Fractionation Of Light Rare Earth Elements Research Articles

RARE-EARTH elements in the topsoils of a Russian industrial city: Sources and human health risk assessment

Research on rare-earth elements (REEs) in urban soils of Russian industrial cities is extremely limited. This study investigates the potential sources and human health risks of REEs contained in the topsoils of the industrial Russian city of Chelyabinsk. The study also takes into account natural sources of REE as the city is located on the border of granites (Urals) and sedimentary rocks (Western Siberia). We analyzed the concentration and distribution of REEs in the soils of four types of locations: residential courtyards, city parks, roadsides, and industrial locations. The total REE concentrations ranged from 44 to 255 mg/kg, with average concentrations of 140, 124, 113 and 89 mg/kg in the courtyards, roadsides, industrial location and city parks, respectively. The REE content in courtyard soils could be influenced by poor cleaning of fallen leaves. The ratios of light REE (LREE) to heavy REE (HREE) ranged from 9.5 to 10.1, revealing an obvious fractionation between them. The fractionation of LREE and HREE, and the REE/ES (European Shale) pattern showed that REE accumulation in Chelyabinsk soils has been disturbed by human activities. It was shown that the dust from industrial emissions was the main anthropogenic source of REE accumulation in urban soil. The largest amounts of REEs are emitted from an electrometallurgical plant and zinc production plant. Fortunately, the estimated daily intakes of REE from soils for children and adults were well below the safety thresholds. At the same time, in order to prevent social tension and reduce the anthropogenic load on the urban area, it is recommended to use phytoremediation technologies, smart landscaping of industrial and residential areas, more thorough cleaning of fallen leaves and road dust. It is also recommended to move the most dangerous production processes outside the urban area.

REE Geochemical Characteristics of the Huri Karst-Type Bauxite Deposit, Irano–Himalayan Belt, Northwestern Iran

The Huri bauxite deposit is located 40 km northwest of Maragheh City, East Azerbaijan province, northwestern Iran. Bauxite horizons at Huri develop within karstic depressions and sinkholes of carbonate footwalls of the Ruteh Formation, overlain by carbonate of the Elika Formation. Powder X-ray diffraction (PXRD) and scanning electron microscope, coupled with energy dispersive X-ray spectroscopy (SEM-EDS) analyses show that the Huri bauxite ores consist of hematite, diaspore, kaolinite, and lesser amounts of halloysite, pyrophyllite, illite, goethite, clinochlore, amesite, rutile, zircon, and monazite. Based on geochemical studies (Eu/Eu* vs. Sm/Nd and U/Th bivariate diagrams), basalt rocks interbedded in limestone of the Ruteh Formation are the possible precursor rocks of the Huri bauxite deposit. The pH variations of weathering solutions, fluctuations in the groundwater table level, the function of carbonate bedrock as a geochemical barrier, simultaneous precipitation of Fe-bearing minerals, and preferential scavenging of light rare earth elements (LREE) by hematite played an important role in the fractionation of LREE from heavy rare earth elements (HREE) in the Huri bauxite ores. Fluctuations in groundwater table level, increasing pH of acidic solutions percolating downward, preferential adsorption of Ce onto hematite at the base of the profile, and the possible presence of Ce-bearing fluorocarbonates played an important role in increasing Ce anomaly from the top of the profile downward.

Редкие и редкоземельные элементы в Cu-Ni рудах Норильского района

The deposits of the Norilsk region are confined to the marginal northwestern part of the Siberian platform and are associated with the manifestation of the Early Mesozoic trap volcanism. Sulfide mineralization is concentrated in the intrusive facies of the volcano-intrusive complex. Four main types of Cu-Ni ores are distinguished according to textural features: massive, vein-disseminated, brecciated, and disseminated. The main types of Cu-Ni ores differ significantly from each other in the chemical composition and content of rock- and ore-forming components. The enrichment ratios of ore trace elements in the Oktyabrskaya and Talnakhskaya vein-disseminated ore mainly reach hundreds (S, Pt, Au, Se,Te, Pb, As, Sn, Mo) and thousands (Ni, Cu, Ag, Pd) times. A rather high enrichment of all types of Bi, Re and Cd ores has been revealed, which are potentially industrially significant as high-tech metals, and can be promising for associated mining. The amount of rare-earth elements (REE) in ores is several times less than in the upper crust and more than in the primitive mantle. The concentration of rare earths in the main types of ores, with the exception of massive ones, exceeds the level of the chondrite standard, and REE fractionation is weak. The distribution spectra of chondrite-normalized REEs have a slight negative slope: the total content of light REEs is higher than that of heavy ones. The similarity of the geometry of the REE spectra of massive, vein-disseminated, brecciated and disseminated types of ores containing a significant admixture of host rocks indicates the inheritance of the REE composition in ores from the host rocks. In the disseminated ores of common taxite gabbro-dolerites, the Eu anomaly is present in the Kharaelakhsky (Oktyabrskoye Deposit) and Talnakhsky (Talnakhskoye Deposit) intrusions, it is positive, and in the Norilsk I Intrusive (in the same rocks), it is negative. Higher concentrations of REEs and significant fractionation of light and heavy rare elements are typical for host contact-metamorphic and metasomatic rocks with vein-disseminated (exocontact) ores. For olivine-free hornfels and skarns in brecciated ores, as well as for vein-disseminated ores, significant fractionation of light REEs with relatively heavy ones and a pronounced Eu minimum are characteristic. In massive ores, the distribution of REEs is determined by the composition of xenoliths.

Petrogenesis of Triassic granitoids in the Fengxian–Taibai ore cluster, Western Qinling Orogen, central China: Implications for tectonic evolution and polymetallic mineralization

The petrogenesis and tectonic setting of the Triassic granitoids, and their relationship with the coeval Au and Pb–Zn polymetallic mineralization in the Western Qinling Orogen (WQO) in central China remain debated. The Fengxian–Taibai Ore Cluster (FTOC) in the eastern part of the WQO hosts large-scale Triassic granitoids and numerous polymetallic deposits, providing an excellent opportunity to constrain the controversies mentioned above. In this work, we choose the Huahongshuping granodiorite pluton, Guji biotite monzogranite pluton, Taoerling biotite monzogranite dike, Dongtangzi granite porphyry dike and Dagou dioritic dikes in the FTOC for a combined study on the zircon U–Pb geochronology, whole-rock major, trace elements and Sr–Nd isotopes. LA–ICP–MS zircon U–Pb dating yields ages of ~225 Ma, ~224 Ma and 227–217 Ma for the Huahongshuping pluton, Guji pluton and granitoid dikes, respectively. All these granitoids have metaluminous and high-K calc-alkaline series affinities, and are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE) and depleted in high field-strength elements (HFSE) with a marked LREE/HREE fractionation ((La/Yb)N = 10.66–32.42), fractionation of LREE ((La/Sm)N = 2.72–6.60), and negative Eu anomalies (δEu = 0.45–0.99). They have initial Sr isotopic ratios of 0.7062–0.7082, and negative εNd(t) values of –9.42 to –8.09. The above features indicate that the FTOC granitoids were mainly derived from the partial melting of the Neoproterozoic meta-volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite, with direct contribution of mantle materials. Combined with previous data on sedimentation, deformation and metamorphism, and tectonics, we suggest that the WQO underwent the Early Mesozoic tectonic evolution from the subduction at 248–235 Ma, through syn-collision at 235–225 Ma, to post-collision at 225–195 Ma. Integrating with the ore geology, geochronology, mineralogy, and quartz and carbonate H–O, and in-situ S isotope data of sulfides from different mineralization stages at representative Au and Pb–Zn deposits in literatures, we propose a conceptive model that the post-collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the FTOC.

Biogeochemical Cycle of Lanthanides in a Light Rare Earth Element-Enriched Geological Area (Quebec, Canada)

This work investigated a rare earth element (REE) natural biogeochemical cycle in an area with a light rare earth element (LREE)-rich ferrocarbonatite intrusion. An REE determination in this geological environment allowed us to trace REE natural transfers in order to better manage future REE mining exploitations. Our findings suggest that although REE concentrations in abiotic compartments (soil and freshwater systems) and biotic samples (terrestrial and aquatic plants) were low, the LREE fractionation observed in the parent material was maintained along compartments. Additionally, Nd anomalies observed in the sediment pore water suggest a potential different biogeochemical cycle of this element in aquatic systems. According to the potential bioaccumulation of REEs in the organisms of two studied plants belonging to terrestrial and aquatic compartments, Equisetum arvense L. and Typha latifolia L. (respectively), we observed that REEs were not accumulated and that they showed limited REE transfer inside plants, but with an increased uptake of Eu relative to the other REEs. Our results indicated a low mobility and transfer of REEs from REE-rich bedrocks in a natural area toward terrestrial and freshwater systems, but also pointed to a dilution of the REE content in the different compartments, maintaining the LREE fractionation. Our findings provide new knowledge about the REE biochemical cycle in a natural area (from rocks to plants) and represent a starting point for an environmentally friendly exploitation of future REE mining areas.

Rare earth element fractionations of the northwestern South China Sea sediments, and their implications for East Asian monsoon reconstruction during the last 36 kyr

Rare earth elements (REEs) fractionations in siliciclastic marine sediments are related to the parent rock properties, weathering degree, grain size, and heavy minerals. The relations between marine sediment REE compositions and regional paleoclimate are not fully understood. Here, the REEs for detrital components of a core retrieved from the northwestern South China Sea (SCS) were measured. Results show the REE (LREE/HREE, (La/Yb)N) and Light REE ((La/Sm)N) fractionation degrees have good responses to the regional paleoclimate over the past 36 kyr. Sea level played a key role in REE fractionations during ~36-24 kyr BP, manifesting as lower REE and LREE fractionation degrees correspond to the decreased sea level. The entire trends for LREE/HREE, (La/Yb)N and (La/Sm)N over the past 20.5 kyr are almost in phase with the regional East Asian summer monsoon (EASM) records. Because REEs are mainly concentrated in heavy minerals, the density difference of heavy minerals is probably the main controlling factor on REE fractionations. Clay minerals and feldspars released during the weathering process have a high content of Light REEs (LREEs). In the warm, wet periods (Holocene Optimum, and Bølling-Allerød), more precipitation promoted weathering and strengthened the estuarine hydrodynamic force, which respectively resulted in high content of clay minerals and feldspars, and high-density heavy minerals (e.g., monazite). The process led to LREE enrichment because the enriched minerals contain higher contents of the LREEs. In contrast, the dry, cold climate conditions during the periods Heinrich 1, Younger Dryas, and Last Glacial Maximum resulted in the reversed REE fractionations, as the weak hydrodynamic force can only carry the low-density heavy minerals (e.g., zircon) that are enriched in HREEs. This model does not seem to apply to 36- 24 kyr BP, probably because of the re-transport of ancient shelf sediments. The study facilitates the understanding of REE application in paleoclimate reconstruction within slope sediments.

Geochronology and geochemistry of late Mesozoic volcanic rocks in the Wenkutu area, Great Xing'an Range, China

This paper reports on the petrography, geochemistry, and geochronology of late Mesozoic volcanic rocks in the Wenkutu area of the northern Great Xing'an Range, North‐east China, and discusses the formation age, petrogenesis, and tectonic environment of the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. Results of Zircon U–Pb dating show that the volcanic rocks in the Wenkutu area were formed during the Early Cretaceous (Manketouebo Formation at 145 ± 1 Ma, Manitu Formation at 141.3 ± 1.7 Ma, Baiyingaolao Formation at 128.7 ± 1.1 Ma, and Meiletu Formation at 129 ± 1 Ma). The Manketouebo, Manitu, and Baiyingaolao formations are dominated by intermediate to acid volcanic rocks that belong to the high‐K calc‐alkaline series, with Eu/Eu* values of 0.38–0.84.They are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE). The Meiletu Formation is dominated by intermediate to basic volcanic rocks, belongs to the high‐K calc‐alkaline series, has Eu/Eu* values of 0.89–0.97, and is enriched in Ba, K, Th, U, La, Ce, Sr, Nd, and Hf but relatively depleted in Nb, Ta, Zr, P, and Ti, with no obvious fractionation of LREE and HREE. Results indicate that the magma of the Manketouebo, Manitu, and Baiyingaolao formations had a crustal origin, whereas the Meiletu Formation was sourced from the mantle and subsequently contaminated by crustal materials. The late Mesozoic volcanic rocks in the study area were formed during an orogenic stage, and the compressional environment was caused by subduction of the paleo‐Pacific Plate below the Asian continental plate.

Rare earth elements (REEs) in PM10 and associated health risk from the polymetallic mining region of Nandan County, China

ABSTRACTA total of 26 samples were collected from Dachang, Chehe and Liuzhai in Nandan County, China, in order to investigate the concentrations of rare earth elements (REEs) and associated health risk in particulate matter. The concentrations of REEs in Dachang (23.54 ng/m3), Chehe (20.29 ng/m3) were significantly higher than those in Liuzhai (8.1 ng/m3). The light rare earth elements (LREEs) account for 87.08%, 87.09%, and 86.17% of the total REE burden in PM10 at Dachang, Chehe, and Liuzhai, respectively, indicating that the distribution pattern of REEs in PM10 was characterized by the obvious fractionation of LREEs. Enrichment factor calculation indicated that EFs for La, Ce, Nd, Pr, and Nd in PM10 from Dachang and Chehe were greater than 2, indicating moderate enrichment. Source identification indicated that REEs in PM10 from Dacheng and Chehe originated from anthropogenic activities such as mining and smelting activities, while REEs in PM10 from Liuzhai were associated with natural sources, like soil erosion. Noncarcinogenic and carcinogenic risks associated with the exposure of REEs in PM10 were negligible based on the health risk assessment models. However, greater noncarcinogenic risk for children was found in studied areas compared with the adults.

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

Paola Ridge, along the NW Calabrian margin (southern Tyrrhenian Sea), is one of the few reported deep sea sites of precipitation of authigenic carbonates in the Tyrrhenian Sea. Here, the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite, siderite and other carbonate phases. The occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate-methane transition zone (SMTZ).Concretions of authigenic minerals formed in the near sub-bottom sediments of the Paola Ridge were investigated for their geochemical and isotopic composition. These concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs. The mud diapirs are dotted by pockmarks and dissected by normal faults, and are known for having been a site of fluid seepage for at least the past 40 kyrs. Present-day venting activity occurs alongside the two alleged mud volcanoes and is dominated by CO2-rich discharging fluids. This discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area.In this study, we used stable isotopes (carbon and oxygen) of carbonates coupled with rare earth element (REE) composition of different carbonate and non-carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates. The analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea-water.Authigenic calcite/aragonite concretions, from surficial sediments on diapiric structures, have depleted 13C isotopic composition and slightly positive δ18O values. By contrast, siderite concretions, generally found within the first 6 m of sediments on the alleged mud volcanoes, yielded positive δ13C and δ18O values. The siderite REE pattern shows consistent LREE (light REE) fractionation, MREE (medium REE) enrichment and positive Gd and La anomalies. As shown by the REE distribution, the 13C-depleted composition and their association with chemosymbiotic fauna, calcite/aragonite precipitated at time of moderate to high methane flux close to the seafloor, under the influence of bottom seawater. Authigenic siderite, on the other hand, formed in the subseafloor, during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13C-rich dissolved inorganic carbon (DIC) and 18O-rich water, likely related to methanogenesis and intermittent venting of deep-sourced CO2.

Geochemical and Hf isotopic compositions of Late Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China: petrogenesis and tectonic implications

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

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

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

Age and origin of Pan-African granites and associated U–Mo mineralization at Ekomédion, southwestern Cameroon

Various granites in the Pan-African Neoproterozoic Fold Belt of Cameroon were investigated in an area around Ekomédion (southwestern Cameroon) in order to set constraints on their genesis and age. The studied granites were likely emplaced in relation with the Central Cameroon Shear Zone (CCSZ). Hence dating the granites provides an age for the dextral transcurrent tectonics event responsible for the CCSZ. The interest in these granitic rocks is spurred by noticeable Mo and U mineralization in a pegmatite that is hosted by two-mica granite. The studied granites comprise post-collisional syn-D4 two-mica granite, alkali-feldspar granite, biotite-rich granite and porphyritic granite. Based on lithogeochemical data, the granites can be characterized as ferroan, peraluminous and high-K calc-alkaline. The biotite-rich and alkali feldspar granites show I-type characteristics, whereas the porphyritic and two-mica granites reveal S-type affinity. All of them are marked by strong light rare earth element fractionation and a strong negative Eu anomaly, and they are strongly depleted in Ba, Nb, P, Sr and Ti, pointing to a crustal origin. Crust assimilation is also evident from zircon xenocrysts in the two-mica granite with upper intercept U–Pb ages of 2051±44Ma and 1550±24Ma. U–Pb dating of zircon, monazite and xenotime grains/domains yielded indistinguishable ages for both the two-mica granite (578±4Ma) and the porphyritic granite (579±3Ma). An identical age (578±11Ma) was obtained by Re–Os dating of molybdenite that occurs together with uraninite in pegmatite within the two-mica granite, thus clearly attesting to a late-magmatic origin of the ore. The studied granites have many similarities, both in composition and age, with granites in Nigeria and northeastern Brazil, supporting the continuation of a large Pan-African/Braziliano magmatic province from west-central Africa to northeastern Brazil.

Within plate, arc, and collisional Neoproterozoic granitic magmatism in the Araxá Group, Southern Brasília belt, Minas Gerais, Brazil

Three Neoproterozoic granitic episodes associated with the Araxa Group, in Southern Brasilia Belt between the cities of Araxa and Cascalho Rico, Minas Gerais State, Brazil, are characterized from the geologic, geochemical, and geochronological points of view. The earliest episode, of 833 Ma, generated in a within-plate environment, is represented by the Quebra Anzol Granite, which has: low 87Sr/86Sr = 0.706 ratios and eNd(833) = -3.98; high levels of SiO2, Na2O, MnO, Nb, Y and Co; and low concentrations of Sr and Ba, with the absence of anorthite and presence of diopside, acmite, and positive Eu anomaly, therefore it may have been originated from mantle sources with small crustal contamination. The second episode, of 790 Ma, is represented by the pre-collisional Monte Carmelo Granitic Complex, which was partially remobilized in a collisional event around 630 Ma. It is metaluminous to peraluminous with low 87Sr/86Sr ratios of 0.706, high light rare earth elements fractionation relative to heavy rare earth elements, and negative Nb, Ta, Sr, P, and Ti anomalies. Their eNd(790) = -2.218, with T DM = 1.29 Ga, indicate an origin from juvenile sources and reveal the existence of a late Mesoproterozoic magmatic arc in this region. The last granitic episode has taken place in a collisional tectonic environment between 642 and 630 Ma, generating peraluminous granites with muscovite, garnet, and tourmaline denominated Serra Velha, Tamandua, Pirapetinga, Galheirinho, Perdizes, Estrela do Sul, and Cascalho Rico. They have negative eNdt, Nd model ages from Neo to Mesoproterozoic, and variable rare Earth elements enrichment patterns, with strong negative Eu anomalies.

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

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

Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances

The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites (T=1,200°C; P=1.5 GPa) into fine-grained garnet–amphibole peridotites (T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. cpx/opx D Li=3–7 and cpx/ol D Li=2.7–8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene Pb N /Nb N =16–90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive anomalies in Cs, Ba, Pb and U and moderate enrichment in Li, indicating addition of a crustal component to the mantle rocks. Amphibole hosts most of these trace elements. Clinopyroxene displays high LILE/HFSE (Pb N /Nb N =300–600), low Ce/Pb (1.4–2.7 in garnet-facies clinopyroxene compared with 2.6–24.5 in the spinel-facies one) and variable LILE and LREE enrichments. The coupled increase of modal amphibole, Sr and Pb, together with positive Pb–Sr and Pb–U correlations, further indicate that incompatible element influx in these samples was fluid-mediated. In the garnet-facies samples, amphibole and, interestingly, olivine have similarly high Li concentrations as clinopyroxene, leading to cpx/amph D Li=0.7 and cpx/ol D Li=0.7–0.8, the latter being up to ten times lower than in the spinel-facies rocks. Due to its high modal abundance, olivine is the main host of Li in the garnet–amphibole peridotites. The observed metasomatic features provide evidence for the infiltration of an aqueous fluid in the mantle wedge above a subducting slab. This fluid most likely derived from subducted crustal rocks that underwent partial melting. Successive retrograde re-equilibration during exhumation of the garnet peridotite is accompanied by garnet and clinopyroxene breakdown and amphibole formation. This process produced minor changes, such as an increase of HREE and Li in amphibole, and an increase of Li in olivine. The general trace element signature remains essentially unchanged during retrogression and further hydration, indicating that fluids with a similar composition to the one present at the garnet–amphibole peridotite formation, were responsible for increased amphibole formation. The combined evidence from the metamorphic and metasomatic evolution indicates that the peridotites experienced first corner flow in a mantle wedge, followed by subduction and finally entrapment and exhumation within a crustal slab. During their entire history the Ulten peridotites were percolated first by melts and then by aqueous fluids, which added recycled crustal components to the mantle wedge.

REE geochemistry of the recent playa sediments from the Thar Desert, India: An implication to playa sediment provenance

The playas (saline lakes) situated in the Thar Desert, north-west India, provide prominent examples of alkaline brine and varying assemblages of detrital and evaporite mineralogy. The eastern margin of the desert is relatively semi-arid, whereas the central to western region is arid to hyper-arid in nature. Rare earth elements (REEs) systematics in the sediments of nine different playas of the Thar Desert were studied to understand the provenance of the sediments and the intensity of chemical weathering in the region. Based on the REE patterns, fractionation of light REE (LREE) (La/Sm) N and heavy REE (HREE) (Gd/Yb) N, and Eu anomaly (Eu/Eu*), the upper continental crust normalised playa sediments are divided into two different groups. The eastern margin playa sediments show homogeneous REE contents, relatively positive Eu anomaly and depleted HREE values, whereas the western arid core playa sediments have highly variable REE contents, relatively negative Eu anomaly and similarly fractioned LREE and HREE patterns. The dissimilarity in the degree of HREE fractionations both in the eastern and western playa sediments is attributed to the differential distribution of minerals, depending upon their resistance to chemical weathering. It is believed that the relatively higher abundance of REE bearing heavy minerals and the presence of higher amounts of evaporites influence the large variation of REE distribution and enriched HREE in the western playa sediments. Apart from the relatively higher abundance of heavy minerals, the presence of rock fragments of variable petrographic character and roundness mirror the lower rock–water interaction in the arid western region. The presence of well-rounded metamorphic rock fragments and minerals, sourced from the eastern margin Aravalli mountains, indicates that the playas of the entire desert get the detrital and dissolved material mainly from the Aravalli mountains. Additionally, the western playas receive sediments from their surrounding Proterozoic and Mesozoic formations. This interpretation is supported by the presence of angular rock fragments of basalt, rhyolite and limestone in the western playas.

U?Pb zircon geochronology and petrology of granitoids from Mitwaba (Katanga, Congo): implications for the evolution of the Mesoproterozoic Kibaran belt

In Katanga (Congo), Mesoproterozoic Kibaran granitoids were previously divided into: (1) type A or G1 granitoids (1370±25 Ma); (2) type B or G2 granitoids (1308±25 Ma); (3) type C or G3 granitoids (1094±50 Ma); and (4) type D or G4 tin granites (ca. 977±18 Ma). Our field investigations indicate that igneous bodies previously described as G1 (A), G2 (B) and G3 (C) granitoids in the Mitwaba region were emplaced during the same early Kibaran compressional orogenic event (D1). The heterogeneous distribution of strain during D1 produced orthogneissic rocks along high strain zones versus slightly to unfoliated granitoids in low strain zones. New SHRIMP U–Pb zircon analyses yield the following igneous crystallisation ages for Mitwaba G1–G3 granitoids: Kisele monzogranite gneiss 1386.3±8.1 Ma, Kabonvia granodioritic augen gneiss 1385.5±7.4 Ma, Nyangwa monzogranite 1383.3±4.8 Ma, Kungwe-Kalumengongo monzogranite 1377±10 Ma and Fwifwi foliated leucomonzogranite 1372±9.6 Ma. All these granitoids were emplaced over a short period of time and could be coeval at 1381±8 Ma. This age dates the early Kibaran orogenic event (D1) in the Mitwaba region. Metamorphic zircons from the strongly deformed Kisele monzogranite gneiss yield an age of 1079±14 Ma, which possibly dates the D2 orogenic event in the Mitwaba area. The tin granites are younger and were emplaced at ∼1.0 Ga. The Mitwaba granitoids are two-micas and ilmenite-bearing strongly peraluminous S-type granitoids. The rare-earth elements (REE) patterns show a strong Eu negative anomaly, an enrichment and fractionation of light REE (LREE) and slightly fractionated to almost flat Heavy REE (HREE) patterns. Primordial-mantle normalized spidergrams are characterized by negative anomalies of Ti, Sr, Nb and subsidiary Ba. The emplacement of arc mafic magmas into the crust triggered the dehydration melting, at high temperatures, of a highly fertile and heterogenous metasedimentary source material producing a felsic melt which was mixed with the arc mafic magmas in variable proportions to produce these granitoids. The petrogenesis of these rocks and similar coeval occurrences defining a ca. 3000 km granitoid belt stretching from SE Congo to NE Congo, Burundi, Rwanda and Uganda indicates similar processes throughout the Kibaran orogenic system in central and eastern Africa.

U–Pb zircon geochronology and petrology of granitoids from Mitwaba (Katanga, Congo): implications for the evolution of the Mesoproterozoic Kibaran belt

In Katanga (Congo), Mesoproterozoic Kibaran granitoids were previously divided into: (1) type A or G 1 granitoids (1370±25 Ma); (2) type B or G 2 granitoids (1308±25 Ma); (3) type C or G 3 granitoids (1094±50 Ma); and (4) type D or G 4 tin granites (ca. 977±18 Ma). Our field investigations indicate that igneous bodies previously described as G 1 (A), G 2 (B) and G 3 (C) granitoids in the Mitwaba region were emplaced during the same early Kibaran compressional orogenic event (D 1). The heterogeneous distribution of strain during D 1 produced orthogneissic rocks along high strain zones versus slightly to unfoliated granitoids in low strain zones. New SHRIMP U–Pb zircon analyses yield the following igneous crystallisation ages for Mitwaba G 1–G 3 granitoids: Kisele monzogranite gneiss 1386.3±8.1 Ma, Kabonvia granodioritic augen gneiss 1385.5±7.4 Ma, Nyangwa monzogranite 1383.3±4.8 Ma, Kungwe-Kalumengongo monzogranite 1377±10 Ma and Fwifwi foliated leucomonzogranite 1372±9.6 Ma. All these granitoids were emplaced over a short period of time and could be coeval at 1381±8 Ma. This age dates the early Kibaran orogenic event (D 1) in the Mitwaba region. Metamorphic zircons from the strongly deformed Kisele monzogranite gneiss yield an age of 1079±14 Ma, which possibly dates the D 2 orogenic event in the Mitwaba area. The tin granites are younger and were emplaced at ∼1.0 Ga. The Mitwaba granitoids are two-micas and ilmenite-bearing strongly peraluminous S-type granitoids. The rare-earth elements (REE) patterns show a strong Eu negative anomaly, an enrichment and fractionation of light REE (LREE) and slightly fractionated to almost flat Heavy REE (HREE) patterns. Primordial-mantle normalized spidergrams are characterized by negative anomalies of Ti, Sr, Nb and subsidiary Ba. The emplacement of arc mafic magmas into the crust triggered the dehydration melting, at high temperatures, of a highly fertile and heterogenous metasedimentary source material producing a felsic melt which was mixed with the arc mafic magmas in variable proportions to produce these granitoids. The petrogenesis of these rocks and similar coeval occurrences defining a ca. 3000 km granitoid belt stretching from SE Congo to NE Congo, Burundi, Rwanda and Uganda indicates similar processes throughout the Kibaran orogenic system in central and eastern Africa.