Shale-normalized REE Research Articles

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

Biogenic and abiogenic low-Mg calcite (bLMC and aLMC): Evaluation of seawater-REE composition, water masses and carbonate diagenesis

The ΣREE and shale-normalized (PAAS) REE SN values of modern brachiopods (biogenic low-Mg calcite: bLMC) represented by several species from high- to low latitudes, from shallow- to deep waters and from warm- and cold-water environments, define three distinct average ‘seawater’ trends. The warm- and cold-water brachiopods define two indistinguishable ( p < 0.050) groups that mimic open-ocean seawater REE chemistry, exhibiting the typical LREE enrichment with a slightly positive to negative Ce anomaly followed by an otherwise invariant series. Other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ΣREE and REE SN trends from the previous two populations, showing a slight enrichment in the MREEs and an increasing trend in the HREEs. Other groups of modern brachiopods are characterized by elevated REE SN trends relative to the ‘normal’ groups as well as by complexity of the series trends. The most characteristic feature is the decrease in the HREEs in these brachiopods from areas of unusual productivity (i.e., such as upwelling currents, fluvial input and aerosol dust deposition). Preserved brachiopods from the Eocene and Silurian exhibit REE SN trends and Ce anomalies similar to that of the ‘open-ocean’ modern brachiopods, although, their enriched ΣREE concentrations suggest precipitation of bLMC influenced by extrinsic environmental conditions. Preservation of the bLMC was tested by comparing the ΣREE and REE SN trends of preserved Eocene brachiopods to those of Oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric-water. The altered bLMC is enriched by approximately one order of magnitude in both ΣREE and REE SN trends relative to that in bLMC of their preserved counterparts. Similarly, the ΣREE and REE SN of preserved Silurian brachiopod bLMC were compared to those of their enclosing altered lime mudstone, which exhibits features of partly closed system, phreatic meteoric-water diagenesis. Despite these differences in the diagenetic alteration systems and processes, the ΣREEs and REE SN trends of the bLMC of altered brachiopods and of originally mixed mineralogy lime mudstones (now diagenetic low-Mg calcite) are enriched by about one order of magnitude relative to those observed in the coeval and preserved bLMC. In contrast to the changes in ΣREE and REE SN of carbonates exposed to phreatic meteoric-water diagenesis, are the REE compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids. The ΣREE and REE SN trends of the burial cements mimic those of their host lime mudstone, with all showing slight LREE enrichment and slight HREE depletion, exhibiting a ‘chevron’ pattern of the REE SN trends. The overall enrichment or depletion of the cement REE SN trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system, but also strong differences in the elemental and REE compositions of the burial fluids. Evaluation of the (Ce/Ce*) SN and La = (Pr/Pr*) SN anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids.

Rare earth elements in authigenic methane-seep carbonates as tracers for fluid composition during early diagenesis

Authigenic carbonates forming at an active methane-seep on the Makran accretionary prism mainly consist of aragonite in the form of microcrystalline, cryptocrystalline, and botryoidal phases. The δ13Ccarbonate values are very negative (− 49.0 to − 44.0‰ V-PDB), agreeing with microbial methane as dominant carbon source. The δ18Ocarbonate values are exclusively positive (+ 3.0 to + 4.5‰ V-PDB) and indicate precipitation in equilibrium with seawater at bottom water temperatures. The content of rare earth elements and yttrium (REE + Y) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution ICP-MS varies for each aragonite variety, with early microcrystalline aragonite yielding the highest, cryptocrystalline aragonite intermediate, and later botryoidal aragonite the lowest REE + Y concentrations. Shale-normalised REE + Y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation: Microcrystalline aragonite shows high contents of middle rare earth elements (MREE), reflecting REE patterns ascribed to anoxic pore water. Cryptocrystalline aragonite exhibits a seawater-like REE + Y pattern at elevated total REE + Y concentrations, indicating higher concentrations of REEs in pore waters, which were influenced by seawater. Botryoidal aragonite is characterised by seawater-like REE + Y patterns at initial growth stages followed by an increase of light rare earth elements (LREE) with advancing crystal growth, reflecting changing pore fluid composition during precipitation of this cement. Conventional sample preparation involving micro-drilling of carbonate phases and subsequent solution ICP-MS does not allow to recognise such subtle changes in the REE + Y composition of individual carbonate phases. To be able to reconstruct the evolution of pore water composition during early diagenesis, an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases. High-resolution analysis of seep carbonates from the Makran accretionary prism by LA-ICP-MS reveals that pore fluid composition not only evolved in the course of the formation of different phases, but also changed during the precipitation of individual phases.

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.

Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator

A first study of REE + Y distribution in a variety of Neoproterozoic (Cryogenian and Ediacaran) carbonates from different settings in the Saldania, Gariep, Damara and West Congo Belts in southwestern and central Africa revealed systematic differences that can be explained by varying palaeoenvironmental factors. The majority of samples display relatively unfractionated, flat shale-normalised REE + Y patterns that cannot be ascribed solely to shale contamination but are interpreted as resulting from the incorporation of near-shore colloids, possibly related to Fe-oxihydroxide scavenging. Only few carbonate units yielded trace element distributions that conform to a typical seawater composition. Those carbonates that were affected by stratiform, syn-sedimentary hydrothermal mineralisation are distinguished by Eu anomalies. Considering the similarity in residence time between REE and carbon, the strong influence of river-born particles on the REE + Y distribution in the analysed carbonates casts considerable doubt over the usefulness of these carbonates for stratigraphic correlation of Neoproterozoic sediment successions based on carbon isotopes.

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.

Hydrothermal plume-particle fluxes at 13°N on the East Pacific Rise

We have investigated the geochemical flux to sediment traps deployed close to the Totem vent site, 13°N EPR. An important emphasis has been to investigate what proportion of this settling flux derives from direct co-precipitation of vent-fluid material as polymetallic sulphides and what proportion is in the form of Fe oxyhydroxide material which not only co-precipitates vent-fluid metals but can also scavenge dissolved material from seawater. Mass fluxes and major element compositions (Fe, S, Al, Mn, CaCO 3 and C org) for our near vent samples compare well with results from previously reported Pacific hydrothermal sediment trap studies, both at this site and on the Endeavour Ridge. Our samples record large fluxes of Cu, Zn and Pb, as well as V and P, all of which are in excess over typical open-ocean trap values. If P and V are transported to the traps as sinking Fe-oxyhydroxide material from the neutrally buoyant plume, we calculate that 10–20% of the Fe entering the near vent traps occurs as oxidised material with the remaining 80–90% being supplied by polymetallic sulphides. Shale-normalised REE distribution patterns for near-vent trap samples are similar to those for local vent fluids and sulphidic sediments. Detailed mass balance calculations, however, reveal evidence for additional input from hydrothermal Fe-oxyhydroxide material with a scavenged REE composition that is less “evolved” than that reported for local neutrally buoyant plume particles. U fluxes into the near vent traps are high and consistent with uptake by sulphides. 210Pb fluxes are also high and appear dominated by co-precipitation direct from vent-fluids as Pb-sulphides. In contrast, Fe-oxyhydroxide scavenging from seawater can account for the entire 230Th and 232Th fluxes reported. If the scavenging processes identified here were similarly active in neutrally buoyant plumes, we would predict hydrothermal scavenging to impact ocean biogeochemical cycles significantly, e.g. causing removal of ca. 10% of the dissolved 230Th production from the deep water column, out to a distance of ca. 10–100 km off-axis, along the entire 60,000 km global ridge-crest.

Thermal effects on rare earth element and strontium isotope chemistry in single conodont elements

A low-blank, high sensitivity isotope dilution, ICP-MS analytical technique has been used to obtain REE abundance data from single conodont elements weighing as little as 5 μg. Sr isotopes can also be measured from the column eluants enabling Sr isotope ratios and REE abundance to be determined from the same dissolution. Results are comparable to published analyses comprising tens to hundreds of elements. To study the effects of thermal metamorphism on REE and strontium mobility in conodonts, samples were selected from a single bed adjacent to a basaltic dyke and from the internationally used colour alteration index (CAI) “standard set.” Our analyses span the range of CAI 1 to 8. Homogeneous REE patterns, “bell-shaped” shale-normalised REE patterns are observed across the range of CAI 1 to 6 in both sample sets. This pattern is interpreted as the result of adsorption during early diagenesis and could reflect original seawater chemistry. Above CAI 6 REE patterns become less predictable and perturbations from the typical REE pattern are likely to be due to the onset of apatite recrystallisation. Samples outside the contact aureole of the dyke have a mean 87Sr/ 86Sr ratio of 0.708165, within the broad range of published mid-Carboniferous seawater values. Our analysis indicates conodonts up to CAI 6 record primary geochemical signals that may be a proxy for ancient seawater.

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.

Rare earth elements in Holocene reefal microbialites: a new shallow seawater proxy

The concentration of rare earth elements and yttrium (REE + Y) was determined in Holocene Mg-calcite microbialites from shallow reef framework cavities at Heron Reef, Great Barrier Reef. Shale-normalized REE + Y patterns of 52 microbialite samples show: (1) uniform heavy REE enrichment (Nd SN/Yb SN = 0.236, SD = 0.026); (2) consistent negative Ce and positive La anomalies; (3) marine Y/Ho ratios (56.17, SD = 2.66); and (4) slightly positive Gd anomalies. All of these features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater. REE partition coefficients were calculated relative to shallow Coral Sea seawater. They are uniform (relative SD = 10.2%) across the entire mass range and almost two orders of magnitude higher than those between coral and seawater. Hence, terrigenous detritus-free, modern microbialites are a more reliable proxy for seawater REE chemistry than are skeletal carbonates. Ancient limestones have been considered largely problematic as sources for REE proxies owing to perceived problems with diagenesis, partly on the basis of relatively high REE concentrations in some limestones compared to modern skeletal carbonates. However, high REE concentrations in modern microbialites suggest that ancient limestones with relatively high REE concentrations, if not contaminated by terrigenous detritus, may reflect original seawater chemistry. Terrigenous contamination, if present, is readily detectable on the basis of co-occurring trace element concentrations (Sc, Hf, Th) and Y/Ho ratio. Hence, ancient, particularly reefal, limestones may provide reliable seawater REE proxies. The occurrence of microbialites in clean limestones as old as 3.5 Ga suggests the possibility of reconstructing shallow marine REE chemistry over most of Earth history with important implications for paleogeography and paleoredox studies.

Geology, mineralogy, and geochemistry of the Middle Proterozoic Wafangzi ferromanganese deposit, Liaoning Province, China

Wafangzi is the largest ferromanganese-ore deposit in north China, consisting of southeastern and northwestern regions containing 11 ore blocks (districts). Middle Proterozoic and Cambrian sedimentary rocks occur in the mine area, and Jurassic–Cretaceous intrusive igneous rocks subjected the ores to local metamorphism. The ore-bearing sequence is located in the upper part of the Middle Proterozoic Tieling Formation, which is composed of three ore beds and associated rocks. The ore-bearing sequence and the ores display lithological and mineralogical changes from manganite ore and red and dark-brown mudstone in the southeast to rhodochrosite ore and black shale and silty dolostone in the northwest. The industrial type of Wafangzi ores is ferromanganese containing 16 to 33% Mn, 15 to 20% Fe, and Mn/Fe ratios of 1 to 2. Genetic types include primary sedimentary and diagenetic type which includes manganite and rhodochrosite ores; contact metamorphic type, which includes bixbyite–braunite ore, Mn silicate–Mn oxide ore, and pyrosmalite–rhodochrosite ore; and secondary oxidized type, which includes pyrolusite ore and psilomelane–vernadite–goethite ore. Mean contents of Co, Ni, and Zn in rhodochrosite ore are two to four times higher than those of the other ores. Strontium and Ba contents in manganite ores are 16 to 27 times higher than in the rhodochrosite ores, indicating that Sr and Ba probably occur in barite in manganite ores. The mean total REE content of the various ore types is 129 ppm, LREE/HREE is 6.5, La/Yb is 12.3, Eu/Sm is 0.18, and Sm/Nd is 0.22, comparable with those of continental and subcontinental crust. Chondrite-normalized REE patterns of the various ore types have small negative Eu anomalies, whereas the shale-normalized REE curves have weak negative Eu anomalies. Organic carbon contents in rhodochrosite ores range from 1.18 to 1.77%, whereas contents in the host rocks range from 0.58 to 0.88%. The H/C vs. O/C diagram shows that organic matter in ores experienced catagenesis and metagenesis. The mean δ 13 C PDB of the rhodochrosite ore is −6.5‰ (range −3.9 to −10.5‰), and δ 18 O PDB is −7.2‰ (range −4.7 to −9.4‰). The following conclusions are made: (1) The source of the ore components is mainly terrigenous, derived from the weathering of continental crust; (2) both manganite and rhodochrosite ores are of primary sedimentary origin, and precipitated in a shallow-marine basin; (3) the presence of bacterial textures in the black matrix of spheroidal rhodochrosite ore and C isotope data indicate that microbial activity played a role in formation of the ores; (4) some evidence indicates that hydrothermal solutions contributed to the ores, such as the presence of adularia in manganite ore and pyrosmalite in rhodochrosite ore. Hydrothermal fluids were generated by igneous intrusions.

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.

Rare earth element and isotope (C, O, Sr) characteristics of hydrothermal carbonates: genetic implications for dolomite-hosted talc mineralization at Göpfersgrün (Fichtelgebirge, Germany)

The talc deposit at Göpfersgrün (Fichtelgebirge, Germany) was formed by hydrothermal alteration of the Wunsiedel marble probably during Permian times. The Wunsiedel marble consists of calcite (cc1) and dolomite (do1) layers. Low REE contents, flat shale-normalized REE n patterns with a negative Ce anomaly, δ 13 C PDB values of −1.5 to 2‰ and a low 87 Sr/ 86 Sr ratio (0.7086) of the hydrothermally unaltered marble are typical for a sedimentary protolith formed within a marine environment. Hydrothermal alteration and talc mineralization of the marble occurs along a major fault zone and is associated with formation of massive saddle dolomite 2 (do2) replacing cc1 and do1. Massive dolomitization is followed by vug-filling carbonates, dolomite 3, calcite 2 and calcite 3. Each carbonate generation has a distinct chemical composition. Dolomite 2 is enriched in REE, depleted in 18 O ( δ 18 O SMOW =10.8 to 11.9‰) and shows a higher 87 Sr/ 86 Sr value (0.7103) compared to the Wunsiedel marble ( δ 18 O SMOW =14.3 to 18.1‰). Late stage vug-filling carbonates differ from the preceding (replacement) dolomite 2 by higher REE contents, a clear roof-shaped REE n pattern, enhanced radiogenic Sr isotope composition ( 87 Sr/ 86 Sr =0.7115 and 0.7117) and lower δ 13 C PDB values (−2.6 to −11.5‰). Main talc mineralization result from decarbonation reactions at low X CO 2 and temperatures between 250 and 400°C. It is proposed that main stage hydrothermal dolomitization, talc mineralization and late stage carbonate vug-fillings are related to formation brines or crustal fluids that interact with graphite-bearing metapelites under acid conditions. Changes in chemical composition between (early) replacement dolomite and (late) vug-filling carbonates are mainly ascribed to increasing fluid/rock ratios and decreasing temperatures.

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.

Hydrothermal scavenging on the Juan de Fuca Ridge: 23OTh xs, 10Be, and REEs in ridge-flank sediments

We have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the N.Cleft Segment, Juan de Fuca Ridge, ∼8km west of the “MegaPlume” area previously identified near 45°N. The core contains varying biogenic, lithogenic, and hydrothermal components, as reflected in CaCO 3, Al, and Fe contents, respectively. Horizons of pronounced hydrothermal input, in core-top sediments and at depth, exhibit increased concentrations of Fe, Cu, Zn, and Pb and shifts in Pb isotopic compositions toward nonradiogenic (MORB/hydrothermal) values. REE concentrations co-vary with hydrothermal Fe down-core, and shale-normalised REE distribution patterns exhibit both negative Ce-anomalies and positive Eu-anomalies, indicative of input from plume-particle fall-out. Unsupported 230Th xs activities down-core are consistent with continuous slow sediment accumulation rates of 0.54 cm/ky for ∼200 ky since the deposition of the deeper Fe-rich horizon. 10Be (0) and 9Be isotope concentrations also co-vary with hydrothermal Fe down-core and exhibit 10 Be (o) 9 Be ratios which approach that of Pacific Ocean deep water, indicative of a seawater-scavenging source. 10 Be (0) 230 OTh xs(0) ratios throughout most of Core GC88-6 are greater than mean Pacific Ocean values, indicating that hydrothermal scavenging can lead to significant net removal of dissolved 10Be into ridge-flank sediments.

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.

Silicalites of hydrothermal origin in the Lower Cambrian black rock series of South China

A silicalite bed was found in the hanging wall and foot wall of the sulfide-rich bed of the Lower Cambrian black rock series in South China. Its origin was not described before. On the oxide (SiO2-Al2O3, SiO2-MgO, SiO2-K2O + Na2O) diagrams for discriminating silicalites of chemical, biological and volcanic origins (Liu Xiufeng, 1991), most of the data points of silicalites fall within the areas representing silicalites of chemical and volcanic origins. On the Al-Fe-Mn diagram for discriminating silicalites of hydrothermal and biological origins (Yamamoto, 1987), the data points fall within the areas representing silicalites of hydrothermal and hydrothermal-biological origins. On the SiO2-Al2O3 diagram for discriminating silicalites of hydrothermal and hydrogenous origins (Bonatti, 1975), the data points mostly fall within the hydrothermal area. The ratios of SiO2/Al2O3, SiO2/(K2O+ Na2O), SiO2/MgO, and K2O/ Na2O in the silicalites stand between those of volcanic sediments and of seafloor hydrothermal sediments. The total amount of rare-earth elements in the silicalites is low; the North American Shale-normalized REE patterns decline leftward with obvious negative Ce anomaly. The trace elements Mo, Zn, As, Sb, Se, U, and Ba are higher than those in non-hydrothermal sediments and U/Th ≥1. The present authors think that the silicalites are derived from seafloor hot brines which had attracted elements from igneous rocks.