Fe Mn Oxyhydroxides Research Articles

Geochemistry of uranium in the Ganga (Hooghly) River estuary, India: The role of processes in the water column and below the sediment-water interface

In this study, we discuss the cycling of uranium (U) in the Ganga (Hooghly) River estuary through investigation of the composition of the water samples of six periods of contrasting water discharge, the bulk and exchangeable phases of the coexisting suspended particulate matter (SPM) and the bed sediments. In addition, we evaluate data on urban and industrial effluent waters that drain into the estuary. Similar U concentrations of freshwaters and effluent waters do not indicate the anthropogenic source to be an important contributor of U to the estuary. The removal of dissolved U in the low-salinity zones, observed for five of six study periods, is primarily linked to the cycling of the FeMn oxyhydroxide phases. The compositions of exchangeable phases of SPM provide supportive evidence for the removal of dissolved U. Mass balance calculations show that the degree of removal of dissolved U can be sustained by the observed enhancement of exchangeable U in the SPM. Loss of U from the bed sediments is inferred from the distributions of exchangeable U concentrations as a function of salinity. Multiple observations based on compositions of the exchangeable and bulk phases of bed sediments indicate mobilization and loss of both U and Mn through suboxic diagenesis in the sediment columns followed by tidally-induced sediment resuspension.We estimate that 40–60% of riverine U is removed in the low-salinity zone during the monsoon and post-monsoon periods. More importantly, the salinity range over which U removal occurs varies between the periods of similar water discharge. The estimated U flux from the estuary is ∼70% higher in 2012 than in 2013. A significant difference between the annual U fluxes, driven mainly by considerable variation in the magnitude of U removal, suggests that estimates of meaningful annual U fluxes would require data at higher temporal resolution. This study highlights the importance of processes operating in the water column and below the sediment-water interface in the cycling of U in the Hooghly River estuary, where the solute-particle interaction driven by high SPM load is known to influence the cycling of trace metals.

Base metals- and Lithium- rich ferromanganese oxide deposits from the South Andaman Sea, Northeastern Indian Ocean: Mode of occurrence and genesis

The Andaman Sea hosts ferromanganese (Fe-Mn) oxide deposits as crusts and nodules over seamounts and ridges shallower than 2000 m water depths. Fe-Mn crust and nodules were recovered from West Sewell Ridge (WSR), Sewell Rise (SR) and Invisible Bank (IB) in the South Andaman Sea. The samples from WSR and SR are enriched in Ni (1628 – 7681 ppm), Pb (500 – 2935 ppm), Zn (372 – 7351 ppm), Li (3 – 781 ppm), Ba (710 – 8672 ppm) and As (119 – 414 ppm) and depleted in Co and Cu. Base metals (Ni + Pb + Zn) are enriched to a maximum concentration of 1.6 % (with an average of 0.7 %) in the WSR and a maximum of 0.7 % (with an average of 0.5 %) in the SR. Nodules from both WSR and SR have consistently higher content of Li, Ba and base metals than Fe-Mn crusts. Fe-Mn nodules and crusts from WSR and crusts from SR are formed predominantly by hydrogenetic precipitation. Fe-Mn nodules from SR have mixed hydrogenetic-hydrothermal origin. Samples from IB are Fe-rich ironstones enriched in Cr, Ni and As that precipitated from hydrothermal solutions percolated through ultramafic rocks. Saturation of deeper waters with metal ions emanating from volcanic-hydrothermal sources and oxygen supply to the basin by intermediate waters together facilitated rapid precipitation of Fe-Mn oxyhydroxides over seamounts and ridges shallower than 2000 m water depths in the Andaman Sea. The rapid precipitation of Fe-Mn oxy-hydroxides in turn constrained the enrichment of elements (especially Co and REEs) that are generally incorporated by slow-hydrogenetic precipitation.

Boundary processes and neodymium cycling along the Pacific margin of West Antarctica

Neodymium (Nd) isotopes have been utilized as a tracer of water mass source in the modern ocean and in palaeoceanographic studies, though the oceanic cycling of Nd is not yet fully constrained. Recent studies have highlighted the importance of processes that occur near the seawater – sediment interface in altering the Nd isotopic composition of bottom waters. The two major observed processes “boundary exchange” and “benthic flux” have been suggested as playing an important role in setting water mass compositions, however, more studies are needed to constrain their chemical mechanism and the extent to which these processes set the composition of deep waters. The Antarctic continental margin is an important place to study these processes because Antarctic-sourced waters dominate the Southern Ocean and ventilate the global deep ocean. This study is the first to measure and compare seawater, porewater and sediment data from along the margin of Antarctica to examine the nature of potential boundary processes. We show that a process similar to boundary exchange seems to be occurring within porewaters, modifying porewater chemistry by shifting its Nd isotopic ratios to more radiogenic values without significantly increasing the concentration of dissolved Nd. We hypothesize that this shift results from partial dissolution of radiogenic detrital particles, such as smectite, amphibole and/or volcanic glass, while re-scavenging maintains low Nd concentrations. We infer the existence of benthic flux of porewaters to deep waters by examining chemical gradients in porewaters and show that it is much lower on the Antarctic margin compared to other studies. Benthic flux appears to be slightly higher along the Antarctic Peninsula than in the Bellingshausen Sea due to partial degradation of organic matter and associated dissolution of Fe-Mn oxyhydroxides. Taken together, boundary processes do not significantly change the Nd isotopic composition of Antarctic margin seawater because while the porewaters have an altered Nd isotopic composition the Nd concentration of these porewaters is low compared to other settings.

Spatial distribution of Mo and δ98Mo in waters of the northern Indian Ocean: Role of suboxia and particle-water interactions on lighter Mo in the Bay of Bengal

The dissolved Mo concentrations and Mo isotopic composition (δ98Mo) have been determined in the water column of the northern Indian Ocean to assess the impact of water column anoxia/suboxia, seasonal hypoxia, and enormous riverine particulate fluxes from the Ganga-Brahmaputra river system on Mo isotope composition of seawater. All the samples collected from the Arabian Sea and the central and southern Bay of Bengal display conservative and uniform Mo concentrations and δ98Mo, 112 ± 2 nmol/kg, and 2.25 ± 0.13‰, respectively, similar to the other global oceanic basins. Overall, this study suggests no modification in both Mo concentrations and Mo isotopic composition in the water columns of the Arabian Sea and the open ocean locations of the Bay of Bengal under the influence of suboxic/denitrifying conditions and high primary productivity. However, we report, for the first time, lighter Mo isotope composition in the water column of the northern coastal Bay of Bengal. These waters from the northern Bay of Bengal continental margin show significantly depleted/lighter δ98Mo (average = 1.64 ± 0.42‰; 1σ) than the global open ocean average value, with conservative Mo concentrations. In addition, δ98Mo of bulk sediments and leachable Fe-Mn oxyhydroxide phases from this region show a lower δ98Mo than the overlying water column, with the bulk sediments containing ∼ 5–11% of leachable Mo fraction adsorbed on Fe-Mn oxyhydroxide fraction. Intermediate water anoxia/suboxia in this region extends up to the sediment–water interface. The depletion in δ98Mo in the northern coastal Bay of Bengal waters could result from the in situ release of relatively lighter Mo from the reductive dissolution of Fe-Mn oxyhydroxide phases adsorbed on suspended particulate matter. Repeated cycles of deposition and reductive dissolution of sedimentary Fe-Mn oxyhydroxide phases in the shelf/slope regions could supply lighter Mo to the porewaters. Furthermore, the subsequent diffusion/advection of these porewaters to the water column and/or dissolution of Fe-Mn oxyhydroxide phases in the suboxic water column could supply lighter Mo to the waters of the northern coastal Bay of Bengal.

Sediment quality of the Ridracoli fresh water reservoir in Italy: Insights from aqua regia digestion and sequential extractions

The inter-element relationships and the forms in which metals exist strongly influence their mobility and, in turn, have a signature on the environment and human health. Located in the northern Apennines within the Emilia-Romagna region, the Ridracoli artificial lake is one of Italy's most important reservoirs that provides drinking water for about one million people. This work characterized the reservoir sediments by ICP-MS after aqua regia digestion (ARD), comparing the limits by law to assess environmental compliance and XRF data from the same sample-set taken as total concentrations. The Degree of Extraction (DE) from pseudo-total concentrations of ARD analysis allows assessing elements mobility and the associated environmental risk. Principal Component Analysis (PCA) on the obtained data helped to investigate inter-element relationships better; for example, we observed carbonate-sourced sediments, many trace elements (e.g., Ni, Zn) linked to FeMn oxyhydroxides, the importance of the grain size in elements distribution, and the central role of the organic matter in element partitioning. In addition, a Sequential Extraction Procedure (SEP) was applied to the sediment samples to understand the partitioning of many analytes, including Potentially Harmful Elements (PHE) such as Fe, Mn, Cu, Cr, Ni, Pb, and Zn. The results indicated that the most easily mobilized forms were predominant in the area near the dam, in correspondence to sediments affected by the formation of a seasonal anoxic layer.

Metallogenesis of Co-rich ferromanganese nodules in the northwestern Pacific: Selective enrichment of metallic elements from seawater

Ferromanganese nodules in the northwestern Pacific are newly discovered submarine solid mineral resources with high economic value. To systematically ascertain the metallogenesis of these nodules, we investigate their microstructure, mineralogy, and elemental concentrations, as well as their Nd and Hf isotopic compositions. The seabed nodules in the study area are formed in a low-temperature marine environment, with low crystallinity and detrital components. The minerals identified by XRD contain a large amount of vernadite (mean 63.6%) and a small amount of todorokite (mean 7.2%), indicating an oxidized metallogenic environment. Moreover, the high contents of Co (0.31–0.51 wt%, mean 0.46%), Ti (mean 1.28%), rare earth elements and yttrium (REY, mean 1969 ppm), as well as the high δCe, and Fe/Mn ratios, differ from those of diagenetic and mixed-type nodules, confirming the typical hydrogenetic chemical composition of the nodules. Extended exposure to oxidizing deep-water environments, as a consequence of oligotrophic, continuous bottom current activities together with constant low sedimentation rates, provides optimal conditions for the Co-rich ferromanganese nodules (Co-rich nodules) to enrich multiple elements from seawater. Notably, Mn, Co, Ce and Pb are scavenged elements with low contents that decrease with water depth, and are primarily regulated by Mn–Fe oxyhydroxide in the water column. Nutrient-type elements, such as Fe, Ti, Cu, Ni, and REY (except Ce) can also be highly enriched in Co-rich nodules; however, they are mainly controlled by biological particles in water column. Collectively, coulombic interactions represent the major driving force for selective enrichment of metallic elements for Co-rich nodules from seawater. εNd and εHf also show that the Co-rich nodules are hydrogenetic ranging from −6.33 to −4.41 and from +5.00 to +6.18, respectively. The εNd of the Co-rich nodules was significantly lower than that of the Co-rich ferromanganese crusts observed on seamounts, indicating that the nodules are more affected by bottom currents. The nodules and crusts in the northwestern Pacific have rich radioactive Nd isotopic compositions, which is consistent with the high εNd observed along the Pacific margins, caused by the weathering of terrigenous components.

Drainage chemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Québec, Canada

Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64–89 wt %) and fluorapatite (2–22 wt %), as well as biotite (6–17 wt %) and chlorite (0–7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 μg/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 μg/L in tailings pore water although values in effluent are usually less than 20 μg/L. At the ambient pH, Zn is strongly adsorbed by Fe–Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 μg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 μg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 μg/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.

Appraisal of groundwater arsenic on opposite banks of River Ganges, West Bengal, India, and quantification of cancer risk using Monte Carlo simulations.

This study was conducted to inspect the spatial distribution, source identification, and risk assessment of groundwater arsenic (As) in different blocks that lie on the opposite banks of river Bhagirathi (a distributary of river Ganges), Murshidabad, West Bengal, India. It has been observed that the blocks that lie towards the eastern bank of river Bhagirathi have elevated arsenic and comparatively more reducing groundwater (lower oxidation-reduction potential and high iron). About 66% of groundwater samples across the district have arsenic concentration higher than the World Health Organization (WHO) permissible limit. Speciation of groundwater arsenic reveals that about 90% of arsenic species were present as arsenic (III). Further, principal component analysis (PCA) was employed to identify the controlling factors that favor the release of arsenic. PC1 comprises EC, TDS, As, Fe, TOC, and HCO3- with moderate loadings, which suggests microbially mediated degradation of organic matter (OM), helps in reductive dissolution of arsenic-bearing Fe-Mn oxyhydroxides. Results pointed out severe groundwater arsenic poisoning; hence, a health risk assessment was performed for the exposure of arsenic in groundwater, using incremental lifetime cancer risk (ILCR) models coupled with Monte Carlo simulations. On the eastern bank of river Bhagirathi, incremental lifetime cancer risk (ILCR) due to oral exposure (5th to 95th percentile values) ranged from 1.30538E - 04 to 9.31398E - 03 with a mean of 2.84194E - 03 for adults, which is 2841 times higher than the USEPA high safety risk guidelines of one in 1 million. The outcomes of the results will be useful for the policymakers and regulatory boards in defining the actual impact and deciding the pre-remediation goals.

Study of trace metal contamination and ecological risk assessment in the sediments of a tropical river estuary, Southwestern India

The present study aims to assess the extent of trace metal pollution in the sediments of Sita-Swarna estuary, west coast of India, and investigate their possible ecological risk on the aquatic environment. The sediment cores were analyzed for sand, silt, clay, organic carbon, and trace metals (Al, Fe, Mn, As, Cd, Co, Zn, Pb, Ni, Cr, and Cu) at 2-cm intervals. The study revealed that sediments have deposited in relatively violent to very violent hydrodynamic energy conditions. Factor analysis indicated that the metal distribution is mainly controlled by Fe–Mn oxyhydroxides and organic carbon. Further, the geochemical approach, pollution indices, and statistical evaluation revealed moderate pollution in the catchment. From an ecotoxicological perspective, the estimated risk index (RI) value was found to less than 150, indicating low risk for aquatic life. Thus, this baseline study would help to adopt strategies in pollution control and protect the fragile marine environment.

Sources and internal cycling of dissolved barium in a tropical coastal lagoon (Chilika lagoon, India) system

The abundance and distribution of barium (Ba) serve as a reliable proxy for oceanic biogeochemical processes. Its cycling in the coastal regions, however, is complicatedly regulated by multiple sources/sinks. In this study, the spatial distribution of dissolved barium concentrations in a large coastal lagoon (Chilika lagoon, India) and its source waters have been investigated for three different seasons (pre-monsoon (May 2017), monsoon (August 2017), and post-monsoon (January 2018)). Additionally, contents of Ba and other elements in the bed and suspended sediments in their different fractions (bulk, clay, and exchangeable) have also been investigated. These data were used to constrain coastal processes in regulating coastal Ba inventory. The dissolved [Ba] of the Bay of Bengal (median ~ 37 nmol/kg (range: 30–53 nmol/kg)) are lower than that observed for the river (median ~ 319 nmol/kg (range: 256–583 nmol/kg)), and groundwater (median ~ 464 nmol/kg (range: 20–4428 nmol/kg)) samples. The salinity-weighted barium concentrations of the Chilika lagoon for the pre-monsoon (1052 nmol/kg) and monsoon (652 nmol/kg) seasons are higher compared to that during the post-monsoon period (332 nmol/kg). Covariations between salinity and [Ba] confirm the non-conservative behavior of barium during all three seasons. The salinity-[Ba] trends show barium removal in the low-salinity regions, attributable to its authigenic scavenging onto Fe-Mn oxyhydroxides. Mass balance calculations show that the relative contribution of barium via rivers (~18%), (freshwater) submarine groundwater discharge (SGD; ~25%), and desorption from particulate matters (~19%) to the lagoon during pre-monsoon seasons is nearly the same. During the monsoon season, the SGD supply of barium decreases to ~2%, whereas riverine (~20%) and desorption (~24%) contributions remain similar to that observed during the pre-monsoon. The sediment chemistry (Mg, Al, Fe) data and their correlations with Ba concentrations reveal that the desorptive release of barium to the lagoon is mainly via its cationic replacement from the suspended sediments by seawater Mg. In addition to these sources, our results show that about half of the dissolved barium during pre-monsoon and monsoon seasons remains unaccounted and may have been supplied by an additional source, which may be linked to subsurface supply of recirculated seawater and/or organic matter remineralization. Our quantitative approach emphasizes the importance of these additional sources in regulating the coastal barium inventory.

Mesoporous Mn–Fe oxyhydroxides for oxygen evolution

Mesoporous Mn–Fe oxyhydroxides exhibit excellent OER performance with overpotential of 275 mV, where the surface doping of Fe onto the MnOOH can favorably destabilize the surface bridge O atoms and promote the formation of surface oxygen vacancies.

Geology and geochemistry of jasperoids from the ‘Montaña de Manganeso’ district, San Luis Potosí, north-central Mexico

Large outcrops of jasperoids occur in the ‘Montaña de Manganeso’ mining district in north-central Mexico. They range from massive manganiferous jasperoids to highly brecciated, hematitic jasperoid. The jasperoids of ‘Montaña de Manganeso’ occur mainly as replacements of limestone, sandstone and shale, commonly nearby high-angle fault systems. The mineralogy of the jasperoids consist of quartz and its polymorphs (chalcedony, tridymite and cristobalite), Fe-Mn oxyhydroxides, calcite and minor barite. Many outcrops show evidence of several periods of brecciation and silicification. The geochemical signature of the jasperoids suggests that silicification was product of hydrothermal activity. The jasperoids display enrichment in elements of hydrothermal provenance such as Ba, Sr, As, Cr, Mo, Sb, Ni, Zn and Cu, whereas are strongly depleted in the elements indicative of clastic sources such as Ti, K, Th and Zr. Element ratios such as (Fe+Mn)/Ti, Al/(Al+Fe+Mn), Fe/Mn and U /Th, along with the Al-Fe-Mn and Fe-Mn-(Ni+Co+Cu)×10 ternary diagrams confirm a hydrothermal origin. Low ∑REE, an enrichment of LREE over HREE, negative Ce anomalies and positive Y anomalies (YPASS/HoPAAS) also support the hydrothermal processes. The geological evidence, in the form of a feeder zone and extensive hydrothermal alteration, show that the silica forming the rocks originated from ascending hot fluids.

The chemostratigraphy and environmental significance of the Marlstone and Junction Bed (Beacon Limestone, Toarcian, Lower Jurassic, Dorset, UK)

AbstractTwo fallen blocks of the Marlstone and stratigraphically overlying Junction Bed sampled on the beach below Doghouse Cliff in Dorset, UK (Wessex Basin) have been examined for carbon and oxygen isotopes of bulk carbonate as well as for strontium, carbon and oxygen isotopes and Mg:Ca ratios in the contained belemnites. The sequence, which contains most of the Toarcian zones and subzones within a metre or less of grey to yellow to pink, red and brown fossil-rich nodular limestone, is extremely condensed and lithologically similar to pelagic red limestones of the Tethyan Jurassic that are locally mineralized with Fe-Mn oxyhydroxides (e.g., Rosso Ammonitico). Strontium-isotope ratios of the contained belemnites are compatible with existing reference curves and both blocks show a rise to more radiogenic values post-dating the Pliensbachian–Toarcian boundary. The high degree of correlation between the relatively negative carbon and oxygen isotopes of the bulk carbonate is compatible with significant diagenetic overprint, and contrasts with higher carbon-isotope values in coeval condensed coccolith-rich limestones elsewhere. Evidence for the characteristic signature of the Toarcian Oceanic Anoxic Event, as represented by organic-rich sediment, is absent, possibly owing to a stratigraphic gap. Both blocks exhibit abrupt carbon-isotope shifts to lower values, one of which could represent the limbs of an incompletely recorded negative excursion associated with the Toarcian Oceanic Anoxic Event. That the Toarcian Oceanic Anoxic Event was also a significant hyperthermal is illustrated in both blocks by a drop in oxygen-isotope values and rise in Mg:Ca ratios of belemnites close to the base of the Junction Bed in the lowest part of theserpentinumzone.

Petrography and Mineral Chemistry of Monte Epomeo Green Tuff, Ischia Island, South Italy: Constraints for Identification of the Y-7 Tephrostratigraphic Marker in Distal Sequences of the Central Mediterranean

The 56 ka Monte Epomeo Green Tuff (MEGT) resulted from the largest volume explosive eruption from Ischia island (south Italy). Its tephra is one of the main stratigraphic markers of the central Mediterranean area. Despite its importance, a detailed characterisation of the petrography and mineral chemistry of MEGT is lacking. To fill this gap, we present detailed petrographic description and electron microprobe mineral chemistry data on samples collected on-land from the MEGT. Juvenile clasts include pumice, scoria, and obsidian fragments with porphyritic/glomeroporphyritic, vitrophyric, and fragmental textures. The porphyritic index is 13–40 vol.%, and phenocryst phases include alkali-feldspar, plagioclase, clinopyroxene, ferrian phlogopite, and titano-magnetite, in order of decreasing abundance; accessory phases include sphene, hydroxy-fluor-apatite, and rare edenite. Plagioclase varies from predominant andesine to subordinate oligoclase, whereas alkali-feldspar is more variable from sanidine to anorthoclase; quasi-pure sanidine commonly occurs as either rim or recrystallisation overgrowth of large phenocrysts due to hydrothermal alteration. Secondary minerals include veins and patches of carbonate minerals, Fe-Mn oxyhydroxides, clay minerals, and zeolites. Clinopyroxene is ferroan diopside (En45–29Fs7–27) and never reaches Na-rich compositions. This feature allows the discrimination of MEGT from aegirine-bearing, distal tephra layers erroneously attributed to MEGT, with implications for the areal distribution of Ischia explosive deposits.

Geochemical Fractions of Heavy Metals in Bottom Sediments of the Pobeda Hydrothermal Cluster in the Mid-Atlantic Ridge (17°07′–17°08′ N)

In this study, to better understand the influence of hydrothermal processes on ore metal accumulation in bottom sediments, we examined distribution of Fe, Mn, Cu, Zn, As, and Pb in core of metalliferous sediments from the Pobeda hydrothermal cluster, and in core of non-mineralized (background) carbonate sediments (located 69 km northwards). Mechanisms of Fe, Mn, Cu, and Zn accumulation in sediments (12 samples) were evaluated based on sequential extraction of geochemical fractions, including a conditional mobile (F-1, exchangeable complex; F-2, authigenic Fe-Mn oxyhydroxides and associated metals; F-3, metals bound to organic matter/sulfides), and residual (F-4), fixed in crystalline lattices ones. The element contents were determined by the XRF and AAS methods, total carbon (TC) and total organic carbon (TOC) were determined using a Shimadzu TOC-L-CPN. Mineral composition and maps of element distribution in sediment components were obtained using the XRD and SEM-micro-X-ray spectrometry methods, respectively. In metalliferous sediments, according to our data, the major Fe mineral phase was goethite FeOOH (37–44% on a carbonate-free basis, cfb). In the metalliferous core, average contents (cfb), of Fe and Mn were 32.1% and 0.29%, whereas those of Cu, Zn, Pb, and As, were 0.74%, 0.27%, 0.03%, and 0.02%, respectively. Metalliferous sediments are enriched in Fe, Cu, Zn, Pb, and As, relatively to background ones. The exception was Mn, for which no increased accumulation in metalliferous core was recorded. Essential mass of Fe (up to 70% of total content) was represented by the residual fraction composed of crystallized goethite, aluminosilicates, the minerals derived from bedrock destruction processes mineral debris. Among geochemically mobile fractions, to 80% Fe of the (F-1 + F-2 + F-3) sum was determined in the form of F-2, authigenic oxyhydroxides. The same fraction was a predominant host for Mn in both metalliferous and background sediments (to 85%). With these Fe and Mn fractions, a major portion of Cu, Zn, and Pb was associated, while a less their amount was found in sulfide/organic fraction. In the metalliferous sediment core, maximal concentrations of metals and their geochemically mobile fractions were recorded in the deeper core sediment layers, an observation that might be attributed to influence of hydrothermal diffused fluids. Our data suggested that ore metals are mostly accumulated in sediment cores in their contact zone with the underlying serpentinized peridotites.

Neodymium Isotopes in Glauconite for Palaeoceanographic Reconstructions at Continental Margins: A Preliminary Investigation From Demerara Rise

Contourite sediment accumulations at continental margins are related to strong bottom water circulation, where intense winnowing can result in neoformation of authigenic grains of glauconite at the seafloor. In this study, we investigated whether such glauconite grains could faithfully record ambient bottom-water neodymium (Nd) isotopic compositions, and hence be used as paleoceanographic archives. To this purpose, we measured Nd isotopic compositions (εNd) in a series of glauconitic grains, foraminiferal assemblages, leached Fe-Mn oxyhydroxide phases, and detrital clays separated from a contourite sediment record at the Demerara slope off French Guiana (IG-KSF-11; 2370 m water depth), at a location where the present-day εNd distribution along the water column is well characterised. We show that the εNd composition of core-top glauconite grains (−12.0 ± 0.5) agrees with the expected NADW-like seawater signature at the same location and water depth (−11.6 ± 0.3), while departing from measured εNd values for corresponding detrital clays (−11.3 ± 0.2), foraminiferal (−10.9 ± 0.2), and Fe-Mn oxyhydroxide fractions (−9.2 ± 0.2). This finding indicates that glauconitic grains at this particular location are probably best suited for paleoceanographic reconstructions than foraminifera and leached Fe-oxyhydroxide fractions, which appear to be influenced by sediment redistribution and the presence of terrestrial continental Fe-oxides, respectively. Using rare earth elements (REE), we tentatively propose that the acquisition of seawater Nd isotopic signatures by glauconite is controlled by the presence of authigenic REE-bearing phosphate-rich phases intertwined within clay mineral sheets, while confirming previous findings that the process of glauconitisation results in the progressive loss of REE within glauconitic grains. Preliminary paleoceanographic implications suggest strengthened bottom-water circulation of the glacial analogue of NADW at this particular location and water depth, with a εNd signature (between −10.8 and −11.5) similar to that of modern NADW.

Geochemistry of fracture coatings in Athabasca Group sandstones as records of elemental dispersion from the McArthur River Uranium deposit

A mineralogical and geochemical study of minerals on seven types of fracture fillings from near and above the McArthur River unconformity-related uranium deposit has been carried out to determine if fracture filling materials and wall rocks record evidence of primary and secondary dispersion of elements from the deposit. This study shows that Pb extracted by weak acid leaching (2% HNO3) provides Pb-Pb model ages near 1100 Ma for fractures with any of the three REE patterns: 1) LREE-enriched patterns, 2) REE patterns that are concave and similar to uraninite in the deposit, and 3) fractures with flat REE patterns. The REE patterns and ages indicate the fractures are an open system to post-mineralization dispersion of radiogenic Pb from a U-rich source, with a major mobilization of Pb from the uranium deposit at ca. 1100 Ma.Brown (Type 1) fractures, which host Mn-Fe oxides and clay minerals, provide the best evidence of U mineralization through post-mineralization dispersion, containing elevated concentrations of pathfinder elements Co, Ba, Tl, Mn and radiogenic Pb near the surface and at depth. Post-mineralization dispersion is also observed within brown (Type 1) and pink (Type 7; clay minerals + hematite) fractures and wall rock, showing low 207Pb/206Pb values of 0.29–0.50 that were subsequently overprinted by fluids with more common Pb, as recorded in fractures with higher 207Pb/206Pb values of 0.51–0.84.Fracture fillings were analyzed by continuous leach inductively coupled plasma mass spectrometry (CL-ICP-MS) to determine the relationship between trace elements and elemental release from specific mineral phases. CL-ICP-MS results indicate that common Pb (with 207Pb/206Pb values > 0.7) and anomalous Co (360 ppm) and V (220 ppm) contents are resident in organic phases that are dissolved during the 10% H2O2 leach phase. CL-ICP-MS also indicates that radiogenic Pb, as well as Ni and V, are resident in clay minerals and Fe-Mn oxy-hydroxides that break down during leaching with 30% HNO3. Uranium contents are low in the leachate during release of Fe, Al, Mn, and Pb using the 10% and 30% HNO3 leaches, reflecting post-mineralization dispersion of radiogenic Pb, but not U. Thus, secondary dispersion of elements into fractures enhances the size of the deposit footprint, particularly in deep fractures near the P2 fault, but also in some fractures near the surface. The results of this study strongly suggest that fractures did indeed act as conduits for dispersion of elements from the deposit, reaching all the way from deposit to the surface, providing viable exploration targets for U mineralization in sedimentary basins.

Mechanism and Influencing Factors of REY Enrichment in Deep-Sea Sediments

Deep-sea sediments with high contents of rare-earth elements and yttrium (REY) are expected to serve as a potential resource for REY, which have recently been proved to be mainly contributed by phosphate component. Studies have shown that the carriers of REY in deep-sea sediments include aluminosilicate, Fe-Mn oxyhydroxides, and phosphate components. The ∑REY of the phosphate component is 1–2 orders of magnitude higher than those of the other two carriers, expressed as ∑REY = 0.001 × [Al2O3] − 0.002 × [MnO] + 0.056 × [P2O5] − 32. The sediment P2O5 content of 1.5% explains 89.1% of the total variance of the sediment ∑REY content. According to global data, P has a stronger positive correlation with ∑REY compared with Mn, Fe, Al, etc.; 45.5% of samples have a P2O5 content of less than 0.25%, and ∑REY of not higher than 400 ppm. The ∑REY of the phosphate component reaches n × 104 ppm, much higher than that of marine phosphorites and lower than that of REY-phosphate minerals, which are called REY-rich phosphates in this study. The results of microscopic observation and separation by grain size indicate that the REY-rich phosphate component is mainly composed of bioapatite. When ∑REY > 2000 ppm, the average CaO/P2O5 ratio of the samples is 1.55, indicating that the phosphate composition is between carbonate fluoroapatite and hydroxyfluorapatite. According to a knowledge map of sediment elements, the phosphate component is mainly composed of P, Ca, Sr, REY, Sc, U, and Th, and its chemical composition is relatively stable. The phosphate component has a negative Ce anomaly and positive Y anomaly, and a REY pattern similar to that of marine phosphorites and seawater. After the early diagenesis process (biogeochemistry, adsorption, desorption, transformation, and migration), the REY enrichment in the phosphate component is completed near the seawater/sediment interface. In the process of REY enrichment, the precipitation and enrichment of P is critical. According to current research progress, the REY enrichment is the result of comprehensive factors, including low sedimentation rate, high ∑REY of the bottom seawater, a non-carbonate depositional environment, oxidation conditions, and certain bottom current conditions.

Investigations on mineralogy and geochemistry of a black shale profile on the northern Yangtze platform, China: Weathering fate of rare earth elements and yttrium (REY) and its implications

The mineralogical and geochemical compositions of an 11.5 m black shale weathering profile on the northern Yangtze platform, China, are documented to investigate the fate of rare earth elements and yttrium (REY) and their potential implications during chemical weathering. Our results signify that the profile has experienced an expected weathering trend in initial to moderate weathering stages based on the chemical index of alteration (CIA) values (54.45–75.50). Major elements (Ca, Mg, Na and K) have been leached to various degrees throughout the profile, roughly coupled with the weathering trends of primary minerals. Most REY were released from the black shale matrix, transported downwards and partially precipitated in a fracture. Lanthanum, Ce and Pr exhibited less mobility than other REY, which was connected with the weathering-resistant rhabdophane ((La,Ce)PO4). The REY enrichment in the fracture resulted from the adsorption of Fe–Mn oxyhydroxides and kaolinite. Negligible Ce anomalies (Ce/Ce* values of 0.95–1.12) and prominent negative Eu anomalies (Eu/Eu* values of 0.40–0.77) are observed throughout the profile. The Ce/Ce* values are covariant with the sizes of the first tetrad effect, while Eu anomalies were inherited from the bedrock, and the variation in Eu/Eu* values is related to plagioclase dissolution. LREE/HREE, Sm/Nd and Y/Ho fractionation occurred during black shale weathering. The LREE/HREE fractionation was mainly controlled by primary and secondary minerals (e.g., rhabdophane and kaolinite). The Sm–Nd and Y–Ho fractionation was affected by the tetrad effect. Fracture zones in black shales would be new potential targets for REY prospecting and extraction. Highly REY-enriched zones may occur in soils and underground aquifers, which would pose potential threats to crops and humans. Our findings, combined with previous studies, suggest that LREE/HREE fractionation is indicative of the weathering degree of continental rocks.

The temporal variability of hydrothermal activity of Wocan hydrothermal field, Carlsberg Ridge, northwest Indian Ocean

The Wocan is a basalt-hosted hydrothermal field on the slow-spreading Carlsberg Ridge (CR), northwest Indian Ocean. It consists of two hydrothermal sites: Wocan-1 which is active, and Wocan-2 which is inactive. Three push-cores collected by the manned submersible Jiaolong from these two sites were investigated on their mineralogy, geochemistry, Pb-Sr isotopes, and chronology to understand the characteristics of hydrothermal sediments and their hydrothermal history. Core JL125-S17 (26-cm long) was collected from the Wocan-1 site with its lithology characterized by metalliferous sediments, mainly composed of sulfides, sulfates, Fe-oxides, and Fe-oxyhydroxides. Cores JL123-S03 (18-cm long) and JL124-S04 (20 cm long) were retrieved from the Wocan-2 site, mainly composed of calcareous oozes. Geochemically, Wocan-1 sediments exhibit strong metal and S enrichment. Ag, As, Cd, Co, Cu, Fe, Mg, Pb, Zn, and S are mainly incorporated into sulfides, while As, Ga, Mo, P, U, and V are mainly scavenged from seawater by Fe-Mn oxyhydroxides. In contrast, Wocan-2 sediments are rich in Ca and have much lower heavy metal content. The temporal variations of hydrothermal metal fluxes, which corrected for lithogenic input with Ti show that the intensity of hydrothermal activity at the Wocan-1 site has been extremely strong over the past ~1069 years, with hydrothermal Fe (FeH) fluxes varying from 9483 to 10834 mg·cm−2·kyr−1. During this period, 699–1069 yr BP and 0–411 yr BP represent two stages of increased hydrothermal activity. The Wocan-2 site has been inactive since 4864 ± 114 years before present (yr BP) but the cores still contained weak hydrothermal signatures evidenced by a slightly elevated flux of FeH at the base of the cores, which could be explained by the mineralization of low-temperature diffuse flow and distal plume fall-out from Wocan-1.