Redistribution Of Rare Earth Elements Research Articles

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.

Closure temperature in cooling bi-mineralic systems: I. Definition and with application to REE-in-two-pyroxene thermometer

Closure temperature is an important concept to many diffusion related problems involving cooling. The basic idea and formulation were outlined in the seminal work of Dodson for cooling mono-mineralic systems. The Dodson’s equation has been widely used to calculate closure temperatures for igneous and metamorphic rocks that contain more than one mineral. The purpose of this study is to examine closure temperatures in cooling bi-mineralic systems and to investigate the physical meaning of temperatures calculated using the REE-in-two-pyroxene thermometer. We conduct numerical simulations of diffusive redistribution of trace elements between two coexisting minerals under prescribed cooling using temperature-dependent diffusion coefficients and mineral–mineral partition coefficients. Following Dodson’s treatment, the closure temperature in bi-mineralic systems can be defined by the evolution of either average trace element concentrations in the two minerals or their ratio. The latter defines an effective partition coefficient. Closure temperatures calculated based on the two definitions are compared for a range of cooling rates, grain sizes, mineral proportions, and temperature-dependent partition coefficients and diffusion coefficients. Temperatures defined by the effective partition coefficient are recommended. Application to diffusive redistribution of rare earth elements (REE) in orthopyroxene–clinopyroxene systems demonstrates that closure temperature differences among REE are small and hence their average value may be used as the closure temperature for the cooling two-pyroxene system. The average closure temperature of REE in the two-pyroxene system is essentially the same as the temperature calculated using the REE-in-two-pyroxene thermometer and practically independent of pyroxene modal abundance in the system. Differences in temperatures calculated using the REE- and major element-based two-pyroxene thermometers can be used to infer cooling rate of two-pyroxene bearing mafic and ultramafic rocks.

An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites

The distribution of rare earth elements (REEs) in mantle minerals depends on temperature, pressure, and mineral composition. We developed parameterized lattice strain models for rare earth element partitioning among mantle minerals (olivine, orthopyroxene, clinopyroxene, and garnet) using mineral–melt partitioning models that were calibrated against experimentally determined high quality mineral–melt REE partition coefficients. The validity of the mineral–mineral REE partitioning models were independently tested using measured REE abundances in minerals from well-equilibrated mantle peridotites and eclogites reported in the literature. Using the new partitioning models, we critically assessed the extent of subsolidus re-equilibration on REE distribution among major rock-forming minerals in spinel and garnet peridotites. During subsolidus re-equilibration, REEs are preferentially partitioned into clinopyroxene in peridotites. The extent of REE redistribution in clinopyroxene depends on the modal abundance of clinopyroxene in the peridotite. Subsolidus exchange with orthopyroxene and olivine can increase heavy REE abundances in clinopyroxene from spinel harzburgite and middle REE abundances in clinopyroxene from garnet harzburgite by a factor of three. The higher than expected heavy REE abundances in clinopyroxene from spinel harzburgite may give rise to a false garnet signature in residual peridotites. The extents of melting for abyssal peridotites based on REE abundances in clinopyroxene can be significantly over-estimated without proper correction for subsolidus re-equilibration. REE abundances in both clinopyroxene and orthopyroxene are needed to assess the melting processes from residual peridotites.

Trace element budgets and (re-)distribution during subduction-zone ultrahigh pressure metamorphism: Evidence from Western Tianshan, China

We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an ultrahigh pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM.We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.

Hydrothermal mobilization of pegmatite-hosted REE and Zr at Strange Lake, Canada: A reaction path model

Petrological and geochemical observations of pegmatites in the Strange Lake pluton, Canada, have been combined with numerical simulations to improve our understanding of fluid–rock interaction in peralkaline granitic systems. In particular, they have made it possible to evaluate reaction paths responsible for hydrothermal mobilization and mineralization of rare earth elements (REE) and Zr. The focus of the study was the B-Zone in the northwest of the pluton, which contains a pegmatite swarm and is the target of exploration for an economically exploitable REE deposit. Many of the pegmatites are mineralogically zoned into a border consisting of variably altered primary K-feldspar, arfvedsonite, quartz, and zirconosilicates, and a core rich in quartz, fluorite and exotic REE minerals. Textural relationships indicate that the primary silicate minerals in the pegmatites were leached and/or replaced during acidic alteration by K-, Fe- and Al-phyllosilicates, aegirine, hematite, fluorite and/or quartz, and that primary zirconosilicates (e.g., elpidite) were replaced by gittinsite and/or zircon. Reaction textures recording coupled dissolution of silicate minerals and crystallization of secondary REE-silicates indicate hydrothermal mobilization of the REE. The mobility of the light (L)REE was limited by the stability of REE-F-(CO2)-minerals (basnäsite-(Ce) and fluocerite-(Ce)), whereas zirconosilicates and secondary gadolinite-group minerals controlled the mobility of Zr and the heavy (H)REE. Hydrothermal fluorite and fluorite–fluocerite-(Ce) solid solutions are interpreted to indicate the former presence of F-bearing saline fluids in the pegmatites. Numerical simulations show that the mobilization of REE and Zr in saline HCl–HF-bearing fluids is controlled by pH, ligand activity and temperature. Mobilization of Zr is significant in both saline HF- and HCl–HF-bearing fluids at low temperature (250°C). In contrast, the REE are mobilized by saline HCl-bearing fluids, particularly at high temperature (400°C). The LREE are more mobile than the HREE in saline HCl-bearing fluids due to the greater stability of LREE-chloride complexes. The simulated mineralogy is consistent with the zonation observed in the pegmatites and with fluid–rock interaction at conditions that were rock-buffered in the pegmatite borders (low fluid/rock ratio; and pH>4) and fluid-buffered in the cores (high fluid/rock ratio; pH⩽2). We propose a model in which saline HCl–HF-bearing fluids created pathways during acidic alteration from the pegmatite cores outward. This led to the mobilization of REE and Zr due to progressive alteration of primary silicate minerals and increased acidity upon cooling. The key requirement for REE and Zr mobilization in peralkaline igneous intrusions is the formation of an acidic subsystem with high fluid/rock ratios that increases the overall permeability of the rocks. In these zones, the extent of late stage hydrothermal redistribution and concentration of REE and Zr depends on the buffering capacity of the rocks and the availability of fluids that may produce autometasomatic rock alteration, interact with external rock units and/or mix with fluids from other sources.

THE REDISTRIBUTION OF RARE-EARTH ELEMENTS IN SECONDARY MINERALS OF HYDROTHERMAL VEINS, SCHWARZWALD, SOUTHWESTERN GERMANY

Minerals of the rare-earth elements (REE) occur as supergene phases in the Schwarzwald ore district, southwestern Germany. They form by alteration of hydrothermal fluorite – barite – quartz – carbonate veins with various associations, including Cu– Pb, Pb–Zn and Co–Bi–Ag–U assemblages in sandstones, gneisses and granites. The REE minerals, including mixite-group minerals ([REE,Bi,Ca,Pb]Cu 6 (AsO 4 ,AsO 3 OH) 3 OH 6 •3H 2 O), rhabdophane and churchite (REEPO 4 •H 2 O and REEPO 4 •2H 2 O), chukhrovite (Ca 3 REEAl 2 SO 4 F 13 •10H 2 O) and bastnasite (REECO 3 F), were analyzed by electron microprobe and LA–ICP–MS. In addition, REE concentrations in secondary fluorite, calcite and Mn oxides cogenetic with the REE minerals were determined by LA–ICP–MS. Results of analyses of 74 mixite-group samples from 20 localities in the Schwarzwald ore district show that continuous miscibility is possible between REE and Ca and between Bi and Ca end-members. In contrast, no miscibility seems to exist between the Bi and REE end-members, and only Ca-rich members can accommodate small amounts of both Bi and REE. The REE phosphates churchite and rhabdophane do not occur at the same locality in the Schwarzwald, which is probably dependent on which REE (light or heavy) predominate at a certain locality. Whereas churchite incorporates heavy REE (HREE), rhabdophane prefers light REE (LREE). Dependent on the source of the REE, either HREE or LREE dominate in an alteration fluid and, consequently, only one type of REE phosphate forms. The HREE dominate in such a fluid if REE originate from dissolved “pitchblende”, whereas the LREE dominate if the altered host-rock or dissolved fluorite are the source of the REE. The REE contents of Mn oxides and calcite cogenetic with REE-bearing mixite-group minerals show that the REE distribution of secondary minerals can be influenced by each other. Whereas the Mn oxides incorporate Ce 4+ resulting in a positive Ce anomaly, a cogenetic mixite-group mineral develops a negative anomaly, which shows that Ce anomalies are coupled. Calcite intergrown with a mixite-group mineral can only incorporate those REE that are not consumed by the latter. These processes involve the distribution of REE between minerals on the micrometer scale and do not take place until minerals precipitate. However, sorption and complexation processes also take place during transport on a larger scale and produce decoupled Ce anomalies. This interplay between large- and small-scale processes results in a complex redistribution of REE during remobilization.

How mineralogy and slope aspect affect REE release and fractionation during shale weathering in the Susquehanna/Shale Hills Critical Zone Observatory

Abstract To understand the factors that control rare earth elements (REE) release and fractionation during shale weathering, we investigate the REE contents in solid (bedrock, regolith, stream sediments), and natural waters (stream, and pore waters) from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. Up to 65% of the REE (relative to parent bedrock) is depleted from the weathering profiles in the acidic and organic-rich soils due to chemical leaching. In addition, newly formed fine particles were also lost along with the down-slope movement of soil waters. Weathering profiles on the south-facing slope show less depletion of REE than those on the north-facing slope (33% vs. 45% on average). We hypothesize that the different degrees of REE depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates, controlled by contrasting slope aspect-induced microclimate conditions. In addition, weathering profiles, natural waters and sequential extractions all show a preferential removal of Middle REE (up to 22% more) relative to Light REE and Heavy REE during shale weathering, due to preferential release of MREE from rhabdophane. Furthermore, the long-term phosphate mineral dissolution rates (e.g., rhabdophane) were estimated at 10 − 15 to 10 − 14 mol m − 2 s − 1 under field conditions, based on REE depletion profiles. Strong positive Ce anomalies (average [Ce/Ce*] N value: 1.79) observed in the regolith, stream sediments, and regolith extractions point to the fractionation and preferential precipitation of Ce as compared to other REE, due to the generally oxidizing conditions during release, transport, and redistribution of REE in the surface and subsurface environments. Positive Eu anomalies (average [Eu/Eu*] N value: 1.30) observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock. This study highlights the use of REE as natural tracers for low-temperature geochemical processes.

REE redistribution during hydrothermal alteration of ores of the Kalahari Manganese Deposit

The Kalahari Manganese Deposit (KMD) is the largest land-based manganese deposit, hosting approximately 80% of the world's known, mineable manganese resources. The deposit, located near Kuruman in the Northern Cape Province of South Africa, is one of five erosional relics of the Paleoproterozoic (ca. 2.2Ga) Hotazel Formation, with sedimentary manganese ores occurring as up to 50m thick beds interbedded with banded iron-formation (BIF) and hematite lutite.The study focuses on the manganese ores of the Nchwaning–Gloria mining area of the northern KMD. In this area, pronounced mineralogical and major element alteration was imparted on the sedimentary manganese ores by a structurally-controlled hydrothermal fluid flow event. Most notable effects of hydrothermal alteration are the decomposition and leaching of Ca- and Mg-carbonate, and marked residual enrichment of manganese. On the basis of mineral assemblage, grade, texture and geochemical characteristics, three ore types were distinguished in the studied sample set, classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. Advanced altered ores may be further classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. The rare earth element (REE) geochemistry of these fundamental ore types was studied in detail, to document REE mobility during hydrothermal alteration.Total REE concentrations in LA ores were found to be very low (14–22ppm) and remarkably uniform, within the range typically observed for BIF. Hydrothermal alteration results in residual enrichment and a much larger scatter in REE contents. A small Ce anomaly observed in the protolith remains similar in magnitude when observed in PAAS-normalised REE plots. The data define, however, a power trend in the (Ce/Ce*) vs (Pr/Pr*) diagram. Such behaviour is interpreted in terms of a conservative system that was predominantly protolith-buffered. Local remobilisation of REE during hydrothermal alteration is attributed to the dissolution of diagenetic apatite and redistribution of hydrothermal trace minerals, including neoformed apatite, monazite and cerianite.

Behaviour of rare earth elements and yttrium in ferromanganese concretions, gibbsite spots, and the surrounding terra rossa over dolomite during chemical weathering

The behaviour of rare earth elements (REEs) and yttrium (Y) during chemical weathering processes has been investigated on a 4.05 m thick terra rossa profile over dolomite on the Yunnan–Guizhou Plateau, China. In this profile, ferromanganese concretions and gibbsite spots coexist in the terra rossa solum. Analyses of REEs, Y, Zr, F, S, and TOC, as well as mineralogical studies, were conducted on a suite of ferromanganese concretions, gibbsite spots, terra rossas, dolomites, and insoluble residues from the underlying dolomite. These analyses helped us to understand the mobilization, redistribution, and fractionation of REEs and Y during chemical weathering. The REEs and Y are mobilized and redistributed during the terra rossa formation. REEs, except for Ce, are removed from the upper and middle profiles, transferred downwards, and then precipitated in the base profile, resulting in remarkable enrichment in the terra rossa near the weathering front. The significant increase of pH near the weathering front was responsible for REE (excluding Ce) and Y enrichment in the base profile. Y is quite mobile during extreme chemical weathering, and most of it was carried away from the profile. Because of very efficient oxidative fractionation of Ce, a significant positive Ce anomaly in terra rossa was found in the middle of the profile, whereas the terra rossa near the weathering front exhibited notably negative Ce anomalies. Moreover, the presence of cerianite and its content in the ferromanganese concretions can explain the markedly positive Ce anomaly and the variation of Ce/Ce * values with depth. In profile, these Ce anomalies are characteristic of the geochemical environment, especially the redox condition. Studies of REE distribution in the ferromanganese concretions, gibbsite spots, and surrounding terra rossa revealed that significant REE fractionation occurred. REEs and Y were preferentially concentrated in the ferromanganese concretions rather than in the gibbsite spots. The water–rock interaction resulted in M-type tetrad effects in some of the ferromanganese concretions, gibbsite spots, and terra rossa samples. In addition, the tetrad effect in terrestrial weathering processes likely played an important role in the fractionation of REEs and Y, such as Y–Ho, Sm–Nd, and the Eu anomaly change.

Crystallization of REE minerals and redistribution of U, Th, and REE at contact boundary between granite and gabbro during hydrothermal alteration

In this study, redistribution behaviors of U, Th, and rare earth elements (REE) in association with igneous activity in the Unazuki region of southwest Japan were investigated. At the contact boundary between granite and gabbro, REE minerals such as monazite, xenotime, and REE oxides formed with Fe–Ti oxides in K-feldspar assimilated by gabbro. U was enriched in xenotime (UO 2 > 0.72 wt.%) and REE oxides (UO 2 > 0.10 wt.%), whereas Th was concentrated in REE oxides (ThO 2 > 1.32 wt.%). Considering the similarity between the ionic radii of U 4+ and Y 3+, U was incorporated into xenotime and REE oxides together with heavy REE. Th incorporation into xenotime (eightfold coordinated site) and monazite (ninefold coordinated site) was constrained by the lower contents of Gd and Dy, respectively. Saussuritization of plagioclase in gabbro and chloritization of biotite in granite at the contact boundary suggest low-temperature hydrothermal alteration at late stages of gabbro intrusion that occurred at 253 ± 1 Ma. In contrast, zircon in the chloritized granite contained altered domains characterized by a dark response between back-scattered electron and cathodoluminescence images; this was caused by a low average atomic number due to the release of Zr and Hf; the incorporation of non-formula elements such as Ca, Fe, and Al; and low crystallinity because of radiation damage. The altered domains showed enrichments of Th (ThO 2 > 1.50 wt.%) and light REE (LREE) to middle REE (MREE) as well as positive Eu anomalies; this suggests the preferential incorporation of Th and LREE–MREE (particularly Eu 2+) with Ca occurring during the hydrothermal alteration. The redistribution behaviors of U, Th, and REE during low-temperature hydrothermal alteration strongly depend on their ionic radii. Thorite in zircon was probably formed by the exsolution of high-Th altered domains.

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO2-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge.

Elemental distribution in apatite, titanite and zircon during hydrothermal alteration: Durability of immobilization mineral phases for actinides

In situ isotopic analyses of U, Pb and rare earth elements (REE) in apatite and titanite were conducted for tuff samples collected from Bidoudouma, the Republic of Gabon, in order to assess the stability of accessory minerals during hydrothermal alteration and to deduce the alteration conditions. The U–Pb results reveal that titanite of detritus origin (2.8–2.7 Ga) released Pb by igneous activity at ca. 560 Ma, in association with the dolerite dyke intrusion. The hydrothermal fluid associated with the igneous activity interacted with titanite and zircon; this resulted in elemental redistribution. The Ti content of the corroded titanite is 9–24% lower than that of unaltered titanite, whereas the Al, Fe and Ca contents are higher than those of unaltered titanite (Ca: >10% of the Ca contents in unaltered titanite; Al and Fe: >twice the Al and Fe contents in unaltered titanite); this indicates that Ca was incorporated into the corroded titanite together with Ti substitution for Al and Fe. Ca enrichments in the corroded titanite and altered zircon led to the incorporation of Pb and Eu 2+ during the alteration, and this resulted in positive Eu anomalies of the corroded titanite and altered zircon. The U–Pb data and REE distributions suggest that Pb was incorporated into both the minerals; however, U was released from the corroded titanite and incorporated into the altered zircon during the hydrothermal alteration. On the other hand, apatite grains were not affected by the hydrothermal alteration at ca. 560 Ma. Analyzing the mobilization of Ti and Zr and the solubility of the apatite, it can be inferred that the redistribution of REE, U and Pb in titanite and zircon occurred under alkaline conditions. The U–Pb and REE data suggest that apatite is a good immobilization phase for REE, Pb and U under alkaline conditions (pH > 8).

Redistribution of REE, Pb and U by supergene weathering studied from in-situ isotopic analyses of the Bangombé natural reactor, Gabon

The isotopic analyses of rare earth elements (REE), Pb and U in several kinds of minerals from the clay and black shale layers above the Bangombé natural reactor, Gabon, were performed using a sensitive high-resolution microprobe (SHRIMP) to investigate the migration and retardation processes of fission products released from the reactor. REE isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays, which reveals that the clays play an important role in preventing fission products from spreading. Zircon crystals in the clays heterogeneously contain high-U regions (up to 28.3 wt%) with normal 235U/ 238U ratios (=0.00725) and significant amounts of fissiogenic REE, which suggest the occurrence of significant chemical fractionation between REE and U during the dissolution of reactor uraninite and the recrystallization of secondary U minerals. The Pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event, and that a significant amount of Pb was derived from 2.05-Ga-old original uraninite rather than reactor uraninite. The U–Pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2.8 Ga and geochemical evidence of the incomplete mixing of independent Pb and U sources. This result is consistent with previous chronological results in this area.

Redistribution of rare earth elements in aquifers in Sikhote-Alin Range, Russia

Redistribution of rare earth elements in aquifers in Sikhote-Alin Range, Russia

The effect of oxygen on segregation-induced redistribution of rare-earth elements in silicon layers amorphized by ion implantation

A model of segregation-induced redistribution of impurities of rare-earth elements during solid-phase epitaxial crystallization of silicon layers amorphized by ion implantation is developed. This model is based on the assumption that a transition layer with a high mobility of atoms is formed at the interphase boundary on the side of a-Si; the thickness of this layer is governed by the diffusion length of vacancies in a-Si. The Er concentration profiles in Si implanted with both erbium and oxygen ions are analyzed in the context of the model. It shown that, in the case of high doses of implantation of rare-earth ions, it is necessary to take into account the formation of Rm clusters (m = 4), where R denotes the atom of a rare-earth element, whereas, if oxygen ions are also implanted, formation of the complexes ROn (n = 3–6) should be taken into account; these complexes affect the transition-layer thickness and segregation coefficient.

Hydrothermal redistribution of rare-earth elements in Pingxiang Dacite, Guangxi

Distribution of the rare-earth elements (REE) in dacite has been studied so as to get a better understanding of the migration behavior of REE during alteration. Both unaltered and altered samples were collected in an unpolluted area of Guangxi Zhuang Autonomous Region, southwest China. The REE concentrations were analyzed by ICP-MS. It is concluded that the REE were enriched during dacite alteration in varying degrees. The chondrite-normalized REE patterns of altered samples approximately maintain the characteristics of unaltered samples. However, if we normalize the REE concentrations of altered samples with unaltered dacite, fractionation of REE will appear. The LREE are more enriched than HREE in all altered samples with the LREE possibly precipitated as carbonate minerals. Both positive and negative Eu anomalies exist. Enrichment, immobility and depletion are noticed for the element Lu. Heavy mineral alteration, difference in stability constant between carbonate LREE and HREE complexes, downward migration of weathering fluid and microenvironment change may be responsible for the fractionation of REE in the altered dacite.

Origins of Nd–Sr–Pb isotopic variations in single scheelite grains from Archaean gold deposits, Western Australia

Accessory gangue scheelite (CaWO 4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique. Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/ 86Sr and ε Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/ 86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone. The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/ 86Sr ratios and ε Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/ 86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite. A positive correlation between Na and REE suggests that REE 3+ are accommodated by the coupled substitution REE 3++Na +=2 Ca 2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration. The very high- ε Nd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnäsite were found within micro-fractures of one scheelite grain containing an extremely high- ε Nd signature. The hydroxyl-bastnäsite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high- ε Nd i values observed in scheelite from Western Australia.

CO2, Alkalies and REE Systematics in Hydrothermally Altered Gabbro Hosting the Cordova Gold-bearing Veins, Ontario

The earliest gold-mining operations in Ontario included the Cordova, Deloro, and Sophia mines north of Lake Ontario (see for example, Miller and Knight, 1914). The Cordova deposit, worked intermittently between 1891 and 1940, is reported to have yielded 22 774 oz of gold from 127 670 tons of ore averaging 0.19 oz/ton Au (Gordon et al., 1979). The property remained inactive for many years. In 1988, Gunnar Gold Inc. and Mill City Gold Inc. conducted a drilling program along the main Cordova vein. The present study is based on core from three drill holes through the Cordova gabbro (Fig. 1⇓) and the carbonate alteration zones enveloping the quartz-carbonate veins. The relatively narrow width of the alteration zone and the availability of continuous sections of core provided an opportunity to conduct a compact mineralogical-chemical study of carbonate alteration and its effect on the redistribution of rare earth elements (REEs). Although the Cordova deposit is not of great economic importance, this study has revealed a number of interesting features which are interpreted as resulting from processes that may be applicable to gold-bearing quartz vein deposits elsewhere. Fig. 1. Location and general geology of the Cordova area (after Bartlett and Moore, 1985). The Cordova gabbro is layered in places, varying from fine-to coarse-grained and in color from light gray (feldspathic) to dark gray-green (amphibolitic). However, layering within the section of the gabbro considered here and which hosts the Cordova gold deposit is mainly textural rather than compositional. Samples of unaltered gabbro selected for analyses were medium- to coarse-grained; amphibolitic and anorthositic phases were avoided as being less representative of the general chemical composition of the gabbro. The Cordova intrusion has undergone regional metamorphism and consists dominantly of actinolite and saus-suritized plagioclase with smaller amounts of granular clinozoisite and flaky chlorite; minor quartz, ilmenite, …

Redistribution of fissiogenic and non-fissiogenic REE, Th and U in and around natural fission reactors at Oklo and Bangombé, Gabon

Elemental abundances and isotopic compositions of rare earth elements (REE), Th and U of twenty eight samples from four kinds of natural fission reactors at the Oklo and Bangombé uranium ores were determined by mass spectrometry. Fissiogenic and non-fissiogenic REE components of each sample were calculated from the isotopic compositions. The geochemical distribution of both fissiogenic and non-fissiogenic REE differs among the four reactors. Each reactor shows the following features possibly associated with the local geochemical conditions. (1) Reactor 9 at the shallow depth of the Oklo deposit contains higher amounts of non-fissiogenic REE than the other reactors. The distribution of non-fissiogenic REE does not reflect the primordial REE abundance patterns, and might have been disturbed in and around the reactor due to supergene alteration. Our results suggest that significant amounts of fissiogenic lighter REE (LREE), such as La, Ce and Pr, were mobile in the reactor; (2) Reactors 10 and 13 located deep underground have much lower contents of non-fissiogenic REE than reactor 9. All fissiogenic REE show high retention in the reactors. Non-fissiogenic REE of reactor 10 inform of the primordial ore formation without any geochemical disturbance. Little redistribution of both fissiogenic and non-fissiogenic REE in the reactor 10 was observed; and (3) The Bangombé reactor located at shallow depth has apparently well preserved fissiogenic REE. Non-fissiogenic REE in the Bangombé reactor might have been disturbed, possibly by hydrothermal processes. The distribution behavior of fissiogenic REE is considered to be related with physico-chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor, but clear relationship between nuclear parameters and the REE distribution could not be obtained in this study.

Mineralogic controls on REE mobility during black-shale diagenesis

ABSTRACT The Llandeilo-Caradoc black shales of southwest Wales, U.K. record a paragenetic sequence that requires redistribution of rare earth elements (REE) on a mineralogic scale during diagenesis. The main evidence for this redistribution is formation and replacement of REE-rich apatite and monazite and variations in whole-rock REE patterns. The processes that produce these mineralogic changes create a diagenetic overprint on the Llandeilo-Caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition. Periods of open-system conditions existed during the diagenetic alteration of the Llandeilo-Caradoc black shales, which allow an evaluation of the extent and timing of the REE redistribution inferred from petrography. The observed variations in La/Sm ratios in the whole-rock REE patterns are strong evidence that REE were redistributed on a scale greater than an individual hand sample. This change in the whole-rock REE pattern altered the Cerium anomaly by as much as 20% and produced a range in the Sm/Nd ratio from 0.14 to 0.20. This change in the whole-rock Sm/Nd ratio indicates a disturbance in the Sm-Nd isotopic system at about 460 Ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis. One effect of this disturbance is to alter the range in Nd model age for the Llandeilo-Caradoc black shales from 1.6 to 1.8 Ga to about 1.4 to 2.25 Ga, thus complicating any interpretation of provenance. The ability to link petrographic observations from a black shale to measurable whole-rock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction.