Shale-normalized REE Patterns Research Articles

Geochemistry, facies characteristics and palaeoenvironmental conditions of the storm-dominated phosphate-bearing deposits of eastern Tethyan Ocean; A case study from Zagros region, SW Iran

Phosphate deposits in south-western Iran are part of the South Tethyan Phosphogenic Province, a huge carbonate-dominated strata that extends to the Middle East. The Tethyan phosphorites of Iran are dated Eocene-Oligocene (Pabdeh Formation) and categorized as low-grade ore deposits on a global scale. Depositional conditions of the facies indicate that the Pabdeh Formation was deposited on a carbonate ramp setting as a distally steepened ramp. Under such an environment, turbidity currents transported phosphate particles from the back-shoal setting to the deeper middle and outer ramp of the ocean where they were suspended and deposited as shell-lag and phosphate lamination. Microfacies studies demonstrate that all the phosphatic ooids and phosphatized foraminifera, fish scales, bones and phosphatic intraclasts reworked from shallow parts of the Tethyan Ocean to deeper parts with the help of turbidity currents. Analysis and interpretation of the data reveal positive correlation between. REE+Y and P2O5 in all studied sections which attests to their strong coherence as a geochemical group. The shale normalized REE patterns of Mondun phosphorites are characterized by negative Ce anomalies. This anomaly indicate that the depositional environment was oxic and highly reworked, bioturbated with higher energy realm during phosphate deposition, conversely Nill section with Ce enrichment reflect conditions of relatively deeper water sedimentation. These geochemical findings are in accordance with microfacies studies which indicate shallow and high energy condition for Mondun section with negative cerium anomalies and a deep ramp setting for Nill and Siah sections which denote a positive cerium anomalies in REE patterns.

REEs and U distribution in P-rich nodules from Gelasian Apulian Tethyan carbonate: A genetic record

In the Tethyan realm the carbonate-dominated Meso-Cenozoic South Tethyan Phosphogenic Province is of considerable economic importance since it represents the greatest accumulation of sedimentary phosphorites. In southern Italy, in the Salentine peninsula (the southern part of the Apulian Carbonate Platform, ACP), is well documented the occurrence of Cenozoic P-rich levels consisting of nodules and pebbles and showing a large P2O5 fluctuation (4.07–22.07 wt%), due to variable calcite abundance. The mainly observed P-bearing minerals are hydroxyapatite and carbonate fluorapatite and U, together with Sr and Pb, preferentially acts as Ca substitutes in both lattices. Minor authigenic monazite (LREE-bearing phosphate) and xenotime (HREE+Y-bearing phosphate), likely formed during sediment burial compaction and diagenesis. The total REEs' abundances and the shape of the shale-normalized REE-patterns in the P-rich nodules are in the range of those typically observed in Paleocene-Eocene through Pleistocene-Recent P-rich sediments, supporting the idea of a broadly consistent ocean chemistry in this span of geological time. The (La/Yb)N proxy is within the modern seawater range, signifying early diagenetic adsorption played only a minor role in affecting the REEs' distribution. The Ce and Pr anomalies suggest some P-rich nodules experienced very localized hypoxic to anoxic conditions promoting Ce/Ce* spurious results in a generally oxic to hypoxic environment causing real negative Ce anomaly. This scenario is reinforced by the lack of the coupled uranium-vanadium enrichment typically observed in an anoxic environment. Since the Pliocene the eastern Mediterranean was variously affected by oxygenation and productivity and it is likely the ACP phosphate-rich sediments formed under low sedimentation rates and authigenesis in a bottom current-dominated regime, as also indicated by the glauconite occurrence. Transgressions and sea levels rising following major glaciations may have favored the deposition of phosphate sediments by creating new restricted basin configurations and increased nutrient input promoted by upwelling processes.

Fine-scale LA-ICP-MS study of redox oscillations and REEY cycling during the latest Devonian Hangenberg Crisis (Moravian Karst, Czech Republic)

Trace elementsand REEY distribution of thinly laminated limestone (laminite) and underlying pre-event calciturbidite (Siphonodella praesulcata conodont Zone) deposited in association with the Hangenberg Black Shale Event were analyzed using laser ablation-inductively coupled plasma-mass spectrometry. These deposits were also analyzed by carbonate petrography and scanning electron microscopy. Our study documents the origin of authigenic microbial limestones during the latest Devonian Hangenberg extinction event, when unusual oceanic conditions appear to have enhanced precipitation of calcite within microbial carbonates. Detrital contamination of the calciturbidite and laminated limestone is very low, as demonstrated by low contents of Al, Zr, Ti and Th. These deposits display only limited degree of early diagenetic overprint of limestones, and some of them fall within or very close to the seawater field. Elevated U and Mo enrichments and covariation patterns reflect the episodic establishment of oxic to euxinic bottom condition during deposition of the laminite. The oxic conditions that existed during accumulation of the underlying calciturbidite were likely the result of turbidity current introduction of oxygen into the bottom environment. REE contents of both the laminite and calciturbidite are nearly ten times higher than those of most modern carbonates and similar to Devonian microbialites from the Canning Basin. Calciturbidite display a prominent MREEN bulge. Shale-normalized REE patterns of the overlying laminite are similar to those of modern seawater and Devonian microbialites from the Canning Basin (LREE depletion and progressive enrichments in heavier REE) reflecting the dominance of authigenic carbonate phases. Minor contamination is bound to apatites, siliciclastic minerals, pyrites and organic matter. The element content of authigenic phases benefited from Fe and Mn oxyhydroxide redox cycling. Negative Ce and positive Eu anomalies, low Y/Ho ratios, and very low Al/(Al+Fe+Mn) ratios suggest an influence of diluted high- temperature hydrothermal fluids perhaps related to latest Devonian rift magmatism in the eastern part of the Rhenohercynian zone. The proliferation of anachronistic facies following the Devonian-Carboniferous (D-C) extinction event recognized in Moravia and traced worldwide seems to share some features in common with anachronistic carbonate sequences documented in the aftermath of the end-Permian mass extinction.

Origin of the Breno and Esino dolomites in the western Southern Alps (Italy): Implications for a volcanic influence

The Esino Limestone of the western Southern Alps represents a differentiated Ladinian-Lower Carnian (?) carbonate platform comprised of margin, slope and peritidal inner platform facies up to 1000 m thick. A major regional subaerial exposure event lead to coverage by another peritidal Lower Carnian carbonate platform (Breno Formation). Multiphase dolomitization affected the carbonate sediments. Petrographic examinations identified at least three main generations of dolomites (D1, D2, and D3) that occur as both replacement and fracture-filling cements. These phases have crystal-size ranges of 3–35 μm (dolomicrite D1), 40–600 μm (eu-to subhedral crystals D2), and 200 μm to 5 mm (cavity- and fracture-filling anhedral to subhedral saddle dolomite D3), respectively.The fabric retentive near-micritic grain size coupled with low mean Sr concentration (76 ± 37 ppm) and estimated δ18O of the parent dolomitizing fluids of D1 suggest formation in shallow burial setting at temperature ∼ 45–50 °C with possible contributions from volcanic-related fluids (basinal fluids circulated in volcaniclastics or related to volcanic activity), which is consistent with its abnormally high Fe (4438 ± 4393 ppm) and Mn (1219 ± 1418 ppm) contents. The larger crystal sizes, homogenization temperatures (D2, 108 ± 9 °C; D3, 111 ± 14 °C) of primary two-phase fluid inclusions, and calculated salinity estimates (D2, 23 ± 2 eq wt% NaCl; D3, 20 ± 4 eq wt% NaCl) of D2 and D3 suggest that they formed at later stages under mid-to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity, which is supported by higher estimated δ18O values of their parent dolomitizing fluids. This is also consistent with their high Fe (4462 ± 4888 ppm; and 1091 ± 1183 ppm, respectively) and Mn (556 ± 289 ppm and 1091 ± 1183 ppm) contents, and low Sr concentrations (53 ± 31 ppm and 57 ± 24 ppm, respectively).The similarity in shale-normalized (SN) REE patterns and Ce (Ce/Ce*)SN and La (Pr/Pr*)SN anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor. Positive Eu anomalies, coupled with fluid-inclusion gas ratios (N2/Ar, CO2/CH4, Ar/He), high F− concentration, high F/Cl and high Cl/Br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks, which is consistent with the co-occurrence of volcaniclastic lenses in the investigated sequence.

Geochemical characteristics and metal element enrichment in crusts from seamounts of the Western Pacific

Elemental geochemistry is an essential part of understanding mineralization mechanisms. In this paper, a data set of 544 cobalt crust samples from seamounts of the Western Pacific are used to study the enrichment characteristics of metal elements. REE normalization is utilized to reveal the origin of the crusts; effects of water depth on Co enrichment and impacts of phosphatization on mineral quality are discussed to obtain the evolution of these marine mineral deposits, which gives support to further resource assessment. Conclusions are reached as follows: 1) Elemental abundances, inter-element relationships, and shale-normalized REE patterns for phosphate-poor crusts from different locations reflect hydrogenetic origin of the crusts. EFs (enrichment coefficients) of REE exhibit exponential increase from surface sediments to phosphorite to polymetallic nodules to crusts, suggesting that the improved degree of hydrogeneous origin induces the enrichment of REE. 2) The crusts in the Western Pacific, formed through hotspot produced guyots trails, have relatively lower REE than those in the Mid-Pacific. The latter could be attributed to the peculiar submarine topography of seamounts formed by intraplate volcanism. 3) The non-phosphatized younger crust layers have 40% higher Co than the phosphatized older layers. This indicates the modification of the elemental composition in these crusts by phosphatization. A general depletion of hydroxide-dominated elements such as Co, Ni, and Mn and enrichment of P, Ca, Ba, and Sr is evident in phosphatized crusts, whereas non-phosphatized younger generation crusts are rich in terrigenous aluminosilicate detrital matter. 4) Co increases above the carbonate compensation depth (CCD) from less than 0.53% to over 0.65% in seamount regions with water depth of less than 2,500 m, suggesting the significance of the dissolution of carbonate in the sea water column to the growth and composition of crusts.

Petrological and geochemical features of the Jingtieshan banded iron formation (BIF): A unique type of BIF from the Northern Qilian Orogenic Belt, NW China

The Jingtieshan banded iron formation (BIF) is located in the Northern Qilian Orogenic Belt (NQOB) in NW China. The BIFs are hosted in Mesoproterozoic Jingtieshan Group, a dominantly clastic-carbonate sedimentary formation, and was metamorphosed to lower greenschist facies. The Jingtieshan BIFs include oxide-, carbonate- and mixed carbonate–oxide facies, and consist of alternating iron-rich and silica-rich bands. The BIFs are composed essentially of specularite and jasper, with minor carbonate minerals and barite. The SiO2+Fe2O3 content is markedly high in the oxide facies BIF, followed by FeO, CO2 and Ba, with the other elements usually lower than 1%, suggesting that the original chemical sediments were composed of Fe, Si, CO32− and Ba. The positive correlation between Al2O3, TiO2 and Zr in the BIFs indicates that these chemical sediments incorporate minor detrital components. Oxide facies BIF shows low HFSE, low ∑REE and low Y/Ho. The Post Archean Australian Shale-normalized REE patterns for Jingtieshan BIFs are characterized slight LREE depletion, strong positive Eu anomalies and lack of significant negative Ce anomalies. Siderite in the carbonate- and mixed carbonate–oxide facies BIF shows negative δ13C values varying from −8.4‰ to −3.0‰, and δ18O values show a range of −16.6‰ to −11.7‰. The geochemical signatures and carbon–oxygen isotopes suggest origin from high-temperature hydrothermal fluids with weak seawater signature for the sediments of Jingtieshan BIFs. The absence of negative Ce anomalies and the high Fe3+/∑Fe ratios of the oxide facies BIF do not support ocean anoxia. In contrast to the three main types (Algoma-, Superior- and Rapitan-type) of global BIFs, the Jingtieshan BIFs represent a unique type with features similar to those of sedimentary-exhalative mineralization.

Geochemical and Isotopic Signatures of Surficial Sediments from the Western Continental Shelf of India: Inferring Provenance, Weathering, and the Nature of Organic Matter

Abstract Bottom sediments from thirteen transects sampled mainly at depth contours of 30, 100, and 200 m along the western continental margin of India, falling in two offshore sectors from Dwaraka to Goa and Cape Comorin to Goa, were analyzed for calcium carbonate, major elements in silicate fraction, trace and rare earth elements in the bulk fraction, and carbon and nitrogen isotopes in organic matter to understand the provenance of sediments, weathering patterns in the source areas, and nature of organic matter. Major elements such as Si, Ti, Mg, and discrimination plots involving alkalies and silica have shown a distinct north–south provinciality, with the source signatures pointing at Deccan basalt for sediments in the north (from Dwaraka to Goa) and gneisses and granulites of South India in the south. Shale-normalized REE patterns and REE content suggest additional local sources mainly in the southwest coast of India. Degree of alteration determined using Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and A-CN-K plots have suggested that the sediments are moderately weathered despite the intense orographic rainfall in the source area. Variability in degree of chemical weathering is seen with the change in provenance. In general, the sediments north of Goa with a provenance of Deccan basalts show a higher chemical weathering index. Corg/N molar ratios are mostly between 10 and 12 (minimum and maximum being 6.9 and 14 respectively), which are closer to values for marine organic matter. This is corroborated by marine δ13C values (−18.2 to −21.6‰). While they broadly fall within the marine values, heavier values are seen in northern transects off Porbandar and Dwaraka, where the sediments have a provenance different from those in the south. This could be due to a higher contribution from C4-type vegetation in the north, consistent with heavier δ13C values in terrestrial plants in source regions. Heavier δ15N values are observed for the northern transects (north of Ratnagiri), which is supported by the intense denitrification in the northern Arabian Sea.

Geology, geochemistry, and genesis of the hot-spring-type Sipingshan gold deposit, eastern Heilongjiang Province, Northeast China

The Sipingshan gold deposit, located in the eastern part of the Nadanhada Terrane, is hosted within cherts and silicified breccias of the Upper Cretaceous Sipingshan Formation and rhyolites of the Upper Cretaceous Datashanlinchang Formation. The orebodies are composed of gold- and pyrite-bearing cherts, silicified breccias, and quartz veins accompanied by various types of wall rock alteration, including silicification, pyritization, sericitization, chloritization, pyrophyllitization, and carbonatization. LA-ICP-MS U–Pb zircon ages determined for the ore-bearing rhyolites range between 122 ± 1.4 and 135.2 ± 1.9 million years slightly older than the metallogenic age of the Sipingshan gold deposit. The rhyolite has aluminium saturation index values ranging from 0.015 to 1.25 and shows the following features: enrichment in LILE (e.g. Rb, Pb, K, and Th); depletion of Ba, Sm, and Ti; and negative Eu anomalies. These geochemical characteristics indicate that (1) the rhyolite contains features typical of S-type granites; (2) the felsic magma likely originated through partial melting of the continental crust; and (3) plagioclase crystals were present in the partial melt residues in the magma source region, or else magma evolution involved plagioclase fractionation. The host cherts have high Al/(Al + Fe + Mn) ratios (0.23–0.81, averaging 0.60) and low Al2O3 and TiO2 contents. Their North American shale-normalized REE patterns are characterized by flat REE, slightly positive Eu anomalies, no Ce anomalies, and (La/Yb)SN ratios of 1.27–1.38, indicating that these cherts formed in a continental margin environment. In addition, the analysed cherts have low ΣREE (1.56–3.64 ppm) and Zr (9.1–13.5 ppm) contents, suggesting a hydrothermal origin. Fluid inclusions in quartz veins show elliptical to irregular shapes that range from 5 to 12 μm in size and have homogenization temperatures of 118.7–223.4°C, densities of 0.84–0.94 g/cm3, and pressures of 21.2–51.4 MPa, indicating that the hot-spring-type Sipingshan gold deposit is epithermal in origin.

Archean cherts in banded iron formation: Insight into Neoarchean ocean chemistry and depositional processes

This study reports new REE+Y and 3-isotope sulfur data for Archean banded iron formation and volcaniclastic rocks with cherty bed tops in the Neoarchean Abitibi greenstone belt of Canada. The data were analyzed with a view to better constrain Neoarchean ocean chemistry, atmospheric conditions prevalent during weathering and transport, the development of Algoma-type banded iron formation and the overall process of stratigraphic development of greenstone belts. The Abitibi greenstone belt consists of 7 mafic to felsic volcanic cycles, each capped by a sedimentary interface zone consisting of chemical and minor clastic metasediments. We concentrated sampling on the iron formation capping the ca. 2730Ma Deloro assemblage as it occurs over a wide area (300km×600km) and because there is a substantial depositional gap prior to deposition of the overlying volcanic rocks. Volcaniclastic rocks within the ca. 2710Ma Tisdale assemblage were also sampled.Chemical analyses focussed on the SiO2-rich portion of the samples and were conducted by laser ablation ICP-MS. In situ analysis of S isotopes was obtained for pyrite by ion probe. REE data display four types of patterns: (1) hydrothermally influenced marine hydrogenous sediment, (2) contaminated, hydrothermally influenced marine hydrogenous sediment, (3) hydrothermally dominated patterns, and (4) replacement patterns indicating silicification of precursor volcanic units. Contamination and/or the presence of non-chert components were documented with Th, U and Zr content. Non-chert components were defined as: (1) phosphates that led to elevated Th/U, (2) clastic detritus leading to flat shale normalized REE patterns, and (3) volcanic detritus leading to elevated values for Zr. No meaningful difference in REE+Y geochemistry as a function of elevated Th/U was found implying that phosphates have the same REE patterns as the host chert. The cherts within banded iron formation exhibited stratigraphic variation in several localities, progressing from replacement chemistry (flat REE profile) at the base, hydrogenous sediment geochemistry (positive La, Gd, Y/Ho anomalies) in the middle part and hydrothermal patterns (depleted LREE, elevated positive Eu anomalies) in the upper part. The upper parts of some units also display +Ce anomalies possibly reflective of more oxygenated water also supported by S isotope data. A consistent increase in Pr/Yb in the upper parts of units is postulated to reflect shallowing upward of depositional depth to an unknown extent but not above storm wave base. A number of samples with flat REE patterns lacking La and Gd anomalies represent hydrothermal deposition with the largest Eu/Eu* values recorded for Archean iron formation. The main contribution of the Abitibi banded iron formations is that they provide a deeper water perspective on Archean ocean chemistry. The resulting picture is that of slow BIF accumulation, a generally strong hydrothermal input of REE and complex oceanic cycling, possibly involving a chemocline above the sampled water depth.The new sulfur isotope data show a greater extent of mass independent fractionation than previously recorded for the Neoarchean, with Δ33S ranging from −1.4 to +4.2‰. The strongly MIF positive source (Δ33S=+4‰) was apparently similar to Paleoarchean values.

SEA-LEVEL CHANGES RECORDED BY CERIUM ANOMALIES IN THE LATE JURASSIC (TITHONIAN) BLACK ROCK SERIES OF QIANGTANG BASIN, NORTH-CENTRAL TIBET

The Upper Jurassic black rock series of the Amdo area of north-central Tibet have attracted attention because of its organic-rich matter and oil seepage in the rock series. Abundant ammonites of Aulacosphinctes and Virgatosphinctes as well as bivalves including Buchia and Chlamys, are well preserved in the Qiangmuleiqu Formation of the Middle to Late Tithonian (Late Jurassic). The total rare earth elements (∑REE) abundance of the formation varies from 18.814 to 46.818 ppm and is lower than that of an average in marine shales. Inter-element correlations suggest that the shale-normalized REE patterns provide the information about the origin of sedimentary rocks (i.e., not affected by diagenesis), and can be used as a potential indicator for eustatic sea-level changes. The Ceanom values gradually increased from –0.015 to –0.238 from the lower to middle part of the series, indicating a rise in sea level. The Ceanom then dropped to –0.081, suggesting a lowering of sea level. From the middle to upper part of the section, there are two more sea-level fluctuations indicated by variations of cerium anomaly values. Thus, sea-level fluctuations occurred three times in this Late Jurassic section, with major episodes of eustatic rise took place in the lower-middle part of this section. These episodic cycles show

Shift in detrital sedimentation in the eastern Bay of Bengal during the late Quaternary

Down-core variations of granulometric, geochemical and mineral magnetism of a 70-cm long sediment core retrieved from the eastern Bay of Bengal abyssal region were studied to understand sedimentation pattern and sediment provenance during the last ∼12 kyr BP. Based on down-core physical and elemental variations, three units were identified: unit 3 (70–43 cm) is a ∼30 cm thick clayey silt organic carbon-rich (0.5–0.92%) turbidite probably delivered by the Brahmaputra River during the late Quaternary period. Units 2 (43–24 cm) and 1 (24–0 cm) represent enhanced and reduced supply of coarse-grained detrital sediments from the Ganges River during early and late Holocene period, respectively. Increased terrigenous supply dilutes calcium carbonate (CaCO3) and biogenic elements (P, Ba and Cu) in units 3 and 2. On the contrary, a reduction in detrital input enhances CaCO3 and biogenic elements in unit 1. Lithogenic elements (Ti, Al, K and Rb) and shale-normalized REE patterns in all three units suggest terrigenous source. The shift in provenance from the Brahmaputra to the Ganges derived sediments is evident by a sharp increase in sediment grain size, increased concentration and grain size assemblages of magnetic minerals, lithogenic elements concentration and Lan/Ybn ratio. This study highlights terrigenous dilution on biogenic sedimentation in the eastern Bay of Bengal sediments.

Rare Earth and Metal Geochemistry of Land and Submarine Phosphorites in the Baja California Peninsula, Mexico

The geochemical composition of phosphorites and phosphatic sediments in the Baja California peninsula is studied and used to assess the environment in which phosphogenesis took place. The deposits are classified in three groups: (1) stratified phosphorites, (2) phosphatic sandy sediments from beaches and dunes, and (3) submarine sediments. Some of the elements that might have substituted Ca and PO4 during francolite mineralization were studied by means of ICP-AES. Significant differences are seen in the concentration of these metals (e.g., Cr = 72-406 μg g−1 and V = 17-198 μg g−1), indicating that their concentration is not only controlled by the P2O5 concentration, but also by paleo-environmental conditions existing during francolite precipitation. Shale normalized REE patterns suggest two main environments of formation: (1) a strong negative Ce anomaly (< − 0.3) and La enrichment (La/Nd ≥ 1) enrichment, suggesting well oxygenated shelf environments and probably lower light REE weathering, and (2) a weak negative Ce anomaly (> − 0.3) and La depletion (La/Nd ≤ 1) suggesting shallower waters or restricted circulation and probably LREE weathering.

Miocene phosphorites from the Murray Ridge, northwestern Arabian Sea

Phosphorites from the Murray Ridge, NW Arabian Sea comprise nodules, bioclasts, and bone fragments. The nodules are made up of a homogeneous, light-colored phosphate nucleus consisting of Rivulariacean filamentous cyanobacteria and a thin dark-grey colored phosphate cortex showing abundant microbial filaments and microborings. The bioclasts comprise of ∼ 14–14.5 Ma old planktonic foraminifers, accepted as the time of deposition. Spherical to ovoid-shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts. Bone fragments exhibit apatite fillings. The nodules and bone fragments consist entirely of carbonate fluorapatite (CFA) with low Al, K, and Th concentrations suggesting absence of continental detritus. Shale-normalized REE patterns of the samples support a seawater-derived composition. The highly uniform initial ɛ Nd values of − 4.8 to − 5.1 are interpreted as the seawater value at the onset of phosphatization ∼ 14 Ma ago. In contrast, 87Sr/ 86Sr ratios show a large range of 0.709055 to 0.709124 corresponding to unusually young stratigraphic ages of ∼ 1 to 3 Ma. The data are interpreted as evidence for post-depositional Sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater. It is concluded that the phosphorites formed under oxic, shallow-water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of CFA during early diagenesis at the sediment–water interface on different substrates.

Rare Earth, Major and Trace Elements in the Kunimiyama Ferromanganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

Abstract. Rare earth, major and trace element geochemistry is reported for the Kunimiyama stratiform ferromanganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The deposit immediately overlies greenstones of mid-ocean ridge basalt (MORB) origin and underlies red chert. The ferromanganese ores exhibit remarkable enrichments in Fe, Mn, P, V, Co, Ni, Zn, Y and rare earth elements (excepting Ce) relative to continental crustal abundance. These enriched elements/ Fe ratios and Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are generally analogous to those of modern hydrothermal ferromanganese plume fall-out precipitates deposited on MOR flanks. However in more detail, Mn and Ti enrichments in the ferromanganese ores are more striking than the modern counterpart, suggesting a significant contribution of hydrogenetic component in the Kunimiyama ores. Our results are consistent with the interpretation that the Kunimiyama ores were umber deposits that primarily formed by hydrothermal plume fall-out precipitation in the Panthalassa Ocean during the Early Permian and then accreted onto the proto-Japanese island arc during the Middle Jurassic. The presence of strong negative Ce anomaly in the Kunimiyama ores may indicate that the Early Permian Panthalassa seawater had a more striking negative Ce anomaly due to a more oxidizing oceanic condition than today.

Constraints in using Cerium-anomaly of bulk sediments as an indicator of paleo bottom water redox environment: A case study from the Central Indian Ocean Basin

The total rare earth elements (∑REE) abundance along with major and a range of trace element chemistry of twenty-eight sub-sections in a 5 m long sediment core (AAS-05/GC-02) from the Central Indian Ocean Basin shows a distinct REE fractionation. Sediments from the top ∼1 m of the core have comparatively low ∑REE abundance (close to average shale), with a sample / shale REE ratio very close to 1 strongly suggesting a dominantly terrigenous source. The down-core ∑REE abundance shows a nearly two fold increase (∑REE = 167–341 ppm) with depth and the REE strongly co-varies with Mn, Ti, P and smectite. Inter-element correlations and the shale-normalized REE patterns suggest that light, middle and heavy REE are selectively carried by terrigenous, Mn oxide and authigenic phosphate phases respectively suggesting a distinct REE fractionation. These fractionation effects become more pronounced with increasing depth in the core. The Cerium (Ce)-anomaly in marine sediments is used as one of the promising tools to trace paleo bottom water redox conditions and this varies in the sediments from 0.10 to − 0.08. The Ce-anomaly in general is positive in the top 4 m but the top ∼1 m have higher Ce-anomaly, Mn / Ti and Ce / Ti suggest sediment deposition relatively under more oxic bottom water condition. The negative Ce-anomaly between 4 and 5 m depth, which could be interpreted to indicate a suboxic/anoxic environment. The redox environment inferred by the Ce-anomaly is compared to a number of other redox sensitive parameters to test its reliability. These include Mn content, total organic carbon, U / Th, authigenic Uranium, Cu / Zn and V / Cr ratio that all suggestive of deposition of sediments under oxygenated bottom water condition. Thus, the negative Ce-anomaly observed between 4 and 5 m core depth does not represent a suboxic/anoxic environment, but is more likely due to the retention of a negative Ce-anomaly caused by authigenic phosphate and authigenic Fe-rich smectite which is formed by the reaction between iron oxyhydroxide and biogenic opal suggests an early diagenetic process and is in equilibrium with seawater REE. Early diagenetic process is responsible for the part of negative Ce-anomaly observed here. Therefore, we suggest that the Ce-anomaly of bulk sediments as an indicator of paleo-ocean bottom water redox conditions needs to be used with a caution.

Rare earth elements determination and distribution patterns in sediments of a polluted marine environment by instrumental neutron activation analysis

Results obtained from the analysis of sediment core samples taken froma fairly polluted marine environment were analyzed for the REE contents todetermine the concentrations of La, Ce, Sm, Eu, Tb, Dy and Yb using instrumentalneutron activation analysis. Core samples were divided into strata of between2 to 3 cm intervals and prepared in the powdered form before irradiating themin a neutron flux of about 5.0 . 1012 n . cm —2s —1 in a Triga Mark II reactor. Down-core concentration profilesof La, Ce, Sm, Eu, Tb, Dy and Yb in 3 core sediments from three sites areobtained. The shale-normalized REE pattern from each site was examined andlater used to explain the history of sedimentation by natural processes suchas shoreline erosion and weathering products deposited on the seabed and furnishingsome baseline data and/or pollution trend occurring within the study area.The shale-normalized REE patterns also showed that LREE in the sediment samplesexhibit enrichment relative to HREE particularly, La and Sm showing enrichmentcompared to the ratios in shale. REE concentrations of 124 µg/g at thesurface of sediment collected at two of the three sites were found to decreaseto 58 and 95 µg/g, respectively. This was of particular interest whenit is used to explain the anomalies occurring in the marine sediment as aresult of geochemical processes over a long period of time. Changes in concentrationsfrom surface to bottom of the sediments ratioed to Sm concentrations and thecorrelation between concentrations of Sm and these elements were also investigatedand correlation coefficients were calculated for all REEs and sites. Validationof the method used was done using a Soil-7 SRM.

Distribution of rare earth elements and neodymium isotopes in suspended particles of the tropical Atlantic Ocean (EUMELI site)

We analyzed the REE, Mn and Al concentrations and Nd isotopic ratios in marine suspensions collected on filters (0.65 μm porosity) with in situ pumping systems in the tropical northeastern Atlantic (20°N, 18–31°W). Previously we reported the same parameters on large sinking particles collected with moored sediment traps at the sites. Shale-normalized REE patterns of the filtered suspensions are characterized by a larger light REE (LREE) to heavy REE (HREE) enrichment compared to the trapped material and a Ce anomaly that evolves positively with depth. Depth profiles of REE/Al show maximum values at 50–100 m, where the Mn/Al ratio also reaches a maximum. The profile of the Nd isotopic ratios of the filtered suspensions shows variations similar to those of the seawater. These results suggest that the filtered suspensions preferentially scavenge the LREE, especially Ce, and that the particulate Mn oxides are potential REE carriers. The relationship between the Ce anomaly and the Ce/Al ratio demonstrates that the particulate Ce anomaly is formed by (1) the LREE adsorption onto the particulate Mn oxides in the surface water, (2) Ce(III) oxidation to insoluble Ce(IV)O 2 and (3) preferential desorption of strict trivalent REE from the Mn oxides in deep water. Estimated authigenic Nd contents, using Nd isotopic ratios, decrease with depth. This is consistent with the adsorption of the REE in surface water and their desorption in deep water, suggested by the Ce anomaly formation. All the results show that the suspended particles record more clearly the authigenic REE contribution than the trapped material does. The suspended matter plays a key role in the scavenging of particle-reactive elements.

Monoisotopic REE abundances in seawater and the origin of seawater tetrad effect.

REE data for a GSJ reference carbonate sample (JDo-1) by ICP-AES, when normalized by reliable seawater REE data, exhibit fairly smooth abundance patterns except for Ce. This allows us to estimate the monoisotopic REE (Pr, Tb, Ho, and Tm) concentrations compatible with the widely accepted ID-MS data for seawater REE. For the same purpose, reported seawater REE data by ICP-MS have also been used. The monoisotopic REE concentrations estimated by the two methods are in good agreement. Eventually, the seawater REE patterns normalized by chondrite or average shale show obvious concave tetrad effects of W-type. There is no room for doubt as to the seawater tetrad effect, but its origin is open to debate. We proposed here that the tetrad effect originates from REE partitioning described by the ligand-exchange reactions of REE3+ between particulate REE and REE(CO3)2-(aq) in seawater. The particulate REE must be REE(OH)3-like ones rather than REEO1.5. The refined spin-pairing energy theory explains that the logarithmic equilibrium constants for the reactions exhibit a tetrad effect when Racah parameters for 4f electron repulsion in REE3+ ions are different between the pair of REE(III) species. We inferred that Racah parameters of REE(OH)3 and REEO1.5 are similar but they are commonly much smaller than those parameters of REE(CO3)2-(aq). When the reactions occur in the particulate-seawater system with high particulate concentration such as coastal seawater and estuarine water, the signature of concave tetrad effect is inevitably given to the seawater solution. Subsequent removal of excessive particulate matter from the system leaves the detectable tetrad effect in the seawater. Shale-normalized seawater REE patterns correspond roughly to the logarithmic equilibrium concentration ratios by the ligand-exchange reactions. Our proposal gives a consistent explanation for the seawater-like tetrad effects in REE patterns of marine carbonate rocks and the absence of such tetrad effects in those of deep-sea nodules. This is also related to a theoretical interpretation of the Masuda-Coryell plot in REE geochemistry.

Amazon estuary — assessment of trace elements in seabed sediments

The interactive processes operating on the continental shelf adjacent to the river mouth control the amount and the characteristics of the Amazon discharge reaching the Atlantic Ocean. In this study, the distribution of trace elemental concentrations, with emphasis to the rare-earth elements, in sediment cores collected at several stations from the Amazon continental shelf during the falling water period was investigated by instrumental neutron activation analysis. Cores from the terrigenous and blue water zones have relatively uniform REE concentrations throughout the profile. Cerium anomalies for samples of the upper section of the eight stations are consistently positive and of high values (normally >2). Similar variation in the elemental concentration ratios between the seabed sediments and Amazon River suspended sediments was seen for stations located in the biogenic and blue water zones, with an enrichment for Ce, Sm, Fe, Th, and Sc and a depletion for the La, Eu, Tb, Yb, Co, Cr, Cs, Hf, Ta, and Zn. The shale-normalized REE patterns from shelf sediments are enriched in LREE relative to HREE, with enrichment factors varying from 1.5 for stations near the river mouth (terrigenous zone) to 1.9 for stations located far in the blue water zone. Published data for the Amazon River suspended sediment agree remarkably well with this observation of LREE-enrichment.

Geochemical constraints on the hydrothermal origin of ferromanganese encrustations from the Rodriguez Triple Junction, Indian Ocean

Abstract Ferromanganese crusts recovered from a spreading ridge north of the Rodriguez Triple Junction in the Indian Ocean were studied for their mineral and geochemical compositions. The crusts are highly porous, have maximum thickness of 2 cm, and occur on basaltic substrates. The composition of the crusts resembles that of hydrothermal ferromanganese crusts from other oceanic spreading centres. Compared to hydrogenous ferromanganese crusts and nodules, the hydrothermal crusts exhibit very low Mn/Fe ratios, lower transitional metal contents, low ΣREE contents and negaive Ce anomalies, which are all characteristics of hydrothermal origin. Geochemical discriminant plots of crust data show similarities to data for metalliferous sediment and hydrothermal deposits recovered from other areas. Shale-normalized REE patterns indicate that the hydrothermal contribution to the crusts is between 60 and 92%. Crusts have rarely been reported to occur at Indian Ocean spreading centers, and those described here are consistent with the earlier findings on water column temperature and chemical anomalies indicative of hydrothermal activity in the region. Some aspects of the crusts, such as δ -MnO 2 mineralogy and middle REE enrichment, are not typical of hydrothermal Fe-Mn oxides. Microprobe studies show that some layers within the crusts have Mn/Fe ratios close to unity, suggesting a significant input from normal seawater. Growth rates, estimated from an algorithm based on Mn, Fe and Co contents, are orders of magnitude faster than rates determined for hydrogenous Fe-Mn oxides. The outer layers and some internal layers of the crust had relatively faster growth rates indicating that the crusts precipitated from distinct hydrothermal episodes.