Peter Stille

REE fractionation during granite weathering and removal by waters and suspended loads: Sr and Nd isotopic evidence

Very few studies deal with REE (rare earth element) mobility within the system soil–soil solution–streamwater. In this article, we try to characterize the fractionation and the migration of the REE in a granite-derived soil system located in a small catchment of the Vosges mountains. ICP-MS and TIMS measurements were performed on both solid samples (“fresh” granite, soil, and suspended load of the stream) and waters (soil solutions, springwater, and streamwater) to determine their respective REE concentrations and Sr and Nd isotopic compositions. The PAAS-normalized REE pattern of the bedrock is characterized by a strong depletion in HREE (heavy REE) and a negative Eu anomaly (0.46). Similarly, the granite-normalized REE distribution patterns of the soil samples show HREE depletions that become more important with decreasing depth. The correlative behavior between P2O5, Th, and REE with depth indicates that, besides apatite, other phosphate minerals such as monazite are the most important phases controlling the Th and REE budget in the soil profile. On the other hand, at greater depth, zircon seems to be another important mineral phase controlling especially the HREE enrichment as shown by the positive relationship between Zr content and the Yb/Ho ratio. Different grain size fractions show similar REE distribution patterns and are only weakly fractionated, compared with bulk soil sample. However, the finest fraction (0–20 μm) is more enriched in Sr and REE, suggesting a stronger concentration of REE-carrying minerals in this fraction. The suspended and dissolved load of the stream show as a whole an enrichment in HREE if compared with the granite or with the different soil samples. However, compared with the uppermost soil samples, the suspended load is significantly more enriched in HREE. Its REE distribution pattern is more similar to that of the finest fraction of the deeper soil sample and to the “fresh” granite. Thus, most probably the REE of the suspended load originated from a source with REE characteristics found in the deep soil horizons. This source might have been situated in the uppermost soil profile, which is actually REE depleted. The weathering process can be compared with a leaching experiment where the waters correspond to the leachate and the soil to the residual phase of the granite. The Sr isotope data indicate that the suspended load originates from the finest soil fraction. The Sr and Nd isotopic data of the suspended load suggest that it contains up to 3% Sr and Nd from apatite and up to 97% from feldspar. Most of the Sr and Nd in the waters originate from apatite leaching or dissolution.

Diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies: An isotopic and REE study of Cambrian phosphorites

Seawater rare-earth element (REE) distributions show heavy REE enrichment and cerium depletion but these characteristics are only rarely found in ancient, marine authigenic minerals such as carbonate fluorapatite and calcite. This enigma may be due to changing REE distributions in seawater through time or to post-depositional redistribution of REE. Differentiating between these two factors is, however, far from straightforward as their relative importance is likely to vary and evidence is often ambiguous. In this article, we present REE as well as strontium and sulphur isotope data for basal Cambrian phosphorites of South China, which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm, and tend to undergo, therefore, less diagenetic alteration of REE spectra than skeletal apatites and other phosphate-types. This study reveals stratigraphic trends in Ce anomalies (Ce/Ce*) that could be taken to imply the preservation of a palaeoenvironmental signal. However, further investigation of our data shows that such trends in phosphorites can also be caused by post-depositional alteration, which tends to (1) increase REE contents, (2) reverse Ce depletion, (3) alter Eu anomalies, and (4) concentrate preferentially the middle REE (MREE). Further alteration, which may be due to weathering, has led to HREE depletion, and positive La and Y anomalies (tetrad effect). This is the first study where Ceanom. and phosphate-bound sulphate S-isotope values could be compared for one particular stratigraphic section, which provides us with an additional tool for the interpretation of diagenetic redox conditions. Least altered phosphorites from Meishucun that yield a strongly negative Ceanom. constrain early Cambrian seawater δ34S to about +33‰. We conclude that phosphorite Ceanom. primarily records variations in the local conditions of diagenesis, while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom-water redox conditions.

Rare earth elements and Sr and Nd isotopic compositions of dissolved and suspended loads from small river systems in the Vosges mountains (France), the river Rhine and groundwater

Abstract The aim of this study is to characterize the evolution of the rare earth elements (REE) in non-mature streams from small catchment areas in the Vosges mountains downstream to more mature plain rivers including the river Rhine. The dissolved load REE distribution patterns of the low-pH Vosges streams are very different from those of high-pH plain rivers indicating that different physico-chemical parameters control REE transport in these different water systems. The plain rivers and groundwater show similar REE distribution patterns with a strong negative Ce anomaly and heavy rare earth element (HREE) enrichment. In addition, the river Rhine has a positive Gd anomaly which is of anthropogenic origin. Similar to the worlds major rivers the light rare earth element (LREE) enrichment in the plain rivers is mainly pH controlled. Their Sm/Nd and 143 Nd / 144 Nd isotope ratios are close to average continental crust values. This is not the case for the less evolved, non-mature and low pH Vosges streams. Their high Sm/Nd and 143 Nd / 144 Nd ratios but low 87 Sr / 86 Sr ratios suggest that chemical alteration of accessory middle rare earth element (MREE) enriched minerals such as apatites from rocks in the catchment area control the REE abundances of these waters. A comparison of the dissolved load REE distribution patterns with those of the principal lithologies in the corresponding drainage basins illustrates that especially the Eu anomalies of the Vosges streamlets are strongly lithology dependent. Leaching experiments indicate that the suspended load is isotopically very inhomogeneous. The REE distribution patterns of the suspended load leachates have no similarities with those of the corresponding dissolved load indicating that the leachable reservoir not only contains adsorbed REE but also REE from leachable mineral phases. Their 143 Nd / 144 Nd , Sm/Nd and 87 Sr / 86 Sr ratios support this suggestion being always higher and lower, respectively, than those of their corresponding dissolved loads. They rather point to the presence of relic primary apatite in the leachable portion of the suspended load. The flat PAAS normalized REE distribution patterns, the high 87 Sr / 86 Sr and low 143 Nd / 144 Nd isotopic ratios of the corresponding residues suggest secondary silicate phases such as clay minerals in the residual phase of the suspended load. Leachates and corresponding residual phases define alignments in the Sm/Nd isochron diagram whose slopes define ages ranging between 200 and 390 Ma indicating that the mineral phases in the suspended load retain some memory of their primary precursor minerals in the Hercynian granitic source rocks.

The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance

Calcium isotope ratios relative to seawater were studied in natural waters in order to constrain the Ca isotopic flux to the oceans and to discuss the oceanic Ca budget. This study includes the analysis of (1) Ca isotopes in continental waters at the regional scale of the Upper Rhine valley and (2) the determination of the δ44Ca values of some major rivers and hydrothermal vents. The data indicate that the global δ44Ca variability of the studied waters, with a maximal range of 1‰, is limited and must be linked to mass fractionation processes. At the scale of a small watershed (Aubure, Vosges, France), the δ44Ca river flux changes due to the variation of the proportion of waters resulting from chemical alteration of rocks and biologically fractionated soil solutions. Ca isotopes could therefore be an important tool to quantify the impact of the plants and the biosphere on the river water chemistry. At the regional scale of the Rhine valley as well as at the global scale of world rivers, the δ44Ca variability of river samples ranges from 0.5 to 1‰ without any relationship with the lithology of the watershed or the climate. Similarly, the δ44Ca value of hydrothermal vents is also uniform and close to the mean value of river waters. Consequently, the δ44Ca flux to the ocean should remain rather constant through time with a mean of −1.1±0.2‰. This value confirms that the Ca budget of the present-day seawater is in steady-state. This was not necessarily the case in the past. Non-steady-state periods should have been due to intensity variations of incoming and outcoming Ca fluxes rather than to the variations in their isotopic signatures.

Kinetic aspects of basaltic glass dissolution at 90°C: role of aqueous silicon and aluminium

Abstract Steady-state dissolution rates of a synthetic basaltic glass were measured in an open-system mixed flow reactor as a function of solution composition at a temperature of 90°C and over the pH range 7.8 to 8.3. The dissolution is a two-step process. The first of these steps involves the release of the cation modifier elements leading to the formation of a hydrated surface gel (HBG) of which the solubility controls the overall dissolution reaction. The glass steady-state dissolution rates were found to be independent of aqueous aluminium and silicium concentration but to depend on the chemical affinity for the overall hydrolysis reaction. The glass is a rapidly reacting solid, whose dissolution induces a dramatic change in solution concentration, which results readily in small chemical affinities for the dissolution reaction. Consequently, conditions of great undersaturation have not been investigated (affinity max. 9.8 kJ/mol). However, our results strongly suggest that the dissolution rates are controlled by the decomposition of a stoichiometric silico-aluminous surface precursor. The variation of the steady-state dissolution rates can be described using a simple expression based on the concept that the precursor is formed by the simple absorption of reactants: R (mol cm −2 s −1 ) = 3 × 10 −10 (OH − ) 0.39 (1− Q /8.2 × 10 −5 ), where Q , the ion activity quotient is equal to: Q = (H 4 SiO 4 ) (Al(OH) 4 − ) 0.36 (Fe(OH) 3 ) 0.18 (OH − ) −0.36 .

Nd isotope evidence for the evolution of the paleocurrents in the Atlantic and Tethys Oceans during the past 180 Ma

The Nd isotopic composition of Atlantic and Tethys seawater, as deduced from marine phosphorites, varied considerably during the past 180 Ma. The early Tethys and Central Atlantic seawater from 180 to 160 Ma ago (Early-Middle Jurassic) had a Nd isotopic signature identical to that of the Pacific (eNd ≈ −6) suggesting that Pacific seawater entered the newly forming Tethys basin. However, with time continental runoff draining into the young basin became more important and led to a decrease in the Nd isotopic composition and, finally, a decoupling from the Pacific Ocean. During the late Early Cretaceous (120-90 Ma ago) Atlantic and Tethys seawater reached continental crust-like Nd isotopic composition values (eNd ≈ −10) which could reflect high weathering rates induced by the warm and humid climate at that time. In the time span between 80 and 50 Ma (Late Cretaceous-Early Tertiary) the Nd isotopic composition of both Tethys and Atlantic seawater strongly increased towards Pacific seawater values. In the case of the Atlantic Ocean, this change has been correlated with the opening of the South Atlantic, which enabled the more radiogenic Pacific seawater to travel westwards around the southern edge of Africa into the South and finally the North Atlantic. In the case of Tethys seawater we have to assume that, during this period, large masses of Pacific seawater entered the Tethys again directly through the Indian-Tethys seaway. The renewed importance of this seaway might be related to the Late Cretaceous-Early Tertiary first-order, global sea level highstand. The most abrupt change towards lower, more crust-like values occurred in the early Miocene (25-17 Ma ago) in the South and North Atlantic, as well as in the Tethys. During this period, the143Nd/144Nd isotope ratios dropped from ≈ 0.5124 (eNd ≈ −4.2) down to ≈ 0.5122 (eNd ≈ −8). This change indicates a diminishing influx of Pacif seawater into the Atlantic and Tethys Oceans and can be correlated with the breakdown of the circum-equatorial circulation patterns of the world oceans, due to plate tectonics. From 20 to 17 Ma ago (Miocene) the Nd isotopic composition of the Atlantic Ocean increased again and reached values as high as 0.5123 (eNd ≈ −6.6), whereas Tethys seawater remained at low values. This decoupling can be correlated with the incipient complete isolation of the Tethys; the Nd isotopic composition was henceforth controlled by the continental runoff from surrounding land masses. The major increase in the Nd isotopic composition observed in the Atlantic can be related to the opening of the Drake Passage and establishment of the circum-Antarctic current system. This allowed Pacific seawater to enter directly the South Atlantic around Antarctica. A renewed drop in the Nd isotopic composition of Atlantic seawater during the late Miocene can be related to the onset of North Atlantic Bottom Water formation and its subsequent upwelling.

Rare earth element behavior and Pb, Sr, Nd isotope systematics in a heavy metal contaminated soil

Abstract The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure. The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO 3 . The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The 87 Sr/ 86 Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO 3 extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr. The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and Fe Mn oxides for the REE in the HCl and HNO 3 extracts. A part of the HNO 3 extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by 143 Nd/ 144 Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.

Sm-Nd isotopic dating of Proterozoic clay material: An example from the Francevillian sedimentary series, Gabon

Clay fractions of < 0.4 μm and < 0.2 μm and acid leachates of these clay fractions were separated from two different kerogen-rich black shale samples from the Francevillian sedimentary series in Gabon. They yielded identical Sm-Nd isochron ages of2099 ± 115 Ma and2036 ± 79 Ma, with initialδNd values of−6.5 ± 2.0 and−9.3 ± 1.5, respectively. These ages define multi-episode illitization during early diagenesis, as suggested by the differentSm/Nd ratios of the clay fractions. The Sm-Nd isotope data of the kerogen source rock (up to 13% organic carbon) plot between the two isochrons but close to the clay fractions, suggesting that this organic matter reached Nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids. Analyses of fracture-filling bitumen showed that it had very elevated143Nd/144Nd and147Sm/144Nd ratios. These values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons. This indicates that the bitumen reached Nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals. Secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between Sm and Nd with an attendant increase in theSm/Nd ratio. The results suggest that the Sm-Nd isotope method has potential for both dating diagenetically related illitization using leachate-residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil-producing sedimentary basins.

Characterization and migration of atmospheric REE in soils and surface waters

Abstract Rainwater and snow collected from three different sites in France (Vosges Mountains, French Alps and Strasbourg) show more or less similar shapes of their REE distribution patterns. Rainwater from Strasbourg is the most REE enriched sample, whereas precipitations from the two mountainous, less polluted catchments are less REE enriched and have concentrations close to seawater. They are all strongly LREE depleted. Different water samples from an Alpine watershed comprising snow, interstitial, puddle and streamwater show similar REE distributions with LREE enrichment (rainwater normalized) but MREE and HREE depletion. In this environment, where water transfer from the soil to the river is very quick due to the low thickness of the soils, it appears that REE in streamwater mainly originate from atmospheric inputs. Different is the behaviour of the REE in the spring- and streamwaters from the Vosges Mountains. These waters of long residence time in the deep soil horizons react with soil and bedrock REE carrying minerals and show especially significant negative Eu anomalies compared to atmospheric inputs. Their Sr and Nd isotopic data suggest that most of the Sr and Nd originate from apatite leaching or dissolution. Soil solutions and soil leachates from the upper soil horizons due to alteration processes strongly depleted in REE carrying minerals, have REE distribution patterns close to those of lichens and throughfall. Throughfall is slightly more enriched especially in light REE than filtered rainwater probably due to leaching of atmospheric particles deposited on the foliage and also to leaf excretion. Data suggest that Sr and Nd isotopes of the soil solutions in the upper soil horizons originate from two different sources: 1) An atmospheric source with fertilizer, dust and seawater components and 2) A source mainly determined by mineral dissolution in the soil. These two different sources are also recognizable in the Sr and Nd isotopic composition of the tree’s throughfall solution. The atmospheric contributions of Sr and Nd to throughfall and soil solution are of 20 to 70 and 20%, respectively. In springwater, however, the atmospheric Sr and REE contribution is not detectable.

Transfer of rare earth elements (REE) from natural soil to plant systems: implications for the environmental availability of anthropogenic REE

Background and aimsRare Earth Elements (REE) are widely used to trace natural geochemical processes. They are also increasingly used by man (electronics industry, medicine, agriculture) and therefore considered as emerging pollutants. The present study documents REE mobility in non-polluted natural soil-plant systems in order to characterize their environmental availability for future anthropogenic pollution.MethodsThe study is based on a field approach in non-polluted natural sites with contrasting geological environments (limestone, granite, and carbonatite) and highly variable REE contents.ResultsREE concentrations in soils do not directly reflect bedrock concentrations, but depend largely on pedogenetic processes and on the mineralogy of bedrock and soil. The soils of all sites are with respect to bedrock enriched in heavy REE. The REE uptake by plants is not primarily controlled by the plant itself, but depends on the concentration and the speciation in the soil and the adsorbed soil water pool.ConclusionsREE uptake by plant roots are linked with those of Fe. Roots absorb preferentially the light REE. Before translocation, REE are retained by the Casparian strip leading to much lower concentrations in the aerial parts. The transport of the REE within the xylem is associated with the general nutrient flux.