This work studies the potential release of Cr from solids to surface water and groundwater, and the related isotopic compositions, in a nickel laterite ore deposit from Barro Alto in Brazil (in the State of Goiás). This ultramafic system is characterized by elevated concentrations of chromium (Cr). Even largely immobile Cr may be naturally leached from weathering profiles such as laterite to the surface and groundwater. This system is exploited for metal production, and both mining and metallurgical activities result in a significant increase in the trivalent (Cr(III)) and hexavalent chromium (Cr(VI)) concentrations released into the environment via runoff. Among all the samples collected, ores contained higher amounts of chemically and isotopically exchangeable Cr(VI) (ECr(VI)) with values as high as 104 (± 8) mg kg−1. δ53Cr increased from −0.28 (± 0.01)‰ to −0.05 (± 0.01)‰ and the ECr(VI) value was up to 30 times higher in deep soils than at the surface (up to 7 (± 1) mg kg−1). Chemically extracted Cr(VI) (ECr(VI)-KH2PO4) displayed positive δ53Cr value (1.69 (±0.03) ‰) with a similar Cr isotopic composition as the ones measured in freshwater sampled in and around the mine area. It appears that Cr is primarily released as Cr(VI), i.e. the toxic species, and becomes increasingly more available as it moves from the soil profile to the ores and mining residues. Based on the differences in the Cr isotopic composition, this study proves that δ53Cr can be used in environmental studies to trace Cr leaching.

A new purification approach is reported for the highly precise and accurate determination of Sb isotope ratios in geological samples.

Gallium (Ga) isotopic fractionation during its adsorption on calcite and goethite was investigated at 20 degrees C as a function of the solution pH, Ga aqueous concentration and speciation, and the solid to solution ratio. In all experiments Ga was found to be enriched in light isotopes at the solid surface with isotope fractionation Delta Ga-71(solid-solution) up to -1.27% and -0.89% for calcite and goethite, respectively. Comparison of Ga isotopic data of this study with predictions for 'closed system' equilibrium and 'Rayleigh fractionation' models indicates that the experimental data are consistent with a 'closed system' equilibrium exchange between the fluid and the solid. The results of this study can be interpreted based on Ga aqueous speciation and the structure of Ga complexes formed at the solid surfaces. For calcite, Ga isotope fractionation is mainly triggered by increased Ga coordination and Ga-O bond length, which vary respectively from 4 and 1.84 angstrom in Ga(OH)(4)(-) to 6 and 1.94 angstrom in the >Ca-O-GaOH(OH2)(4)(+) surface complex. For goethite, despite the formation of Ga hexa-coordinated >FeOGa (OH)(2)(0) surface complexes (Ga-O distances of 1.96-1.98 angstrom) both at acid and alkaline pH, a similar extent of isotope fractionation was found at acid and alkaline pH, suggesting that Ga(OH)(4)(-) is preferentially adsorbed on goethite for all investigated pH conditions. In addition, the observed decrease of Ga isotope fractionation magnitude observed with increasing Ga surface coverage for both calcite and goethite is likely related to the formation of Ga surface polymers and/or hydroxides with reduced Ga-O distances. This first study of Ga isotope fractionation during solid-fluid interactions suggests that the adsorption of Ga by oxides, carbonates or clay minerals could yield significant Ga isotope fractionation between secondary minerals and surficial fluids including seawater. Ga isotopes thus should help to better characterize the surficial biogeochemical cycles of gallium and its geochemical analog aluminum. (C) 2017 Elsevier Ltd. All rights reserved.

Mining and metallurgical activities may release into the environment an important amount of Cr. However, only hexavalent chromium (Cr(VI)) is known to be highly soluble in water, bioavailable, toxic. In addition, chromium redox changes are known to induce significant change in chromium isotopic signatures. In the present study, the two mining areas of Barro Alto, an active Ni mine, and Cromińıa, a former Cr mine (BA and CA, resp. Goiás State, Brazil) were studied, in order to determine the potential release of Cr(VI) from mining activities and understand the associated processes, through the study of mineralogy, chromium mobility, speciation and isotopic composition. Iron oxy(hydr)oxydes, spinels and serpentines were identified by X-Ray Diffraction as the main mineral phases in ores, whereas in soils the contents of quartz and chlorite were the higher. Chemically and isotopically exchangeable pool of Cr (ECr) were determined in ores, sediments, suspended matters and soils, showing up to 97 mg/kg of exchangeable Cr(VI) in BA ores, while this pool is lower than 19 mg/kg in BA soils. According to ionic chromatography results (LC-HR-ICPMS), exchangeable Cr is mainly under hexavalent form. Both X-Ray Fluorescence and acid digestion analyses reveal higher total Cr in soils of BA (3,15at%) than in CA (0,68at%). However, the amount of extracted Cr(VI) was similar in both sites, with values up to 22 mg/kg in the deeper horizons of soils profiles. Additionally, ECr values determined in both SPM and sediments from BA retention ponds could reach up to 13 and 6 mg/kg, respectively. This suggests that ores and sediments stored into the mining area are the main sources of Cr(VI), potentially mobilized into surface waters and deeper soil profiles. ∗Speaker †Corresponding author: sivry@ipgp.fr sciencesconf.org:segh-brussels:98252

Though an isotope approach could be beneficial for better understanding the biogeochemical cycle of gallium (Ga), an analogue of the monoisotopic element aluminum (Al), the geochemistry of Ga isotopes has not been widely elaborated. We developed a two-step method for purifying Ga from geological (biological) samples for precise measurement of Ga isotope ratio using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Ga was thoroughly separated from other matrix elements using two chromatographic columns loaded with AG 1-X4 and Ln-spec resin, respectively. The separation method was carefully calibrated using both synthetic and natural samples and validated by assessing the extraction yield (99.8 ± 0.8%, 2SD, n = 23) and the reproducibility (2SD uncertainty better than 0.05‰, n = 116) of the measured isotopic ratio (expressed as δ71Ga). The validation of the whole protocol, together with instrumental analysis, was confirmed by the investigation of the matrix effect, the result of a standard addition experiment, and the comparison of Ga isotope measurement on two mass spectrometers—Nu Plasma II and Neptune Plus. Although the measurements using the sample-standard bracketing (SSB) correction method on both instruments resulted in identical δ71Ga values for reference materials, the modified empirical external normalization (MEEN) method gave relatively better precision compared to SSB on Neptune. Our preliminary results showed large variation of δ71Ga (up to 1.83‰) for 10 standards, with higher values in industrially produced materials, implying potential application of Ga isotopes.

Tafassasset is a primitive meteorite, the origin of which is still debated. Its possible relationship to either the CR chondrites – considered among the most primitive meteorites – or the brachinites – complex primitive achondrites – makes it an interesting sample for studying the initial stages of planetary accretion and differentiation in the early solar system. Here, we report tungsten (W) isotope data for bulk rock samples as well as for mineral fractions from Tafassasset, along with micro-computed tomography of a piece of the meteorite. Silicates show mass-independent W isotope anomalies, while the metal phase does not. These nucleosynthetic anomalies are interpreted as reflecting the presence of SiC presolar grains in the matrix of the meteorite, carrying s-process 184W. After correction of the nucleosynthetic anomalies, a correlation is observed between the 182W/184W isotope compositions and the Hf/W ratios of the different fractions. A 182Hf–182W age of ca. 2.9 Ma after CAIs is inferred from the 182Hf–182W chronometer, slightly older than other estimates based on the 53Mn–53Cr, 26Al–26Mg, and Pb/Pb chronometers, but consistent with the difference in closure temperatures of the different isotopic systems. Numerical modeling of the thermal evolution of Tafassasset indicates accretion of a parent-body less than ∼50 km in diameter, ≤1 Ma after the formation of CAIs, at a time when short-lived radio-nuclides induced metal–silicate separation and partial melting of the silicates with extraction of a basaltic component. According to our new data, Tafassasset may represent an inner part of a CR-like parent body, with a differentiation history similar to, but less severe than, that of brachinites.

Cr isotopic compositions have been measured on carbonaceous chondrites (CC): Tafassasset, Paris, Niger I, NWA 5958, NWA 8157 and Jbilet Winselwan. In bulk samples, the Cr-54/Cr-52 ratios (expressed as epsilon Cr-54) range from 0.93 to 1.58 epsilon units. These values are in agreement with values characteristic for distinct petrologic types. Despite this 54 Cr heterogeneity, the variability in the Cr-54/Cr-52 ratios (expressed as epsilon Cr-53) of 0.2 epsilon units and the Mn/Cr ratios is consistent with the previous finding of an isochron in the Mn-Cr evolution diagram.The Mn/Cr ratio in CC corresponds to variable abundances of high-T condensate formed and separated at the beginning of the solar system, thus the canonical Mn-53/Mn-55 ratio can be defined. Based on a consistent chronology for U-Pb and Mn-Cr between the earliest objects formed in the solar nebula and the D'Orbigny angrite we define a canonical Mn-53/Mn-55 ratio and epsilon Cr-53(i) of 6.8 x 10(-6) and -0.177, respectively.The internal Mn/Cr systematics in Tafassasset and Paris were studied by two approaches: leaching technique and mineral separation. Despite variable epsilon Cr-54 values (up to >30 epsilon) linear co-variations were found between epsilon Cr-53 and Mn/Cr ratio. The mineral separates as well as the leachates of Tafassasset fall on a common isochron indicating that (1) cooling of the Tafassasset's parent body occurred at 4563.5 +/- 0.25 Ma, and that (2) Cr-54 is decoupled from the other isotopes even though temperatures >900 degrees C have been reached during metamorphism. In the case of Paris, the leachates form an alignment with a Mn-53/Mn-55 ratio higher than the canonical value. This alignment is not an isochron but rather a mixing line. Based on leachates from various CM and CI, we propose the occurrence of three distinct Cr reservoirs in meteoritic material: PURE54, HIGH53 and LOW53 characterized by a epsilon Cr-53 and epsilon Cr-54 of 0 and 25,000, -2.17 and 8, and 0.5 and -151, respectively. PURE54 has already been described and is carried by highly refractory nano-spinel; HIGH53 is Mn-rich and most probably carried by sulfides in the matrix, whereas LOW53 is characterized by low Mn/Cr ratios and it is sensitive to metamorphism. This component could correspond to mineral phases such as refractory oxides and carbide. Variable mixing proportions of HIGH53 and LOW53 would explain the larger-than-expected uncertainty (MSWD of 5.5) on the CC bulk regression line. A Monte Carlo simulation allows us to evaluate the impact of the dispersion of the initial Cr isotopic ratios (as a function of variable HIGH53). The co-variation of the Mn/Cr ratio and the epsilon Cr-53 defined by the mineral separates from Paris corresponds to an age of 4566.44(+0.66)/(-0.75) Ma, while their epsilon Cr-54 still differ by at least 0.42 epsilon. This age is likely to date the segregation of forsteritic olivines (most probably from type I chondrules) from fayalitic olivines (from type II chondrules) and, given the sampling procedure by handpicking of hundreds of grains, corresponds to the average age of chondrule formation. (C) 2015 Elsevier Ltd. All rights reserved.

This work provides a simple method for isotope ratio drift correction in MC-ICPMS transient signals.

The possibility of establishing an accurate relative chronology of the early solar system events based on the decay of short-lived Al-26 to Mg-26 (half-life of 0.72 Myr) depends on the level of homogeneity (or heterogeneity) of Al-26 and Mg isotopes. However, this level is difficult. to constrain precisely because of the very high precision needed for the determination of isotopic ratios, typically of +/- 5 ppm. In this study, we report for the first time a detailed analytical protocol developed for high precision in situ Mg isotopic measurements ((25)mg/(24)mg and (26)mg/Mg-24 ratios, as well as Mg-26 excess) by MC-SIMS. As the data reduction process is critical for both accuracy and precision of the final isotopic results, factors such as the Faraday cup (FC) background drift and matrix effects on instrumental fractionation have been investigated. Indeed these instrumental effects impacting the measured Mg-isotope ratios can be as large or larger than the variations we are looking for to constrain the initial distribution of Al-26 and Mg isotopes in the early solar system. Our results show that they definitely are limiting factors regarding the precision of Mg isotopic compositions, and that an under- or over-correction of both FC background instabilities and instrumental isotopic fractionation leads to important bias on delta Mg-25, delta(26)mg and Delta Mg-26 values (for example, olivines not corrected for FC background drifts display Delta Mg-26 values that can differ by as much as 10 ppm from the truly corrected value). The new data reduction process described here can then be applied to meteoritic samples (components of chondritic meteorites for instance) to accurately establish their relative chronology of formation.

Mn/Cr isotope systematics in meteorites presents an interesting tool because this isotope system holds double information: the CrMn couple allows us to obtain chronological information while the Cr isotope systematics yields information on the mixing of nucleosynthetically distinct components. Although the variations of the Cr abundances in meteorites are not yet fully understood, they are nevertheless so systematic that they can be used as a classification tool, very similar to the oxygen isotope systematics. We present Mn/Cr and oxygen isotope data obtained on recently discovered and/or unclassified carbonaceous meteorites. The oxygen isotopic compositions were determined by laser fluorination on small fragments; the Cr isotopic compositions were measured on sequential dissolution steps of bulk rock powders of all meteorites and on separated mineral fractions of two of them. The mineral separates of Paris, forsterite, fayalite, fine-grained material around chondrules (FGR), fall on a line with a slope of Mn/Mn = 6.183 x10 and Cri = -0.165. This slope can be translated into an age based on the LEW Cliff 86010 anchor [1] and corresponds to 4566.33 ± 0.63 x 10 y. This old Mn/Cr age shows that the first chondrules formed rapidly (1-2 x10 y) after CAIs. The mineral isochron obtained on Tafassasset indicates a younger equilibration age: 4563.31 ± 0.42 x10 y. The Cr abundances measured in the bulk rocks of all samples studied as well as the isotopic pattern of the sequential leaching steps allow us to classify Tafassasset as CR, Niger I as CI, Paris as CM and NWA TBC as CV chondrites. Although the existence of a correlation between Cr and oxygen in carbonaceous chondrites has recently been questioned [2] O data for Niger I ( O = -0.361 ±1.189), NWA TBC (4.052 ± 0.550) as well as the Cr abundances of Tafassasset (Cr= 1.42± 0.076), Paris (Cr= 0.925 ±0.098), Niger I (Cr= 1.64±0.11) and NWA TBC Cr= 1.08±0.19) seem to match such a correlation.

Nd isotopes are useful tracers for paleoceanography due to the short Nd residence time in seawater and the large differences between the isotopic signatures of various geological reservoirs. Therefore, epsilon(Nd) variations reflect the geological history of individual oceanic basins.Using a differential dissolution technique, which extracts Nd isotopes of seawater trapped in MnO2 coatings and carbonates in marine sediment, we measured almost two hundred samples from ODP Sites 758 and 757 in the Northern Bay of Bengal covering the last 4 Ma. For the first time, we have shown a covariation between epsilon(Nd) and delta O-18 over at least the last 800 ka. We also show that from 4 Ma to 2.6 Ma, epsilon(Nd) is almost constant and starts to fluctuate at 2.6 Ma when northern glaciations increased. From 2.6 Ma to 1 Ma the fluctuation period is close to 40 ka while from 1 Ma to present it is dominantly 100 ka.We attribute these findings to mixing between Himalayan river water (that ultimately originates as Indian summer monsoon rain) and normal Bay of Bengal seawater. Previous studies on seawater, using epsilon(Nd), delta O-18 analyzed on planktonic foraminifera and sedimentary data, can be integrated into this model.A simple quantitative binary mixing model suggests that the summer monsoon rain was more intense during interglacial than glacial periods. During last glacial episode, the monsoon trajectory was deviated to the east. At a large scale, the Indian monsoon is fully controlled by the variations in Northern Hemisphere climate but with a complex response function to this forcing.Our study clearly establishes the large potential of Nd isotope data to evaluate the hydrological river regime during the Quaternary and its relationship with climate fluctuations, particularly when the sediment archive is sampled close to sediment sources. (c) 2010 Elsevier Ltd. All rights reserved.

There is evidence for the existence of differentiated crust early in Earth’s history, but little is known about the timing and nature of the crust and its formation. New samarium–neodymium data from the Dresser Formation in Western Australia point to differentiation of the early crust from the mantle more than 4.3 billion years ago.

The Tafassasset meteorite which was found the Tenere dessert, Niger, in 2000 and 2001 consists of 26 pieces with a total weight of ~110 kg. The well-preserved meteorite has been studied by several groups [1-4], but until today no consensus on the meteorite's classification was obtained. Reasons for this difficulty are:-texture and mineral abundances vary considerably,-different individual stones were studied by different groups and although a few stones were paired some data, i.e. oxygen isotope compositions are contradictory [1, 4],-petrographic observations on the same samples are debated. Tafassasset shows an equilibrated and recrystallized texture. It contains ~ 8 vol % metal, olivine represents the dominant mineral phase; other mineral phases are low-Ca-, Ca-pyroxene and feldspar. The meteorite experienced thermal metamorphism that led to the formation of secondary minerals, i.e. phosphates and chromite. Tafassasset has been classified as:-a equilibrated CR chondrite,-an ungrouped primitive achondrite and-having strong affinities with brachinites. The presence of relict chondrules and oxygen isotopic compositions close to CR chondrites have been arguments to classify it as a CR chondrite [1], while refractory element ratios depleted in respect to CI chondrites, Al/Mg and Mn/Mg more like primitive achondrites led [2] to suggest the classification as primitive achondrite. Mineralogical observations led [3] to put forward the similarities to brachinites. In order to obtain chronological information and to gain information on the origin and thermal history of this unusual meteorite we started an U/Pb and Cr isotopic investigation of Tafassasset. Several fragments representing different lithologies (fine grained metal-poor feldspar-rich fragments, metal-rich pyroxene-rich fragments) were analyzed. All fragments studied so far, show very low Pb, U and Th concentrations. They are characterized by radiogenic Pb isotopic compositions with 206 Pb/ 204 Pb ratios ranging from 43 up to 215. The corresponding Pb/Pb model ages are old, they lie between 4.555 and 4.564 Ga. The Pb/Pb model age of the fragment that is characterized by the most radiogenic Pb isotopic composition corresponds to 4.563 ± 0.001 Ga. A bulk rock Cr isotopic analysis displays excesses of 0.29 ± 0.08 ε and 1.37 ± 0.27 ε for 53 Cr and 54 Cr respectively. These values agree within error with the Cr isotope values published for Renazzo [5]. Tafassasset thus is the first occurrence of a non-carbonaceous meteorite showing a positive 54 Cr isotopic anomaly.

We report here a two-step Zn chemical separation protocol, which allows dilute aqueous samples with low Zn concentrations to be directly loaded onto the chromatography columns, without any preliminary evaporation. This method is particularly suited for river and rain water samples with Zn concentrations between 0.1 and 120 mu g/l. We present the different steps of the protocol on Chelex-100 and AG 1-X4 resins (Bio-Rad) with varying eluants under different matrix conditions. The method results in acceptable procedural blanks and quantitative yields of Zn. The mass bias during isotope measurements by the Neptune Thermo (R) MC-ICP-MS was corrected using empirical external normalization (EEN) with Cu as the internal dopant. We additionally evaluate the sensitivity of the measurement method to instrumental and analytical conditions as tuning, solution pH, Zn concentration, Zn/Cu ratio, temperature, and trace matrix addition. Both the stable introduction system (SIS, double Scott cyclonic spray chamber) and the Apex HF (ESI) were tested for Zn isotope measurement and the Apex was preferred for its higher sensitivity and better stability. External delta Zn-66 reproducibility for measurements of the standard solution (calibrated over two years) and natural water samples was 0.04%. (2 sigma). (C) 2008 Elsevier B.V. All rights reserved.

Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite Res, (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the Kurile-Kamchatka volcanic Island are setting. The sources of this unique Re enrichment have been inferred from studies of Re-Os isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133-204), Rb/Y (16-406) and Th/Zr (0.01-0.25) ratios. They are characterised by high Re (7-210 ppb) and Os abundances (0.4-0.9 ppb), with Os-187/Os-188 ratios in a range 0.122-0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid. Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2) powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5-1.7 wt%, 10 ppm, and 65-252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3-24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial Os-187/Os-188 ratio of 0.32 +/- 0.15 and an age of 79 +/- 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic Os-187 values, even in the limited period of time, with Os-187/Os-188 ratios up to 3.3 for molybdenite and up to 4.4 for rheniite. The Os isotopic compositions of andesite-basaltic rocks from the Kudryavy volcano (Os-187/Os-188 Up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (Os-187/Os-188 = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between Os-187/Os-188 ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Tit relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at similar to 46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate. (c) 2007 Elsevier Ltd. All rights reserved.

The Re-Os distribution and isotopic composition have been studied within different ore facies, host-rocks and sediments from the Alexandrinka volcanogenic hydrothermal massive sulphide deposit, Southern Urals, Russia. The osmium contents increase and the initial (OS)-O-187/(OS)-O-188 isotopic compositions decrease in the ore facial range: sulphide chimneys-stockwork zone-seafloor massive and elastic sulphides-metal I iferous sediments. This range reflects variable degrees of reduced hydrothermal fluid-oxidized seawater mixing during the hydrothermal ore-forming process. The Os isotopic composition of the hydrothermal sulphide chimney (Os-187/(188)s= 1.3) is estimated to be a minimum value of the Devonian hydrothermal fluid, which is an intermediate between initial Os isotopic compositions of island-arc volcanics and interlayered sediments. The initial Os isotopic composition of metalliferous sediments ((OS)-O-187/(OS)-O-188 similar to 0.17-0.2) possibly reflects that of the Devonian seawater. The low rhenium concentrations in metalliferous sediments (about 1-2 ppb) could indicate oxic formation conditions.The Re-Os isotope data define a best-fit line corresponding to a Late Devonian age of 355 +/- 15 Ma (2 sigma) with initial (OS)-O-187/(OS)-O-188 of 0.12 +/- 0.19. This age could indicate a late Os isotope reequilibration due to ongoing hydrothermal fluid flow from the Givetian (stratigraphic age similar to 375 Ma) until the closure of the Ural paleoocean in the Late Devonian, The Os contents are higher and Re/Os ratio is lower within Palaeozoic island-arc hosted Urals VHMS deposits compared with TAG deposit in MOR setting. (c) 2007 Elsevier B.V All rights reserved.