Gérard Gruau

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/Coet-Dan catchment, France). Previous hydrological and hydrochemical (NO3−, SO42−) 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), NO3−-rich groundwater type (86.8 < NO3− < 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 (NO3− 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.

THE INFLUENCE OF ALTERATION ON THE TRACE-ELEMENT AND ND ISOTOPIC COMPOSITIONS OF KOMATIITES

Abstract To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵNd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵNd is far greater ( −6.2 to +8.8). The variable initial ϵNd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵNd of + 2.3.

Release of trace elements in wetlands: role of seasonal variability.

Dissolved concentrations were determined for Fe, Mn, Al, Cu, Zn, La, U, Th, Cd and As in a wetland and its recipient stream to reveal the effect of seasonal changes in environmental conditions on the cycling and transfer of trace elements at the transition between terrestrial and aquatic ecosystems. These preliminary results from the wetland show marked seasonal changes in dissolved concentration for all elements except Zn and Cu. Concentrations are found to be low until about mid-February and then increase abruptly. The onset of trace element release appears to coincide with a marked decline in redox potential and increase of organic carbon content. Because this decline is itself correlated with a pronounced increase in temperature and dissolved Fe. Mn and organic carbon content, we suggest that the microorganisms which use soil iron and manganese oxy-hydroxides as electron acceptors catalyzed the change in redox conditions and induced an increase of DOC. Temporal changes were also observed in the recipient stream which showed marked positive concentration peaks during stormflow events (except Zn). The seasonal processes occurring in the wetland appear to play a major role in determining the amount of trace elements which are transferred from the wetland to the river.

RbSr and 40Ar39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

Abstract The combined use of the RbSr and 40Ar39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar39Ar plateau ages. 40Ar39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

Resetting of SmNd systematics during metamorphism of > 3.7-Ga rocks: implications for isotopic models of early Earth differentiation

Abstract New SmNd isotopic results for sixteen samples from the Garbenschiefer Amphibolite Unit of the > 3700-Ma Isua supracrustal belt of West Greenland are reported. The data illustrate the difficulty in identifying primary ϵNd(T) values in poly-metamorphic and poly-deformed rock sequences. The investigated samples are interpreted as derived from a Mg- and Al-rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement. However, the data do not give uniform ϵNd(T) values even from single outcrops. The wide variation of ϵNd(T) which we obtain (−1.0 to + 5.0) demonstrates that the SmNd systematics of the samples were disturbed by one (or more) metamorphic event(s) which took place some considerable time after the original crystallization of the Isua belt. A SmNd isochron age of 2849 ± 116 Ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at Isua. This result indicates that the earliest Amphbolite-facies metamorphism of the Isua belt for which pressure and temperature conditions can be obtained (P ≈ 5 kbar; T ≈ 500–600°C) is late Archaean, and not early Archaean as had been assumed earlier by some authors. 2850 Ma is also thought to represent the main time of disturbance of the SmNd system in the Garbenschiefer Amphibolite Unit. A consideration of available isotopic data suggests that many of the rocks from Isua underwent some Nd isotopic re-equilibration and/or fractionation of their 147 Sm 144 Nd ratio during the 2850-Ma metamorphic episode. In these rocks, the computed ϵNd(T) are apparent values only and need not represent either the primary isotopic composition of their protoliths or early Archaean mantle values. Only Nd-rich rocks (> 20 ppm) such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain ϵNd(T) values which are close to their original composition. These average +2.0, which may represent a close approximation of the ϵNd value of the Earths mantle at ∼ 3800 Ma. The results presented in this paper show that great care should be exercised in the use of SmNd data from very early Archaean rocks as constraints for isotopic models of early Earths evolution, particularly in terranes which have been affected by several metamorphic episodes.

Mechanisms of Nitrate Transfer from Soil to Stream in an Agricultural Watershed of French Brittany

In French Brittany, water pollution with nitrate due tointensive agriculture has become one of the major environmentalconcerns. In this article, the nitrate, sulfate and chlorideconcentrations from the groundwater and the stream of a first-order agricultural watershed, are analyzed to infer the mechanisms responsible for the distribution and transfer of nitrate within the watershed. The aquifer is constituted by three layers: the thin soil cover, the weathered shale and thefissured shale. The weathered shale groundwater appears to bea large reservoir of nitrate in the watershed. Indeed the amount of nitrate is estimated at about 450 kg N ha-1, 5 to 9 times the total annual nitrate flux in the stream. In the upslope zones, this groundwater exhibited high nitrate concentrations (up to 138.4±10.5 mg NO3- L-1), which decreased along the flow paths towards the stream (77.1±13.8 mg NO3- L-1). Unlike nitrate, sulfate concentrations showed an increase from uphillto downhill (from 6.1±0.8 to 12.5±5.4 mg SO42- L-1) with little change in chloride concentrations. These patterns are presumed to result from upward flows from fissured shale groundwater where denitrification by oxidation of pyrite occurs with sulfate as end product. A scheme of nitrate transfer is proposed where stream discharge would result from the mixing of three end members which are: uphill weathered groundwater, deep groundwater and water in the uppermost soil horizons ofthe bottomlands. Temporal variability of nitrate concentrationsin base flow reflects changes in the relative contribution of each end member.

The origin of U-shaped rare earth patterns in ophiolite peridotites: Assessing the role of secondary alteration and melt/rock reaction

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al2O3 = 0.6–1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low eNd(T) values (+2 to −7), but high to very high 87Sr/86Sr(T) ratios (up to 0.715), whereas rocks with more fertile major element compositions (Al2O3 = 2–3.2%; CaO = 2–2.6%) give REE patterns showing increasing depletion from Lu to La and have high eNd(T) values (+6 to +8), but low 87Sr/86Sr(T) ratios (0.703–0.704). The low neodymium and high strontium isotopic ratios of those peridotites having U-shaped REE patterns demonstrate that the LREE enrichments observed in these rocks are the result of contamination by continental crust and not the consequence of peculiar fractionation during melting or chromatographic effects during mantle ascent. A clinopyroxene concentrate prepared from a bulk sample of lherzolite with eNd(T) = −7 and 87Sr/86Sr(T) = 0.708 gives MORB-like isotopic signatures: eNd(T) = +14 and 87Sr/86Sr(T) = 0.7035. Furthermore, two highly serpentinized samples with U-shaped REE patterns show strongly negative δD values (−95 and −110‰) much typical of peridotites altered by continental metamorphic fluids or/and meteoritic waters. From these results, we conclude that the crustal contamination corresponds to a low-temperature alteration event which took place during or after obduction of the Trinity ophiolite complex onto the continent and is not the consequence of any high-temperature invasion of the ultramafic rocks by melts/fluids derived from subducted continental materials.

Rb-Sr and Sm-Nd geochronology of lower Proterozoic granite-greenstone terrains in French Guiana, South America

Abstract Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis. The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial 143 Nd 144 Nd ratio (INd) of 0.51002±9 (2 σ), corresponding to ϵNd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial 87 Sr 86 Sr ratio (ISr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths. The positive eNd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high eNd(T) value for these rocks excludes any significant crustal contamination during magma genesis. The French Guianese orthogneisses yield a low ISr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition. The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.

Loss of isotopic (Nd, O) and chemical (REE) memory during metamorphism of komatiites: new evidence from eastern Finland

Komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the Archean mantle. However, a weakness of these rocks in tracking the initial compositional heterogeneity of the Earths mantle is the ubiquitous presence of metamorphic recrystallization, which casts some doubt about the preservation of primary chemical and isotopic characteristics. Two spinifex-textured komatiite flows from the 2.75 Ga old Kuhmo greenstone belt (Siivikkovaara area) of eastern Finland document this weakness. Both flows have experienced low to medium grade metamorphism (T=450±50°C), and now consist entirely of secondary metamorphic assemblages of amphibole±chlorite±plagioclase, with minor proportions of magnetite and ilmenite. MgO contents range from 25 to 8%, which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization. However, neither major nor trace elements fall on olivine and/or clinopyroxene control lines. This is particularly well illustrated by the REE as there is an overall 60% variation of (Ce/Sm)N ratios (0.38 to 0.91), which far exceeds that expected from olivine and clinopyroxene fractionation alone. In fact, careful evaluation of petrographic (including mineral composition data) and chemical characteristics shows that most elements of geological interest (including the reputedly immobile REE) were mobile on a whole-rock scale during metamorphic recrystallization of these two flows. This view is fully supported by Sm-Nd isotopic data since both whole-rock and mineral (amphibole and plagioclase) samples lie on a single isochron relationship at T≈ 1800 Ma, an age which corresponds to the time of regional metamorphism. Thus, the meta-komatiite flows from Siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole-rock scale resetting of primary REE patterns and Sm-Nd isotope systematics. As regards the nature of the mechanism responsible for this resetting, it is suggested here that the secondary mineralogy played an important role, as there are correlations between whole-rock 147Sm/144Nd ratios, major and trace element chemistry, oxygen isotopic ratios and modal proportions of secondary minerals. Consideration of oxygen isotopic data, as well as previous results from a komatiite flow from the nearby Tipasjärvi belt, further enable us to propose that the REE carrier was likely a CO2-rich fluid. The process of secondary REE redistribution prevents estimation of the true initial Nd isotopic compositions of the two flows. Taken as a whole, the results presented in this paper show that great care should be be exercised in the use of meta-komatiites as probes of Archean mantle composition.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, 39Ar40Ar) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar39Ar) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids (δ18O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵNd(T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵNd(T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaOAl2O3 and Al2O3TiO2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.