Rare Earth Element Variations Research Articles

Geochemical modelling of the Chilas Complex in the Kohistan Terrane, northern Pakistan

The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite). The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma.

Seasonal and spatial variations of rare earth elements in rainwaters, river waters and total suspended particles in air in South Korea

In order to investigate the seasonal and spatial variations of rare earth element (REE) concentrations in natural waters in the central part of South Korea, rain and river waters were collected during 2003–2004. Total suspended particles (TSP) in air were also sampled to investigate the effect of the Asian dust (the Yellow sand) on the chemistry of rainwaters. All samples showed that the absolute concentrations of the light REEs (LREEs) were higher than those of the heavy REEs (HREEs). The post-Archean Australian shale (PAAS)-normalized REE patterns indicate that the REEs in TSP and rainwaters were affected by Asian dust and anthropogenic contaminant, whereas those of river waters were mainly controlled by the geology of their drainage basin and seasonal changes in water regime. The calculated fluxes and yields of total REEs (REEs plus Y) in the South Han River were much greater than those in the North Han River due to the more widespread distribution of sedimentary rocks in the drainage area and more efficient chemical weathering.

Spatial variations of rare earth elements in north pacific surface water

cating that two or more factors contribute to the REE distribution. We examined four factors making the regional variation of surface REE concentrations mainly; a) particle scavenging, b) atmospheric dust input, c) vertical mixing and d) lateral transport from the coastal region. Flux calculations for Nd showed that the influence of atmospheric dust was less significant than the vertical input even in the western upwelling zone. Moreover, the longitudinal and latitudinal transitions of surface REE seem to reflect the lateral supply from the coastal areas. We constructed the diagram of surface Er/Lu and Er/Yb molar ratios in order to assess the origin and the input processes of the surface REE. Both molar ratios showed increasing trend toward PEW (Er/Lu (>10.5) and Er/Yb (>1.4)) from PSUW (Er/Lu (>7) and Er/Yb (>1.2)). The high Er/Lu and Er/Yb ratios in PEW indicate that the lateral supply of terrestrial materials from the coastal area is possibly the important factor in PEW, because only weathering and dissolution of rocks can explain such high Er/Lu and Er/Yb ratios to our knowledge.

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

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

Major and rare earth elements in rainwaters from Japan and East China Sea: Natural and anthropogenic sources

Rainwater samples collected from Tokyo and nearby locations in Japan as well as from two locations over the East China Sea have been analyzed for their rare earth elements (REEs) and major ions. The major ion compositions of these samples show the contributions of oceanic, crustal, and anthropogenic sources, and suggest the relative importance of these sources. Rainwater samples collected in Tokyo and Chofu (a town about 8 km from metropolitan Tokyo) have distinctly higher SO 4 2− and NO 3 − content than in rainwater samples from the coastal area and the East China Sea, whereas the major ion compositions of the latter two areas are dominated by sea salt. The rainwater samples also have variable REE compositions, with various enrichment of light REEs over heavy REEs. Rainwater samples from Chofu are significantly enriched in light REEs, whereas those collected in Tokyo have loess or shale-like REE composition. Rainwater from both places, however, has similar major ion compositions. The enrichment of light REEs in the Chofu rainwater samples corresponds to the typical signature of an anthropogenic source. The Y/Ho ratios in these samples are all higher than average crustal values, and in general, increase with increasing enrichment of light REEs. This indicates that physical mixing of crustal and oceanic sources of the REEs cannot account for the variations of the Y/Ho ratios. Furthermore, although the anthropogenic source of the REEs is generally characterized by a high Y/Ho ratio, factors such as particle/rainwater interactions, pH, and speciation of REEs may also be responsible for the variable abundance and fractionation of REEs and Y/Ho in these rainwaters.

Whole-rock and Nd isotopic geochemistry of Neoarchaean granitoids and their bearing on the evolution of the Central Hearne supracrustal belt, Western Churchill Province, Canada

The Central Hearne supracrustal belt (CHSB), Hearne domain, Western Churchill Province forms a large (ca. 30,000 km 2), greenschist grade Neoarchaean (2711–2660 Ma) terrane of predominant metavolcanic and less common metasedimentary rocks intruded by three groups of plutonic rocks. Pre-tectonic group 1 plutons (>2690 Ma) include rare gabbro, dominant diorite to tonalite and rare granodiorite and granite. These intrude and incorporate angular inclusions of cogenetic volcanic rocks and have variably developed foliations. Syn-tectonic group 2 plutons (2690–2679 Ma) comprise rare gabbro and diorite, predominant tonalite and granodiorite and rare granite, and exhibit N–S-trending, ductiley deformed supracrustal schlieren-rich intrusive margins. Post-tectonic group 3 plutons are rare, and comprise either potassic, biotite monzogranite or alkalic nepheline syenite and rare carbonatite. Abundant tonalite to granodiorite, biotite ± hornblende – bearing mineralogies and metaluminous and generally low-medium K 2O compositions, indicate that most rocks are I- or M-type granitoids. Molecular Na–Ca–K variations and AFM indices indicate transitional calc-alkaline – trondjhemitic trends with both tholeitic and calc-alkaline affinities. Rare earth and incompatible element variations suggest that most granitoids exhibit volcanic arc- or tonalite–trondhjemite–granodiorite (TTG)-like abundances with multi-element patterns varying gradationally from La-poor (La N/Yb N < 12) to La-rich compositions (La N/Yb N > 12). Nd isotopic values overlap with contemporaneous depleted mantle indicating that the granitoids represent melts derived from predominantly juvenile mantle or crust. La-poor rocks likely formed through low- P anatexis of amphibolitic crust with plagioclase + amphibole ± clinopyroxene present, whereas high-La rocks were generated via high- P partial melting of a garnet + clinopyroxene-bearing protolith (plausibly a subducted slab). Granitoid evolution from early dominant low- P, tholeiitic and calc-alkaline melts, to later, predominant high- P, high-Al 2O 3 TTG melts, reflects a change in the tectonomagmatic setting at ca. 2690 Ma. Construction of proto-arc crust from ca. 2711–2690 Ma, and extension of the leading edge of normal (ca. 40 km) Archaean oceanic crust, in response to lithospheric processes analogous to those of the Eocene SW Pacific Ocean, resulted in asthenospheric upwelling, intrusion of mantle-derived melts into the lower crust, and their subsequent ascent and fractionation. Partial melting at the base of the extended, thick oceanic crust likely yielded sparse high-La melts at this time. At ca. 2690 Ma, a change from an extensional to a shortening regime resulted in initiation of subduction of adjacent oceanic lithosphere, partial melting of the eclogitic downgoing slab and generation of voluminous, high- P, La-rich granitoid magmas. This yielded less abundant low- P, La-poor melts emplaced into the “infant-arc” crust during and following tectonism. The complete sequence was intruded by late potassic granites derived from tonalitic lower crust and alkalic magmas that originated in the lithospheric mantle.

Controls on the distribution of rare earth elements in shallow groundwaters

Shallow groundwater samples (<0.22μm) collected from a small catchment in Western France (Petit Hermitage catchment) were analyzed for their rare earth elements (REE), dissolved organic carbon (DOC) and trace-element (Fe, Mn, Th and U) contents, with the aim to investigate the controlling factors of REE signatures. Two spatially distinct water types are recognized in this catchment based on changes of REE concentrations and variations of Ce anomalies. These include (i) DOC-poor groundwater flowing below the hillslope domains; this type has low REE contents and records conspicuous negative Ce anomalies; (ii) DOC-rich groundwater from the wetland domains, close to the river network; this type displays much higher REE concentrations, and typically lack negative Ce anomalies. Temporal REE concentration variations were assessed in wetland waters. Results show a marked increase of the REE content at the winter–spring transition, along with variations of DOC, Fe and Mn contents and redox potential changes. Using the above data set in conjunction with previously published results on comparable shallow groundwaters, we demonstrate that factors such as aquifer-rock composition or anthropogenic input probably play a minor role in determining the REE signatures of shallow groundwaters. Most likely, the two dominant factors involved are the organic matter content and the redox status of the waters. We suggest that topography might be the ultimate key parameter by its ability to control the DOC content of groundwater at a catchment scale.

Rare earth element sources and modification in the Lower Kittanning coal bed, Pennsylvania: implications for the origin of coal mineral matter and rare earth element exposure in underground mines

In this study, we examine the variations in rare earth elements (REE) from the Lower Kittanning coal bed of eastern Ohio and western Pennsylvania, USA, in an attempt to understand the factors that control mineral matter deposition and modification in coal, and to evaluate possible REE mixed exposure hazards facing underground mine workers. The results of this study suggest that the Lower Kittanning coal mineral matter is derived primarily from a clastic source similar to that of the shale overburden. While highly charged cations like silicon, aluminum, and titanium remained relatively immobile within the coal mineral matter, iron (primarily as pyrite) was added from nonclastic sources, either during deposition of the coal mire vegetation or subsequent to burial. Other mobile cations (e.g., alkali and alkaline earth elements) appear to have been added to and/or leached from the originally deposited clastic mineral matter. Most of the sulfur in the Lower Kittanning coal bed is bound as FeS 2 in the mineral matter, but a majority of samples contain a small excess of S that is most likely organically bound. In general, the total rare earth element content (TREE) in coal ash is greater than that in the shale overburden. If the primary source of mineral matter is the same as that for the overlying shale, then REE must have been enriched in the coal mineral matter subsequent to deposition. The total rare earth element content of Lower Kittanning coals correlates strongly with Si concentration ([TREE]≈0.0024 [Si]), which provides a threshold for evaluating possible mixed exposure health effects. Chondrite-normalized REE patterns reveal a shale-like light rare earth element (LREE) enrichment for the coal, similar to that of the shale overburden, again suggesting a primarily clastic REE source. However, when normalized to the shale overburden, most of the coal ash samples display a small but distinct heavy rare earth element (HREE) enrichment. We surmise that the HREE were added and/or preferentially retained during epigenesis, possibly associated with groundwater flow through the coal unit, but not necessarily in close association with the addition of iron. At least some of the “excess” HREE could be organically bound within the Lower Kittanning coal.

Archean Nb-enriched basalts in the northern Superior Province

Geochemical data from the ∼2.9-Ga Northern Pickle assemblage of the Pickle Lake greenstone belt in the northern Superior Province distinguish three distinct volcanic suites. Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE)-enriched with negative mantle-normalized Nb anomalies relative to Th and La, similar to the CA suite of Sajona et al. [J. Petrol. 37 (1996) 693]. Basalts from Suite III are also LREE-enriched but lack negative Nb anomalies. Normalized Nb abundances are either similar to or higher than Th and La, comparable to Nb-enriched basalts (NEB) from Phanerozoic arcs. Variable high field strength elements and heavy REE (HREE) systematics support the subdivision of Suite III basalts into two spatially distinct suites. Suite IIIa is interpreted to have melted at relatively shallow depths in the presence of Nb–Ti-bearing silicates whereas Suite IIIb melted in the presence of garnet. Variations in the geochemistry of the three suites can be accounted for by interaction among primitive mantle, adakite melts, and subduction-modified mantle. The association of the three suites is interpreted to be the result of rifting in a back-arc environment, characterized by Suite I tholeiites, permitting asthenospheric upwelling of subduction-modified NEB and associated arc-like volcanic rocks (Suites II and III). The association of arc-like basalts (Suite II) with NEB (Suite III) in the Northern Pickle assemblage extends the known occurrence of Archean NEB beyond the ∼2.7-Ga age of previously recognized examples.

Radiolarian Successional Sequences and Rare Earth Element Variations in Late Paleozoic Chert Sequences of South China: An Integrated Approach for Study of the Evolution of Paleo-Ocean Basins

Radiolarian ecological diagnoses and rare earth element (REE) criteria are sensitive indicators of the evolution of paleo-ocean basins. We examined the radiolarian succession and biozonation, as well as the REE, La n /Ce n and Ce/Ce* of chert samples from the Early Carboniferous to Middle Permian chert sequences in South China. Six radiolarian ecological facies (REFs) are distinguished: (1) a clear pelagic REF,with eupelagic radiolarians assemblage and the lowest Ce/Ce* (0.33-0.60, average 0.45); (2) a transitional REF containing mixed radiolarian assemblage of eupelagic and bathyal taxa and a moderate Ce/Ce* (0.67-0.76, average 0.73); (3) a convergent continental margin REF containing bathyal radiolarians and a higher Ce/Ce* (0.73-0.79, average 0.76); (4) a divergent bathyal REF containing bathyal radiolarians and a variable Ce/Ce* (0.52-0.97, average 0.75); (5) a rifted-trough REF, with monotonous radiolarians and the highest Ce/Ce* (0.77-0.85, average 0.81); and (6) a crisis event REF containing mutated radiolarians as well as extreme and variation Ce/Ce* values. Stacking of genesis-related REFs allowed for analysis of a radiolarian successional sequence (RSS) corresponding to an evolutionary cycle in a paleo-ocean basin. The complete RSS consisted of a basal rifted trough REF underlain by a crisis event REF, a lower divergent bathyal REF, a middle clear pelagic REF, and an upper transitional, possibly a convergent continental margin REF.

Rare earth element variations resulting from inversion of pigeonite and subsolidus reequilibration in lunar ferroan anorthosites

We present results of a secondary ion mass spectrometry study of the rare earth elements (REEs) in the minerals of two samples of lunar ferroan anorthosite, and the results are applicable to studies of REEs in all igneous rocks, no matter what their planet of origin. Our pyroxene analyses are used to determine solid-solid REE distribution coefficients (D = CREE in low-Ca pyroxene/CREE in augite) in orthopyroxene-augite pairs derived by inversion of pigeonite. Our data and predictions from crystal-chemical considerations indicate that as primary pigeonite inverts to orthopyroxene plus augite and subsolidus reequilibration proceeds, the solid-solid Ds for orthopyroxene-augite pairs progressively decrease for all REEs; the decrease is greatest for the LREEs. The REE pattern of solid-solid Ds for inversion-derived pyroxene pairs is close to a straight line for Sm-Lu and turns upward for REEs lighter than Sm; the shape of this pattern is predicted by the shapes of the REE patterns for the individual minerals.Equilibrium liquids calculated for one sample from the compositions of primary phases, using measured or experimentally determined solid-liquid Ds, have chondrite-normalized REE patterns that are very slightly enriched in LREEs. The plagioclase equilibrium liquid is overall less rich in REEs than pyroxene equilibrium liquids, and the discrepancy probably arises because the calculated plagioclase equilibrium liquid represents a liquid earlier in the fractionation sequence than the pyroxene equilibrium liquids. “Equilibrium” liquids calculated from the compositions of inversion-derived pyroxenes or orthopyroxene derived by reaction of olivine are LREE depleted (in some cases substantially) in comparison with equilibrium liquids calculated from the compositions of primary phases. These discrepancies arise because the inversion-derived and reaction-derived pyroxenes did not crystallize directly from liquid, and the use of solid-liquid Ds is inappropriate. The LREE depletion of the calculated liquids is a relic of formation of these phases from primary LREE-depleted minerals. Thus, if one attempts to calculate the compositions of equilibrium liquids from pyroxene compositions, it is important to establish that the pyroxenes are primary. In addition, our data suggest that experimental studies have underestimated solid-liquid Ds for REEs in pigeonite and that REE contents of liquids calculated using these Ds are overestimates.Our results have implications for Sm-Nd age studies. Our work shows that if pigeonite inversion and/or subsolidus reequilibration between augite and orthopyroxene occurred significantly after crystallization, and if pyroxene separates isolated for Sm-Nd studies do not have the bulk composition of the primary pyroxenes, then the Sm-Nd isochron age and εNd will be in error.

Provenance of beach gray sands from western México

Twenty-nine surficial beach dark colored sand samples from about 6500 km of the western littoral of Mexico were chemically and petrologically analyzed to search for the relationship between their heavy mineral content and metal and rare earth element variations. Sand samples were dried and grouped according to color in three classes of gray. In order to search for a possible influence on petrological and chemical variations, the sands were also grouped according to grain size, sorting, composition, main sources, wave energy, and climate. It was observed that quartz, feldspars, and rock fragments averaged low values and, as the sand darkened, the heavy mineral content increased. Also, the higher content of heavy minerals in dark gray and very dark gray sands correlated with a humid rather than arid climate. Rare earth element and other trace element contents seemed to be related more to the composition of the source area than to heavy mineral content. However, some elements, such as Zn, Hf, Th, Ni, and Cr, did relate more to heavy mineral content. Geochemical and petrological analyses of these sands suggest that most are derived from juvenile sources. Sedimentary processes such as wave energy appear to play a significant role locally in heavy mineral content, making the energy conditions of the beach environment an important local factor. Concentrations of rare earth and other elements are not of economic interest.

The variation of REE (rare earth elements) patterns in soil-grown plants: a new proxy for the source of rare earth elements and silicon in plants

Rare earth elements (REEs) in five species of soil-grown plants (Taxodium japonicum, Populus sieboldii, Sasa nipponica, Thea sinensis and Vicia villosa) and in the soil on which each plant grew were determined with an inductively coupled plasma mass spectrometer (ICP-MS) in order to observe the variation in the distribution of REEs and to elucidate their source in soil-grown plants. The plant samples were divided into root (secondary root and main root), trunk (stem) and leaf; the soils into water soluble (soilsolublefraction), HCl and HNO3 soluble (soilnon-silicatefraction) and HF soluble (soilsilicatefraction). The REE abundances of samples were compared using REE patterns where the abundances were normalized to those of a chondrite and plotted on a logarithmic scale against the atomic number. All the plants showed similar REE patterns independent of species and location, and a W-shape variation (W-type tetrad effect) and abundance depletion of cerium (negative Ce anomaly) were found in each REE patterns of plants, more conspicuous tetrad effect being observed in HREE (heavier rare earth elements) region than in LREE (lighter rare earth elements) region. The overall variation of REE patterns of each secondary root was not similar to that of soilsolublefraction, but similar to that of soilsilicatefraction except for the tetrad effect and Ce anomaly. The REE patterns can be interpreted by the idea that plants of different species take in REEs and Si from different parts in the soil. The results of this study seem to imply that Sasa nipponica and Vicia villosa take in free REEs and Si rather directly from silicate in the soil, and that a majority of REEs and Si in Taxodium japonicum and Thea sinensis are originated from the soluble fraction in the soil.

The distribution of rare earth elements in groundwaters: assessing the role of source-rock composition, redox changes and colloidal particles

Rare earth element (REE), dissolved organic carbon (DOC) and trace-element (Al, Mn, Fe, Sr, Ba, U and Th) concentrations were measured in fourteen well water samples (<0.22 μm) and one spring located along two transects set up in a catchment from Western Europe (Kervidy/Coët-Dan catchment, France). Previous hydrological and hydrochemical (NO 3 −, SO 4 2−) investigations demonstrated that three chemically and spatially distinct groundwater types are present in this catchment, which is fully confirmed by the REE and DOC results. These include: (i) a shallow, organic-rich groundwater (4.4 < DOC < 34.6 mg/l) from the wetland areas, close to the river network. This first groundwater type, characterized by the development of temporary reducing conditions, records high and variable REE contents (2 < ΣREE < 16 ppb) and displays slight or no negative Ce anomaly (Ce/Ce∗ = 0.8–1.05); (ii) a shallow, organic-poor (DOC < 3 mg/l), NO 3 −-rich groundwater type (86.8 < NO 3 − < 155 mg/l) located in the weathered schists, below the hillslope domains. This second type corresponds to recently recharged, oxidized water and displays also high and variable REE concentrations (2 ppb < ΣREE < 15 ppb), but distinguish from the former by the occurrence of very strong negative Ce anomalies (Ce/Ce∗ = 0.05–0.10); finally (iii) a deep, organic-poor (DOC < 1 mg/l), nitrate-poor (NO 3 − close to 0.2 mg/l; detection limit) groundwater. This third type corresponds to reduced water flowing into the deep fresh schists and yields low to very low REE contents (ΣREE < 0.15 ppb) as well as slight negative Ce anomaly (Ce/Ce∗ = 0.8 to 0.9). Temporal REE concentration variations were assessed using samples regularly collected over a six month period. Results show that the spatially distributed Ce anomaly and REE pattern signatures are preserved throughout the studied period. By contrast, REE concentrations are quite variable through time, especially in the wetland waters where the REE concentrations are seen to vary in phase with both redox changes and DOC, Fe, U and Th content variations. Three REE-rich water samples (one DOC-rich and two DOC-poor) were also filtered through membranes of decreasing pore size (100,000 D, 30,000 D, 5,000 D). The results show that between about 40% to 65% of the REE present in the shallow, DOC-poor groundwater samples are controlled by the colloidal fraction, which is likely to consist in these inorganic waters of a mixture of mineral phases. In the wetland groundwaters, the fraction of REE controlled by microparticles is higher than 65%, which confirms the predominant role of organic colloids as major REE carriers in wetland waters. Using the above data set in conjunction with analyses of soil samples, we show that the deep Ce anomalies found in the upper non organic part of the aquifer are probably not source-rock inherited features: most likely, these anomalies arise from the oxidative precipitation of Ce. The very low REE content displayed by waters flooding the deep fresh schists is interpreted as due to the combined effects of (i) pH variation, (ii) secondary sulfate mineral precipitation and (iii) the trapping of colloids-borne REE by the aquifer-rock pores. Data from wetland groundwaters show that the REE, Fe, U and Th budgets of these waters are mainly controlled by seasonal changes in redox conditions and organic matter content. However, unlike organic-poor waters, it appears difficult to relate the Ce behaviour in these organic-rich waters solely to redox conditions. It is likely that the complexation of Ce by organic colloids in the organic-rich waters may mask redox changes by inhibiting the development of negative Ce anomalies.

Distribution of rare earth elements in seaweed: implication of two different sources of rare earth elements and silicon in seaweed

Rare earth elements (REEs) and Si in five species of seaweed, ambient surface seawaters, and suspended solid particles in the seawaters were determined separately. Inductively coupled plasma mass spectrometry (ICP‐MS) was used for REEs and inductively coupled plasma emission spectrometry (ICP‐ES) was used for Si in order to evaluate REEs as a tracer in seaweeds and to understand the source of inorganic elements, especially Si, in seaweeds. Two different REE patterns, one similar to that of the seawater solution and another resembling that of suspended particles, were observed in seaweeds, and the variation of REE patterns seems to show a clear dependence on the abundance of Si. The REE pattern and Si concentration seem to vary depending on the division: green and red algae showed REE patterns similar to that of suspended particles, but brown algae showed patterns closer to that of seawater solutions and relatively lower Si concentration. The possibility of contamination from silicate particles on the surface of seaweeds was ruled out for several reasons. Silicate particles, not dissolved silicate, have been identified as the direct source of REEs and Si in plants (Fu et al. 1998), and seaweeds are no exception. We have to consider that seaweeds can take up Si from suspended particles through their blade or branches. From the appearance of tetrad‐effect‐like variation of REEs, Si is assumed to enter a dissolved state just before the particles are taken up. From the results of a sonication experiment, REEs, once taken up as silicate particles, seem to be separated from Si in the thallus.

Uptake and fractionation of rare earth elements on hydrothermal plume particles at 9°45′N, East Pacific Rise

Particulate samples (>0.45 μm) from a neutrally buoyant hydrothermal plume at 9°45′N on the northern East Pacific Rise were collected using large volume in situ filtration and analyzed for Fe, Al, Mn, Ni, and fourteen rare earth elements (REE). The Sm/Fe ratio (a proxy for overall REE/Fe) and Nd/Er (light/heavy REE fractionation) increased moderately with decreasing particulate Fe. Chemically, the sense of these relationships matched that documented previously in the TAG plume on the Mid-Atlantic Ridge (German et al., 1990), although particulate Fe was about 10 fold lower at 9°45′N. Spatial trends relative to the vent source, however, were opposite of expectation because slow Fe(II) oxidation and Fe(III) colloid aggregation over this interval led to increased particulate Fe (10–26 nM) with distance from source (Field and Sherrell, submitted). After subtraction of non-plume background particle composition, plume particles at 9°45′N and TAG had indistinguishable ranges of light REE-enriched fractionation relative to ambient seawater and had very similar Sm/Fe (therefore Kd for Fe oxyhydroxides), demonstrating that plume particles in both oceans reflect to a first degree the local seawater REE composition. Within-plume REE variations at 9°45′N were investigated using a simple mixing model which accounts for the bulk Fe-Al-Mn variations in the plume using two endmembers: fresh hydrothermal oxyhydroxide precipitates and ridge-crest background particles (composed largely of locally resuspended sediment). Sm/Fe and Nd/Er plot linearly with mixing ratio (R > 0.96), implying that the observed REE trends result from mixing of these two endmembers. Extrapolation to the composition of pure hydrothermal precipitates suggests that Nd/Er is fractionated relative to seawater by a factor of 1.8 during adsorption onto fresh Fe oxyhydroxide particles. The ridge-crest background particles are 5 fold higher in Sm/Fe and Nd/Er is 2.49 relative to seawater, partly a result of enriched terrigenous component in the resuspended matter. A reinterpretation of REE at TAG reveals that positive curvature in REE vs. Fe plots, argued previously to reflect continuous REE uptake (i.e., increasing Kd; German et al., 1990), may result from local depletion of the dissolved REE pool by partitioning onto Fe particles at Fe > 100 nM. Similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater-source particle-reactive species in plumes that are sampled at high particulate Fe concentration. In sum, REE behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types, than with kinetic uptake control. Precise measurements of REEs in modern ridge-crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long-term changes in REE of the hydrothermal component.

Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He‐Pb‐Nd‐Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges

He‐Pb‐Nd‐Sr isotope systematics in basalts from the Mid‐Atlantic Ridge (MAR) from 65°N to 78°N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He‐Pb‐Nd‐Sr isotope space clearly indicate that the boundary between the zone of influence of the “low 3He/4He” Jan Mayen plume and the “high 3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb‐Nd‐Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion &lt;F&gt; and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He‐Pb‐Nd‐Sr isotopic systematics and incompatible element variations, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume by Mertz et al. [1991] which was based on more limited isotopic data, and the fluid dynamic models of Ito et al. [1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb‐Nd‐Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjörnes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb‐Nd‐Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear‐dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.

Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province

High precision trace element data from ultramafic and mafic volcanic rocks from two units of the ∼3 Ga North Caribou greenstone belt in the northwestern Superior Province reveals new evidence for the contamination of early Archæan komatiites. Spinifex-textured komatiites (MgO=20−-27 wt.%) of the Opapimiskan–Markop unit are characterised by flat to moderately enriched light rare earth element (LREE) trace element patterns (La/Sm n=1–2) in conjunction with negative Nb and Ti anomalies (Nb/La pm=0.48–0.59; Ti/Ti*=0.6–0.8), consistent with contamination of a primary Munro-type Al-undepleted ultramafic melt by felsic material compositionally similar to Archæan tonalitic-trondhjemitic-granodioritic (TTG) suites. The trace element characteristics of these komatiites are distinct from both Proterozoic and Phanerozoic arc and rift picrites. Mg-tholeiites intimately associated with the komatiites display variable rare earth elements (REE) (La/Sm n=0.8–1.5) and Nb and Ti anomalies also consistent with variable degrees of contamination (Nb/La pm=0.58–0.98; Ti/Ti*=0.6–1.0). An intermediate suite of high-Mg andesites (54–60 wt.% SiO 2) associated with both the tholeiites and the komatiites can be derived by fractional crystallisation of a melt with the trace element characteristics of the most contaminated komatiites. Compositionally uniform Mg- to Fe-tholeiites of the South Rim unit are believed to overlie the Opapimiskan–Markop unit, possibly in stratigraphic contact. They are characterised by flat trace element patterns (La/Sm n=0.9–1.0), are uncontaminated, and comparable with modern day oceanic plateaux, such as Ontong Java. The two units likely represent the eruptive products of a single mantle plume that entrained tholeiitic melts from the upper mantle before impinging upon the Archæan crust at ∼3 Ga.

Rare earth element variations in mid-Archean banded iron formations: implications for the chemistry of ocean and continent and plate tectonics

Abundances of major and rare earth elements (REEs) are reported for mid-Archean (3.3–3.2 Gyr) sedimentary rocks including banded iron formations (BIFs) and ferruginous/siliceous mudstone from the Cleaverville area in the Pilbara craton, Western Australia. Geological, lithological, and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments, ranging from that typical of mid-oceanic spreading centers to convergent plate boundary settings; a range of environments most likely caused by plate movements. Except for the mudstone, the REE content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons. Europium anomalies decrease up-section (Eu/Eu∗ values normalized to NASC change from 3.5 to 1.1) as the REE contents and LREE/HREE ratios increase. The striking similarity in these REE signatures of BIFs and modern hydrothermal sediments leads us to propose that the BIFs were in situ hydrothermal precipitates near a mid-ocean ridge. Significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon. The observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics, dominated by horizontal movement, was already operating in the mid-Archean. Distal hydrothermal sediments without a Eu anomaly (when normalized to NASC) suggest that mid-Archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no Eu anomaly (i.e, it had a negative Eu anomaly when normalized to chondrite). In addition to an absence of a Eu anomaly, mid-Archean seawater did not have a Ce anomaly, suggesting less oxic conditions in the mid-Archean than in the modern ocean.

Lunar Ferroan Anorthosite Petrogenesis: Clues from Trace Element Distributions in FAN Subgroups

The rare earth elements (REE) and selected other trace elements were measured in plagioclase and pyroxene from nine samples of the lunar ferroan anorthosite (FAN) suite of rocks. Samples were selected from each of four FAN subgroups previously defined by James et al. (1989). Plagioclase compositions are homogeneous within each sample, but high- and low-Ca pyroxenes from lithic clasts typically have different REE abundances from their counterparts in the surrounding granulated matrices. Measured plagioclase/low-Ca pyroxene concentration ratios for the REE have steeper patterns than experimentally determined plagioclase/low-Ca pyroxene partition coefficients in most samples. Textural and trace element evidence suggest that, although subsolidus equilibration may be responsible for some of the discrepancy, plagioclase compositions in most samples have been largely unaffected by intermineral redistribution of the REE. The REE systematics of plagioclase from the four subgroups are broadly consistent with their derivation through crystallization from a single evolving magma. However, samples from some of the subgroups exhibit a decoupling of plagioclase and pyroxene compositions that probably reflects the complexities inherent in crystallization from a large-scale magmatic system. For example, two anorthosites with very magnesian mafic minerals have highly evolved trace element compositions; major element compositions in plagioclase also do not reflect the evolutionary sequence recorded by their REE compositions. Finally, a noritic anorthosite breccia with relatively ferroan mafic minerals contains several clasts with high and variable REE and other trace element abundances. Although plagioclase REE compositions are consistent with their derivation from a magma with a KREEPy trace element signature, very shallow REE patterns in the pyroxenes suggest the addition of a component enriched in the light REE.