Shale-normalized Rare Earth Element Research Articles

A trace element study of siderite–jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton—Formation from hydrothermal fluids and shallow seawater

Shale-normalised rare earth element and yttrium (REE + Y) patterns for siderite–jasper couples in a banded iron formation of the 3.45 Ga Panorama Formation, Warrawoona Group, eastern Pilbara Craton, display distinct positive Y and Eu anomalies and weak positive La and Gd anomalies, combined with depleted light REE relative to middle and heavy REE. Ambient seawater and hydrothermal fluids are identified as major sources of REE + Y for the BIF. In the case of siderites, strong correlations between incompatible trace elements and trace element ratios diagnostic of seawater indicate variable input from a terrigenous source (e.g. volcanic ash). We propose a volcanic caldera setting as a likely depositional environment where jasper and siderite precipitated as alternating bands in response to episodic changes in ambient water chemistry. The episodicity was either driven by fluctuations in the intensity of hydrothermal activity or changes in magma chamber activity, which in turn controlled relative sea level. In this context, precipitation of jasper probably reflects background conditions during which seawater was saturated in silica due to evaporative conditions, while siderites were deposited most likely during intermittent periods of enhanced volcanic activity when seawater was more acidic due to the release of exhalative phases (e.g. CO 2).

Possible REE constraints on the depositional and diagenetic environment of Doushantuo Formation phosphorites containing the earliest metazoan fauna

The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of what are probably the earliest metazoan fossils (sponges, cnidarians, and bilaterians) in the world that could contribute to a better understanding of early faunal evolution on Earth. The formation spans an interval of dramatic biological, chemical, and climatic change. In Weng'an mine in southwest China, extensive paleontological investigations show embryonic, larval, and adult animal fossils occur in the upper ore bed of the Doushantuo phosphorite, but no animal forms have been found in the lower ore bed. Taphonomy and experiments on fossilization suggest that no fauna lived on or in the lower ore bed as it was deposited. Rare-earth element (REE) data for the Doushantuo phosphorites in Weng'an document the paleoenvironmental condition under which the phosphorites were deposited. The shale-normalized REE patterns of the upper phosphorite bed (UPB) are characterized by “old” (Vendian–Early Cambrian) phosphorite patterns. The REE patterns of the lower phosphorite bed (LPB) are significantly different with less negative Ce anomalies and less LREE and HREE depletion. The REE characteristics suggest that the Doushantuo phosphorites record seawater condition at the time of sediment deposition and are not affected by subsequent diagenesis. The REE compositions of the Doushantuo phosphorites show that the marine depositional environment changed from anoxic in the lower ore bed to oxic in the upper ore bed. The uniquely diverse and abundant Doushantuo eukaryote fossil accumulations at Weng'an thus may be related to a change from anoxic to oxic bottom-water conditions.

REE geochemistry of surface sediments in the Chukchi Sea

Analyses of rare earth elements (REEs) in 26 surface sediment samples obtained from the Chukchi Sea were conducted using ICP-MS. In general, REEs are relatively rich in fine-grained sediments and deplete in coarse-grained sediments in the Chukchi Sea although REEs have large concentration spans in different types of sediments. Except that a few samples have weak en-richments of light or heavy REEs?most samples exhibit flat shale-normalized REE pattern, indi-cating that surface sediments in the Chukchi Sea are composed dominantly of terrigenous com-ponents experiencing weak chemical weathering. In terms of REE concentrations and other char-acteristic parameters, we inferred that sediments on the eastern and western sides of the Chukchi Sea are derived from landmasses of Alaska and Siberia, respectively; the midsouth sediments are possibly related to northward dispersion of the Yukon River materials. The Herald Shoal in the center of the study area is covered with relict sediment, which has large ratios of light-to-heavy REEs (ΣCe/ΣY ratio) and lacks evident negative Ce anomaly; cerium enrichment is possibly re-lated to manganese transfer under oxidizing conditions in early diagenesis.

Elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis

Shale-normalized rare-earth element (REE) patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the Pennsylvanian-age cyclothems in the U.S. midwestern states of Oklahoma, Kansas, Nebraska, Iowa, and Ohio as well as the Mississippian-age Fayetteville Shale of Arkansas. Shale-normalized enrichments in middle REE (MREE) developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic-rich muds. Depletions in MREE (Sm–Ho) in host shales correspond to the MREE enrichments in some nearby concretions. Rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters. Positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating. Flat REE patterns in both host shales and phosphorite concretions probably reflect detrital influence. Phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals. Overall enrichment in REE in phosphorite relative to host shale suggests movement of REE as does the MREE-enriched phosphorite near the MREE-depleted host shale. Diagenetically mobile REE can affect La–Th–Sc relationships used in provenance analysis. The extent of the diagenetic modification of La–Th–Sc distribution is related to the amount of P 2O 5 present. Typical La–Th–Sc distribution in host shale results when P 2O 5 values are below 0.5%. High La abundance substantially skews La–Th–Sc distribution when P 2O 5 is above 5.0%, but when P 2O 5 is between 0.5% and 5.0% the extent of the effect is unclear. Oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions. Most of the concretions nearest to shoreline sources of detrital input have both flat REE patterns and elevated values of Al 2O 3/(Al 2O 3+Fe 2O 3).

Geochemistry of Upper Miocene Kudankulam Limestones, Southern India

Concentrations of major, trace, and rare-earth elements (REE) were measured in shallow marine limestone samples of the upper Miocene Kudankulam Formation, southern India, in order to investigate the geochemical variations among various litho-units. The CaCO3 content is higher in algal limestone (AL; 92 ± 1, n = 3) and clastic limestone (CL; 90 ± 2, n = 3) than sandy shell limestone (SSL; 81 ± 1, n = 3). All trace elements exhibit lower concentrations than post-Archean Australian Shale (PAAS) values, except one SSL sample. Large variations in ΣREE content are observed among CL, SSL, and AL (~14-142, ~68-124, and ~38-98, respectively). Almost all limestone samples analyzed from the Kudankulam Formation show a small negative cerium anomaly (Ce/Ce* ~0.8-0.9), except one AL sample, which lacks this cerium anomaly (Ce/Ce* ~1.04). Variations in Ce anomalies and ΣREE contents in Kudankulam limestone samples are mainly controlled by the amount of terrigenous sediments and diagenetic behavior. Shale-normalized REE patterns and (La/Yb)s, La/Sc, La/Th, and Th/Sc ratios suggest that the detrital sediments present in the limestones were probably derived from felsic source rocks. The observed low content of U (0.9 ± 0.5, n = 9) and U/Th (0.2 ± 0.1, n = 9) ratio in these limestones are probably related to an oxygen-rich environment.

Rare earth element contents of Jurassic fish and reptile teeth and their potential relation to seawater composition (Anglo-Paris Basin, France and England)

The rare earth element (REE) chemistry of Jurassic shelf seawater from western Europe (Anglo-Paris Basin) was investigated by analyzing the fish and reptile teeth deposited in shallow to deeper water (<200 m) environments. REE patterns in apatites are controlled by the host sediments. Vertebrate teeth sampled from the siliciclastic sediments (calcareous sandstones and shales) show flat shale-normalized REE patterns that reflect the dominant influence of the continental source from which the REE were derived. Carbonate deposits, protected from the clastic sources, contain fish and reptile teeth whose REE patterns reflect more accurately the REE composition of the overlying water column. The REE patterns are similar through the Bajocian to the Oxfordian and are characterized by a depletion in heavy rare earth element (Dy/Yb N =1.8–5.0) compared to modern seawater compositions (Dy/Yb N =0.8–0.9). The fractionations among HREE in the Jurassic seawater are more efficient than in modern oceans and increases with depth instead of decreasing. These HREE removal processes operated as early as the Paleozoic and disappeared during the Late Cretaceous. The fractionation of HREE relative to LREE is correlated with the increasing depth of the basin as indicated by two independent criteria that are the sedimentary structures and the oxygen-isotope composition of coexisting brachiopods. The Dy/Yb N ratio of the marine biogenic phosphates can be used as a proxy of paleo-water depths at the scale of a water mass. Ce anomalies ( Ω Ce) in biogenic apatite-bearing limestones are variable and systematically negative. Relative to modern surface seawater and Bathonian values (mean Ω Ce=−0.63±0.10), weak Ce anomalies during the Callovian (mean Ω Ce=−0.21±0.09) suggest the onset of more reducing conditions. This redox change coincided with an increase of seawater temperatures as suggested by the oxygen-isotope compositions measured on the same teeth. We speculated that the Callovian low Ω Ce could result from a decrease in the p O 2 due to the warming of seawater.

The geochemistry of late Archaean microbial carbonate: implications for ocean chemistry and continental erosion history

Trace element concentrations and combined Sr- and Nd-isotope compositions were determined on stromatolitic carbonates (microbialites) from the 2.52 Ga Campbellrand carbonate platform (South Africa). Shale-normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive La and Y anomalies and in being depleted in light rare earth elements. In contrast to modern seawater (and microbialite proxies), the 2.52 Ga samples lack a negative Ce anomaly but possess a positive Eu anomaly. These latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where, unlike today, hydrothermal Fe input was not oxidised, and scavenged and rare earth elements were not coprecipitated with Fe-oxyhydroxides. The persistence of a positive Eu anomaly in relatively shallow Campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated (Archaean) to continental erosion-dominated (Phanerozoic) rare earth element flux ratio. The dominant hydrothermal input is also expressed in the initial Sr- and Nd-isotope ratios. There is collinear variation in Sr-Nd systematics, which range from primitive values (87Sr/86Sr of 0.702386 and εNd of +2.1) to more evolved crustal ratios. Mixing calculations show that the range in trace element ratios (e.g., Y/Ho) and initial isotope ratios is not a result of contamination by trapped sediment, but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed. Calculated Nd flux ratios yield a hydrothermal input into the 2.52 Ga oceans one order of magnitude larger than continental input. Such a change in flux ratio most likely required substantially reduced continental inputs, which could, in turn, reflect a plate tectonic causation (e.g., reduced topography or expansion of epicontinental seas).

In situ ion microprobe Th-Pb dating of Silurian conodonts.

We report in situ ion microprobe U-Th-Pb dating of an index fossil conodont derived from Langkawi Island, northern Malaysia. Fifteen spots on five fragments of an early Silurian conodont yield 232Th-208Pb* isochron age of 429 ± 50 Ma (2σ, MSWD = 0.95). The 238U-206Pb* isochron age of 436 ± 270 Ma (2σ, MSWD = 0.47) has a large uncertainty, since the 238U abundance is significantly small, but the age is comparable to that of 232Th-208Pb*. These ages are consistent with the depositional and early diagenetic ages of the fossil in a Llandoverian sedimentary sequence (430–439 Ma). The success of the method depends on the heterogeneity of diagenetically incorpolated Th in a hundred μm size and the consequent variations in Pb isotopic compositions due to radioactive decay. Shale-normalized rare earth element (REE) abundance shows a concave pattern with middle REE enrichment. There is not a significant anomaly of Ce and Eu. These characteristics are different from those of a Carboniferous conodont, probably due to a different formation environment as suggested by other workers.

A review of the geology of the dolomite-dominated carbonatite complexes of the Kalahari Craton, southern Africa

Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd(t) values (−0.5 ± 0.2 to −2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative CeSN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga.

Dissolved indium and rare earth elements in three Japanese rivers and Tokyo Bay: Evidence for anthropogenic Gd and In

New data on the dissolved (<0.04 μm) rare earth elements (REEs) and In in the Japanese Ara, Tama, and Tone river-estuaries and Tokyo Bay are presented. Unique shale-normalized REE patterns with a distinct positive Gd anomalies and a strong heavy-REE enrichment were seen throughout the data. The dissolved Gd anomaly is caused by local anthropogenic input mainly due to recent use of Gado-pentetic acid as a medical agent for magnetic resonance imaging (MRI) in hospitals. The heavy-REE enrichment may be attributed to fractionation during weathering and transport in the upstream of the rivers, and only partially to removal of light- and middle-REE enriched river colloids by the use of a new ultrafiltration technique. Dissolved In concentrations in the Japanese rivers are extraordinarily high as compared to those in the pristine Chao Phraya river of Thailand reported elsewhere (Nozaki et al., in press). Like Gd, the high dissolved In in the study area can also be ascribed to recent use of In-containing organic compound, In(DTPA) 2− in medical diagnosis. Thus, in the highly populated and industrialized area, dissolved heavy metal concentrations in rivers and estuaries may be significantly perturbed by human activities and the fate of those anthropogenic soluble substances in the marine environment needs to be investigated further.

Influence of Provenance, Weathering, and Sedimentary Processes on the Elemental Ratios of the Fine-Grained Fraction of the Bedload Sediments from the Vembanad Lake and the Adjoining Continental Shelf, Southwest Coast of India

ABSTRACT Geochemical studies, comprising twelve major elements and twenty-four trace elements, including the rare earth elements (REE), have been carried out on the Concentrations normalized to the average upper continental crust (UCC) show that the sediments studied are depleted in Rb, K, Ba, Ta, Hf, Na, and Ca and enriched in Cs, U, Th, REEs, Sc, Fe, Co, and Cr. UCC-normalized patterns are strikingly similar for all the three sediment types, probably because of efficient mixing of sedimentary material during weathering, transport, and deposition. The depletion of Na, K, Ca, and Ba in sediments relative to UCC could be attributed to their mobility during weathering. The depletion of Ta and Hf and enrichment of Th, REE, and Sc are probably related to provenance characteristics, and enrichment of U is attributed to oxidation-reduction processes. Shale-normalized REE values reveal steep HREE patterns [(Tbsample/Tbshale)/(Ybsample/Ybshale)] >> [(Lasample/Lashale)/(Smsample /Smshale)] and flat LREE patterns. The shale-normalized patterns for all the three types of sediments are remarkably similar, with variations in the magnitude of LREE enrichment (Lan/Ybn varies between 1.72 to 2.57 times that of PAAS). LREE enrichment and steep HREE patterns are attributed to felsic source-rock characteristics. Other geochemical characteristics such as high La/Th and low La/Sc and Th/Sc ratios compared to UCC also support a felsic provenance. Eu/Eu* values for all the three types are little more than 1, which is also a characteristic feature of Archean crust. Furthermore, data plotted on discrimination diagrams of La/Th vs. Hf and La-Th-Sc cluster closer to granite and away from basalt and komatiite and are consistent with source-rock geology. Geochemical data have also helped in ascertaining the weathering trends. The chemical index of alteration (CIA) has been used to quantify the degree of weathering. CIA values range between 81 and 94 on a scale of 40-100, indicating a high degree of alteration (fluvial sediments seem to be more altered). On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals kaolinite, gibbsite, and chlorite. A high degree of weathering is also indicated by other plots such as K2O-Fe2O3-Al2O3 and log K vs. log Rb. The geochemical data are consistent with the studies on clay mineralogy (higher kaolinite) carried out by other authors on the sediments of the same area. Secondary processes seem to affect other geochemical discriminants such as Co, Cr, Ni, and U, as revealed by plots of (1) La/Sc vs. Co/Th and (2) Th-Hf-Co and Co/Th, Cr/Th ratios and makes them unsuitable for determining provenance and tectonic setting.

Origin of rare earth element signatures in groundwaters of circumneutral pH from southern Nevada and eastern California, USA

Concentrations of the rare earth elements (REE) were measured in circumneutral pH groundwaters from southern Nevada and Death Valley, CA. Groundwaters from the regional lower Paleozoic carbonate-rock aquifer (Cambrian–Devonian) have flat shale-normalized patterns that closely resemble the shale-normalized patterns of the aquifer rock samples (principally Cambrian). Groundwaters associated with younger carbonate rocks (chiefly Permian) in the study region exhibit heavy REE (HREE) enriched, shale-normalized REE patterns with substantial negative Ce anomalies that also mimic these carbonate rocks. In addition, groundwaters from the felsic volcanic rock aquifers have the same flat to light REE (LREE) enriched shale-normalized patterns with large negative Eu anomalies as the felsic volcanic rocks. The similar REE patterns of all the groundwaters and associated aquifer rocks studied suggest that the groundwaters inherited REE signatures from the host rocks through which they flow. Because negative Ce anomalies are not an uncommon feature of carbonate rocks of marine origin, the negative Ce anomalies reported here for these groundwaters may reflect a Permian marine Ce signature. Previously, we demonstrated that carbonate complexes dominate REE speciation in southern Nevada and Death Valley groundwaters. Moreover, solid–liquid partitioning coefficients ( K d) indicate that the affinity of LREEs to sorb to aquifer surface sites is substantially greater than for the HREEs in the southern Nevada carbonate- and felsic volcanic-rock alluvial aquifers. Consequently, the HREEs enrichments reported here for groundwaters associated with younger Paleozoic carbonate rocks compared to these source rocks is consistent with REE carbonate complexation and preferential removal of LREEs to aquifer surface sites.

Advective excess Ba transport as shown from sediment and trap geochemical signatures

We report the results of a geochemical study of sediment and trap material. Major and trace elements (Zr, Ba, rare earth elements, and Th) were analyzed on bulk sedimentary material collected along the NE Atlantic margin. Our aim is to test the widespread use of Ba-barite as a proxy for paleoproductivity in a continental margin area. This environment is of great interest because atmospheric–oceanic exchanges are important. In sediments, the geochemical signatures remain close to an upper crust reference, with flat shale-normalized rare earth elements patterns and constant elementary ratios. The calculated biogenic fraction of Ba or excess Ba (20–45%) remains lower than the excess Ba record in trap material (80–99%). The evolution of the geochemical signature along the margin reflects variable dilution of a detrital Post Archean Australian Shale-like component by a biogenic carbonaceous seawater-derived component. The trap material displays a wide range of variation in its trace element content (e.g., Ba ∼150–3000 ppm, Zr ∼2–100 ppm), except for the abyssal site, which is characterized by constant signature. In the two other sites, all of the trace element contents increase with water depth and present pronounced seasonal changes at each sampled water depth. The amount of excess Ba also increases in the deepest traps, and its evolution throughout the year mimics the change of the other analyzed trace elements. In contrast, its relationships with particulate organic carbon are not obvious. In term of fluxes, two periods of enhanced excess Ba fluxes are observed: (1) excess Ba flux increases with the detrital-like elements like Th especially during winter, and (2) excess Ba flux is enhanced without any change for the other trace elements during spring. To explain the first case, a supply through lateral advection is proposed. Such transient input of significant excess Ba flux will have a great impact on the yearly averaged estimation of the export production. Indeed, only the second case reflects a bloom in the biological productivity of the water column and must be taken into account in a mean calculation of the export production. Finally, a normalization of the excess Ba by detrital-like element like Th will help to discriminate between a real increase of the excess Ba due to local productivity change (high excess Ba and high excess Ba/Th ratio ≥ 10,000) and any input due to advection process (high excess Ba but low excess Ba/Th ratio < 2000).

Rare earth elements of sinking particulate matter in the Japan Trench

Settling particles were collected with time-series sediment traps at four depths (1174 m, 3680 m, 5687 m, and 8688 m) moored at the deepest site (water depth 9200 m) of the Japan Trench for the period from March 5, 1991 to March 2, 1992. The composition of rare earth elements (REEs) in the sinking particles selected for four seasons were measured to investigate the REE fractionation during scavenging and settling transport through the water column. The REE-particle associations were also investigated by sequential digestion with three chemical treatments (acetic acid, mild HCl/HNO 3, and HF/HNO 3/HClO 4). The measured vertical particulate flux increased with depth suggesting that there is a significant addition of particles, at least, to the deeper traps from horizontal sources. No systematic variation was observed in the REE concentration in the particles with respect to depth and season. The shale-normalized REE data show either flat or slightly heavy REE-enriched patterns with a middle REE enrichment in which the maximum is usually centred at Gd, but sometimes at Eu and a significantly positive Ce anomaly. The chemical digestion experiments indicated characteristic patterns for shale-normalized REEs. The acetic acid leach showed a marked middle REE-enriched pattern with a maximum at Eu and distinctively negative Ce anomaly. The mild HCl/HNO 3 leach had a middle REE-enriched pattern with a maximum at Gd but roughly no Ce anomaly. The residue showed a rather flat REE pattern with a marked positive Ce and often middle REE enrichment with a maximum at Gd. These results are compared with those reported previously for sinking and suspended particles in other parts of the oceans. It is suggested that REEs adsorbed on the suspended particles are altered into some refractory phases prior to or during transport by settling. The possibility exists that authigenic barite formed in the microenvironment around aggregates may effectively coprecipitate the REEs attracted to the particle surfaces.

Behavior of rare earth elements in seawater at the ocean margin: a study along the slopes of the Sagami and Nankai troughs near Japan

Using a flow injection ICP-MS method, yttrium and all the lanthanides in seawater were determined in the coastal/offshore mixing regime near Japan. Marked enrichments in the trivalent rare earth elements (REEs) in the surface waters are ascribed to the influence of coastal waters from Tokyo Bay in March and perhaps somewhere else in September and October to this region. At less than half of the stations where the detailed vertical profiles were obtained throughout the water column, we found marked excess REE(III) concentrations near the bottom. These bottom concentration anomalies are probably caused by resuspension of underlying sediments. Excluding those surface and bottom anomalous concentrations, the distribution of REE(III)s in the midwater column are very similar to those of the open ocean and are highly correlated with each other (R 2 > 0.95). This suggests that the boundary effects on the chemical fractionation of REE(III)s are small. We find no effect of the boundary scavenging known for 210Pb and 231Pa on the REE distribution, whereas there may be preferential release of light and middle REE(III)s over heavy REEs from slope sediments to the seawater much like 227Ac. From the shale-normalized REE patterns, we obtained up to 90% negative Ce and 10% positive Gd anomalies in the water column.

Rare earth elements as geochemical tracers of regional groundwater mixing

The rare earth elements (REE) were analyzed in a groundwater system from south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, the Spring Mountains, Pahranagat National Wildlife Refuge, and the Nevada Test Site) in order to investigate their potential use as tracers of regional groundwater flow. Previous investigations using conservative tracers (e.g., deuterium and uranium isotopes) identified recharge in local mountains as the primary source (60–70% of total discharge) for the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. Initial mixing calculations for these groundwaters using shale-normalized REE patterns agreed well with the previous studies; however, because the REEs are not expected to behave conservatively in natural waters, the effect of both solution complexation, which acts to enhance the stability of the REES in solution, as well as surface complexation, responsible for the particle reactive behavior of the REES, were examined in subsequent mixing calculations. In order to assess the roles of solution and surface complexation, relative partitioning coefficients were estimated for each REE in each groundwater by evaluating the ratio of the ionic strength corrected co 3 β 1REE, co 3 β 2REE, and [CO 3 2−] F to the first hydrolysis binding constants for the REES. The relative partitioning coefficients were then used to calculate REE patterns expected to develop and persist in solution as a consequence of solution and surface complexation. The calculated REE values closely resembled the actual measured REE concentrations, suggesting that the REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subsequently used to determine mixing ratios, the results of which coincided with the initial calculations as well as the previous studies. The results of this study suggest that solution complexation of the REEs is sufficient to overcome, to a certain degree, the affinity of the REEs to be adsorbed onto surface sites in the aquifers such that distinctive REE signatures develop and persist in solution in groundwaters from different aquifers. The ability of solution complexation to overcome surface complexation is likely related to the formation of the negatively charged dicarbonato complex [i.e., Ln(CO 3) 2 −, where Ln is any REE], which accounts for significant fractions of each REE in these groundwaters.

Trace and rare earth elemental variation in Arabian Sea sediments through a transect across the oxygen minimum zone

We have determined the calcium carbonate (CaCO 3), organic carbon (Co org), trace element, and rare earth element (REE) composition of surface sediments collected from a transect on the central western continental shelf and slope of India in the Eastern Arabian Sea. The transect samples across the oxygen minimum zone (OMZ) allows us to compare the relative abundances of trace elements and REEs in the sediments beneath and beyond the OMZ. Shale-normalized REE patterns, La n /Yb n ratios, and Eu/Eu∗ anamolies indicate that the sediments in the study area are either derived from the adjoining Archaean land masses or from distal Indus source. Sediment deposited in the OMZ have high U values from 3.6 to 8.1 ppm, with their U excess (of that can be supplied by continental particles) values ranging between 82–91% of the total U, indicating that the U may be precipitated as U +4 in the reducing conditions of OMZ. Sediments deposited beneath the intense OMZ (<0.2 mL/L) and away from the OMZ (1–2 mL/L) show slight negative Ce anomalies, with no significant differences between these two sets of sediments. The Ce/Ce shale∗ values are poorly related to U and C org which are indicators of suboxic bottom waters. Normative calculations suggest that two sources, namely, terrestrial and seawater (terrestrial ⪢ seawater) contribute to the total Cc anomaly of the sediments. The Cc anomaly values of the calculated seawater derived component are similar to the anomalies reported for other coastal waters and the oxygenated surface waters of the Arabian Sea and do not show any correspondence to the lowered redox state of the overlying water, probably due to the redirection of dissolved Ce into the oxic deeper water. This data on recently deposited sediments from a known oxygen-poor environment suggest that the Ce/Ce sh∗ in high sedimentation areas of continental margin environments may not be a good paleoredox proxy. U/Th ratios above 1.25 noticed in OMZ are in accordance with the earlier usage of this ratio for identifying reducing environment of ancient sediments.

Geochemical constraints on the formation of Late Cenozoic ferromanganese micronodules from the central Arctic Ocean

In order to determine geochemical compositions of Late Cenozoic Arctic seawater, oxide fractions were chemically separated from 15 samples of hand-picked ferromanganese micronodules (50–300 μm). The success of the chemical separation is indicated by the fact that > 97% of the Sr in the oxide fraction is seawater-derived. Rare-earth element (REE) abundances of the Arctic micronodule oxide fractions are much lower than those of bulk Fe-Mn nodules from other ocean basins of the world (e.g., 33 vs. 145 ppm Nd), but the Arctic oxides are enriched in Ce relative to Nd (CeNNdN = 2.2 ± 0.5) and have convex-upward, shale-normalized REE patterns (NdNGdN = 0.61 ± 0.06, GdNYbN = 1.5 ± 0.2, NdNYbN = 0.9 ± 0.2), typical of other hydrogenous and diagenetic marine Fe-Mn-oxides. Bulk sediment samples from the central Arctic Ocean have REE abundances and patterns that are characteristic of those of post-Archean shale. Non-detrital fractions (calcite + oxide coatings) of Recent Arctic foraminifera have REE abundances and patterns similar to those of Recent foraminifera from the Atlantic Ocean.Electron microprobe analyses (n = 178) of transition elements in 29 Arctic Fe-Mn micronodules from five different stratigraphic intervals of Late Cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment-seawater interface. Transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions. Only K is correlated with Si and Al, and ratios of these elements suggest that they are associated with illite or phillipsite. Ca and Mg are correlated with Mn, which indicates variable substitution of these elements from seawater into the manganate phase. The geochemical characteristics of Arctic Fe-Mn micronodules indicate that the REEs of the oxide fractions were ultimately derived from seawater. However, because of minute contributions of Sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure, Sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the 87Sr86Sr ratio of Arctic seawater or to improve the chronostratigraphy.

Petrography and geochemistry of the Paleoproterozoic Hotazel Iron-Formation, Kalahari manganese field, South Africa; implications for Precambrian manganese metallogenesis

New data are presented concerning the petrography and geochemistry of the Paleoproterozoic Hotazel iron-formation of the Kalahari manganese field, South Africa. Mineralogically and texturally, the Hotazel iron-formation is very similar to Paleoproterozoic banded iron-formations of the Superior type, comprising laminae of chert, iron oxides (magnetite, hematite), iron silicates (greenalite, minnesotaitc, stilpnomelane, riebeckite, Fe mica), carbonates (calcite, ankerite, minor siderite), and pyrite. The major chemical constituents of the Hotazel rocks are SiO 2 , Fe oxides, CaO, and MgO, whereas MnO, Al 2 O 3 , Na 2 O, and K 2 O contents are negligible (below 1 wt %). Elements of detrital derivation (Ti, Zr, Rb) and transition metals such as Zn, Cu, Ni, Co, and V are present in low concentrations, on the order of a few tens of parts per million.Stratigraphically higher, the Hotazel banded iron-formation shows a conspicuous increase in CaO contents that is coupled with a subtle decrease in bulk-rock Fe number ratios (Fe (super 2+) /(Fe (super 2+) + Fe (super 3+) )), implying a gradual transition to a more oxidizing environment. Shale-normalized rare earth element (REE) patterns are similar to those of older banded iron-formations of the Transvaal Supergroup, exhibiting light REE depletion and negative Ce anomalies comparable to modern shallow-level seawater. Occasional weak positive Eu anomalies in iron silicate-rich rocks indicate minor admixtures of a hydrothermal component below the chemocline. The similarities between the Hotazel iron-formation and most Superior-type banded iron-formations, the lack of transition metal enrichments, and the absence of strongly positive Eu anomalies in the Hotazel rocks challenge volcanogenic exhalative models proposed previously for the genesis of the interbedded Kalahari Mn deposits.

Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling

Concentrations of dissolved Y and rare-earth elements (REE) are reported for oxic eastern Mediterranean seawater from the western Levantine Basin, and for anoxic hypersaline brine and overlying oxic seawater from the Tyro sub-basin, the latter data allowing a comparative study of Y and REE behaviours during redox-cycling. All oxic waters show shale-normalized Rare-Earths and Yttrium (REYSN; Y inserted between Dy and Ho) patterns with HREE enrichment, negative CeSN anomalies, and positive anomalies of LaSN, EuSN, and, most pronounced, YSN. The anoxic brine in the Tyro sub-basin displays the highest REY concentrations and the least HREE enrichment of all samples, and positive anomalies of LaSN, EuSN, GdSN, YSN, and, in marked contrast to the oxic samples, CeSN. Compared to overlying oxic water, the anoxic brine shows enrichment factors that decrease from La (12.1) to Yb (2.7), with a pronounced positive deviation for Ce (51.2) and a strong negative deviation for Y (2.4, compared to 4.3 for Dy and 3.7 for Ho). The YHo molar ratio decreases from 102 above to only 67 below the seawater-brine interface, due to preferential sorption of Ho with respect to Y on Fe- and Mn-oxyhydroxide particles that eventually dissolve under anoxic conditions. The pronounced Y-Ho fractionation during redox-cycling of Mn and Fe is further evidence for the considerably lower marine particle reactivity of Y compared to Ho, resulting from lower stabilities of surface complexes of Y relative to those of its REE neighbours. Hence, the YHo ratio appears to be a sensitive indicator of the impact of particles on the distribution of dissolved trace elements in natural waters.