases data show that DaG 489 has an ejection age of ∼1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites.In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ∼25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall.In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high‐degree partial melting of a lherzolite‐like material.