δ53Cr Values Research Articles

Processes controlling the chromium isotopic composition of river water: Constraints from basaltic river catchments

We report chromium (Cr) isotope compositions and concentrations (and additional geochemical and physicochemical data) of bedrock, soils and river waters from two geographically distinct basaltic river catchments, the Uruguay River catchment (Uruguay) and the Glenariff River catchment (Northern Ireland, United Kingdom), to investigate the processes that control Cr mobilisation and fractionation during weathering and riverine transport to the sea. Our results show that the Cr isotope compositions of soils are a function of the modal abundance and weathering rates of Cr-bearing minerals. The accumulation of weathering resistant Cr-spinels in the soils of Northern Ireland results in soils which are enriched in Cr and have δ53Cr values within the range of local bedrock (δ53Cr value of −0.21±0.12‰, 2σ, n=4). By contrast, the more easily weathered Cr-silicates in the bedrock of Uruguay results in greater Cr loss from the soil and a depletion in the heavy isotopes of Cr (with average δ53Cr value of −0.32±0.04‰, 2σ, n=4) relative to the local bedrock (δ53Cr value of −0.22±0.08‰, 2σ, n=4). The river waters in both catchments are predominantly enriched in the heavy 53Cr isotope relative to bedrock, although the range and average river water δ53Cr values differ significantly between each. The Uruguay rivers exhibit a restricted range in δ53Cr values, with a mean of +0.08±0.06‰ (2σ, n=5) that represents a positive fractionation of +0.2‰ relative to bedrock, and is best explained by the unidirectional formation of Cr(VI) during weathering that has not been significantly modified by back-reduction to Cr(III). By contrast, the Glenariff stream and river waters (Northern Ireland) exhibit a wide range in δ53Cr values from −0.17±0.3‰ (2σ, n=4) to +1.68±0.3‰ (n=1) that appears to reflect the variable redox conditions of the catchment. In general, the values with the lowest 53Cr enrichment have higher Cr concentrations, the lowest pH, highest dissolved organic carbon (DOC), and exhibit minimal Ce anomalies that is consistent with a greater proportion of Cr(III) released from more reducing (bog influenced) stream waters. The remaining waters have lower Cr concentrations, lower DOC, and more pronounced Ce anomalies, indicating that a greater proportion of Cr(VI) underwent partial back-reduction to Cr(III) either during weathering, or during riverine transport. Overall, the DOC appears to exert a control on the dissolved Cr speciation and magnitude of isotope fractionation in these river waters. The broader implications of these observations are that the prevailing redox conditions in a river catchment strongly influence the δ53Cr value of the river water flux emptying into seawater. Observations for this study need to be considered when developing a framework for interpreting palaeo-climates from the δ53Cr values of ancient authigenic marine sediments.

The chromium isotope composition of reducing and oxic marine sediments

The chromium (Cr) isotope composition of marine sediments has the potential to provide new insights into the evolution of Earth-surface redox conditions. There are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport, the major source of seawater Cr, and with partial Cr reduction during burial in marine sediments, the major sink for seawater Cr. A more comprehensive understanding of these processes is needed to establish global Cr isotope mass balance and to gauge the utility of Cr isotopes as a paleoredox proxy. For these purposes, we investigated the Cr isotope composition of reducing sediments from the upwelling zone of the Peru Margin and the deep Cariaco Basin. Chromium is present in marine sediments in both detrital and authigenic phases, and to estimate the isotopic composition of the authigenic fraction, we measured δ53Cr on a weakly acid-leached fraction in addition to the bulk sediment. In an effort to examine potential variability in the Cr isotope composition of the detrital fraction, we also measured δ53Cr on a variety of oxic marine sediments that contain minimal authigenic Cr. The average δ53Cr value of the oxic sediments examined here is −0.05±0.10‰ (2σ, n=25), which is within the range of δ53Cr values characteristic of the bulk silicate Earth. This implies that uncertainty in estimates of authigenic δ53Cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of Cr in detrital versus authigenic phases, rather than to the Cr-isotopic composition of the detrital pool. Leaches of Cariaco Basin sediments have an average δ53Cr value of +0.38±0.10‰ (2σ, n=7), which shows no dependency on sample location within the basin and is close to that of Atlantic deepwater Cr (∼+0.5‰). This suggests that authigenic Cr in anoxic sediments may reliably reflect the first-order Cr isotope composition of deepwaters. For Peru Margin samples, the average δ53Cr values of bulk sediments (+0.59±0.06‰; 2σ) and leach fractions (+0.61±0.06‰; 2σ) are also comparable with those of Cariaco Basin samples and modern deepwater values. Finally, we found that the δ53Cr of Peru Margin samples correlates with δ15N on glacial–interglacial timescales, which we attribute to secular variation in basinal or global-ocean redox conditions. Thus, the δ53Cr stratigraphic record of Peru Margin sediments indicates that Cr isotopes may be suited to tracking geologically short-term changes in ocean oxygenation.

Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration

Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ53Cr values falling within a narrow range of −0.12±0.13‰ (2SD, n=30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ53Cr values of −0.13±0.13‰, −0.11±0.13‰, and −0.07±0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ53Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ53Cr values up to ~0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ53Cr (−0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust.

Potential leaching of Cr(VI) from laterite mines and residues of metallurgical products (red mud and slag): An integrated approach

We here report the chemical composition of (a) Fe–Ni laterite ores from central and northern Greece and bauxite ores from Parnassos-Ghiona deposit, (b) the corresponding high volume residues (slag and red mud) produced during the metallurgical processing of ferronickel and alumina, (c) experimentally produced water (natural and/or seawater) leachates of these ores and metallurgical leachates and (d) chromium isotope composition (expressed as δ53Cr values) of the leachates, seawater and of Cr (VI) contaminated natural water, in an attempt to assess the role of industrial mining processes on the Cr(VI) contamination of groundwater in ultramafic rock-dominated terrain.The five week lasting laboratory leaching experiments, performed under atmospheric conditions, showed that only small amounts of Cr(VI) (<~2μg·L−1) from Fe–Ni laterites of Lokris (Central Greece) were mobilized into the aqueous phase. In contrast, Cr(VI) concentrations in leachates from Fe–Ni laterites of Kastoria reached values as high as 1300μg·L−1. In addition, unexpectedly high Cr(VI) concentrations were measured in water leachates from red mud samples (2100μg·L−1). The leachates from red mud are also characterized by relatively high V (6200μg·L−1) and Ga (90μg·L−1) concentrations. In contrast to leachates from red mud, the Cr(VI) concentrations in leachates from slag remained below ~2μg·L−1.The δ53Cr values measured in natural water leachates of Fe–Ni-laterite from Kastoria range between +0.03±0.06‰ and −0.21±0.08‰ and differ from the more positively fractionated value of +1.01±0.05 ‰ obtained for the laterite from Lokris. The δ53Cr values in natural water and seawater leachates from red mud returned δ53Cr values of +0.83±0.05‰ and +0.73±0.08‰, respectively, and thus are statistically indistinguishable. Although there is no clear-cut relationship between Cr(VI) concentrations and δ53Cr values in the diverse leachates, there is an indication that the laterite leachates from Kastoria with high Cr(VI) concentrations yielded less positively fractionated δ53Cr values, compared to the laterite leachates from Lokris.The agreement between chromium isotope compositions of artificial leachates of red mud and that of contaminated seawater from the Assopos river extrusion (estuary) forming a heavily Cr(VI) contaminated point source, suggests that the release of Cr(VI) from the red mud rather than from the primary bauxite ore is a major source of toxic Cr(VI), and that insignificant back-fractionation (reduction) of this mobile and toxic chromium along its pathway to the sea takes place.

Stable chromium isotopic composition of meteorites and metal–silicate experiments: Implications for fractionation during core formation

We present new mass independent and mass dependent Cr isotope compositions for meteorites measured by double spike thermal ionisation mass spectrometry. Small differences in both mass independent 53Cr and 54Cr relative to the Bulk Silicate Earth are reported and are very similar to previously published values. Carbonaceous chondrites are characterised by an excess in 54Cr compared to ordinary and enstatite chondrites which make mass independent Cr isotopes a useful tool for distinguishing between meteoritic groups. Mass dependent stable Cr isotope compositions for the same samples are also reported. Carbonaceous and ordinary chondrites are identical within uncertainty with average δ53Cr values of −0.118±0.040‰ and −0.143±0.074‰ respectively. The heaviest isotope compositions are recorded by an enstatite chondrite and a CO carbonaceous chondrite, both of which have relatively reduced chemical compositions implying some stable Cr isotope fractionation related to redox processes in the circumstellar disk. The average δ53Cr values for chondrites are within error of the estimate for the Bulk Silicate Earth (BSE) also determined by double spiking. The lack of isotopic difference between chondritic material and the BSE provides evidence that Cr isotopes were not fractionated during core formation on Earth. A series of high-pressure experiments was also carried out to investigate stable Cr isotope fractionation between metal and silicate and no demonstrable fractionation was observed, consistent with our meteorites data. Mass dependent Cr isotope data for achondrites suggest that Cr isotopes are fractionated during magmatic differentiation and therefore further work is required to constrain the Cr isotopic compositions of the mantles of Vesta and Mars.

Global variability of chromium isotopes in seawater demonstrated by Pacific, Atlantic, and Arctic Ocean samples

Seawater chromium (Cr) isotope and concentration data are presented from multiple sites in the Arctic Ocean, and three locations in the Atlantic and Pacific Oceans. A 2400-m profile illustrates the heterogeneity of δ53Cr in the Arctic Ocean with depth and water-mass source (Pacific vs. Atlantic). The highest δ53Cr values occur in Pacific-sourced waters, which also have the lowest Cr concentration. Chromium concentration and δ53Cr data from these locations, in conjunction with published data for the South Atlantic Ocean, yield a simple logarithmic function, as predicted by Rayleigh fractionation in a closed system. The observed Cr isotope signature is hypothesized to arise from fractionation during the reduction of Cr(VI) in surface waters and oxygen minimum zones, scavenging of isotopically light Cr(III) to deeper water and sediment, and subsequent release of this seawater-derived Cr(III) back into seawater, either as organic complexes with Cr(III) or after oxidation to Cr(VI). The isotopic fractionation factor (ε) associated with Cr cycling in seawater is estimated to be −0.80±0.03‰ (2σ). Samples from the sea-ice affected Surface Mixed Layer of the Arctic Ocean (∼10 m depth) deviate from the general trend, and samples proximal to rivers illustrate geographic variation in δ53Cr values for continental runoff, but prompt loss of this signature away from the source.

Oxidative release of chromium from Archean ultramafic rocks, its transport and environmental impact – A Cr isotope perspective on the Sukinda valley ore district (Orissa, India)

This study investigates Cr isotope fractionation during soil formation from Archean (3.1–3.3Ga) ultramafic rocks in a chromite mining area in the southern Singhbhum Craton (Orissa, India). The Cr-isotope signatures of two studied weathering profiles, range from non-fractionated mantle values to negatively fractionated values as low as δ53Cr=−1.29±0.04‰. Local surface waters are isotopically heavy relative to the soils. This supports the hypothesis that during oxidative weathering isotopically heavy Cr(VI) is leached from the soils to runoff.The impact of mining pollution is observed downstream from the mine where surface water Cr concentrations are significantly increased, accompanied by a shift to less positive δ53Cr values relative to upstream unpolluted surface water. A microbial mat sample indicates that microbes have the potential to reduce and immobilize Cr(VI), which could be a factor in controlling the hazardous impact of Cr(VI) on health and environment.The positive Cr isotope signatures of the Brahmani estuary and coastal seawater collected from the Bay of Bengal further indicate that the positively fractionated Cr isotope signal from the catchment area is preserved during its transport to the sea. Isotopically lighter Cr(VI) downstream from the mine is probably back-reduced to Cr(III) during riverine transport leading to similar Cr-isotope values in the estuary as observed upstream from the mine.

Algoma-type Neoproterozoic BIFs and related marbles in the Seridó Belt (NE Brazil): REE, C, O, Cr and Sr isotope evidence

The Jucurutu Formation in the Seridó Belt, northeastern Brazil, encompasses fine-to coarse-grained amphibolite-facies marbles, locally with cross-bedding and stromatolites. Banded iron formations (BIF) at three localities in this belt comprise itabirites (actinolite- or cummingtonite-itabirite and quartz-hematite itabirite) and iron ores, which are overlain by marbles of the Jucurutu Formation. Diamictites of uncertain stratigraphic position in the Seridó Belt exhibit gneiss and quartzite clasts up to 0.6 m long and a fine-grained metapelitic matrix.The C-isotope stratigraphic pathways for the Jucurutu Formation show negative δ13C values at the base of the formation followed upsection by positive values. At the Ferro do Bonito iron Mine, values as low as −12‰ in carbonates just above the contact with underlying BIF are followed by values of ca. −5‰ and by positive values up section (+4 to +10‰). δ13C values for carbonates of the Jucurutu Formation deposited on top of BIFs at the Riacho Fundo, São João do Sabugi, and Serra da Formiga sections are all positive, the contact surfaces between carbonates and BIFs being covered by weathered material. The 87Sr/86Sr values for carbonate rocks of the Jucurutu Formation are mostly in the 0.7074–0.7075 interval, a ratio commonly observed in the late Cryogenian to the early Ediacaran.The studied BIFs exhibit Cr concentrations one magnitude higher than usual and largely unfractionated Cr isotope composition, similar to average magmatic values. They are characterized by δ53Cr values between −0.42 and −0.12‰, by Ce anomalies (Ce/Ce*) ranging from −0.4 to 0.7, by positive Eu anomalies, and by super-chondritic Y/Ho ratios (>20). The combination of geochemical parameters and association with mafic rocks resemble Algoma-type BIFs. These BIFs possibly formed proximally to hydrothermal vents, in anoxic and acidic deep waters, whereby Fe and Cr were leached from mafic or ultramafic rocks. High concentrations of Cr in the BIF with a magmatic inventory isotope composition, together with pronounced positive Eu anomalies, fit a rift depositional scenario, characterized by narrow basins and strong hydrothermal input, in which anoxic (ferruginous) conditions prevailed. This is substantiated by the occurrence of positive Ce anomalies. Their deposition was followed by carbonates which show “normal” Cr concentrations with slightly positively fractionated δ53Cr values. These carbonates may represent post-glacial cap carbonate sequences deposited in an anoxic to slightly oxic shallow marine environment, as suggested by Ce/Ce* versus Pr/Pr* and Ce/Ce* versus Nd cross-relationships.

Dual Mechanism Conceptual Model for Cr Isotope Fractionation during Reduction by Zerovalent Iron under Saturated Flow Conditions.

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr(VI) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr(VI) to Cr(III) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr(VI) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ(53)Cr values were associated with decreasing Cr(VI) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr(III) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (ε=-0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε=-0.9‰ for the batch experiment, and ε=-1.5‰ for the column experiment).

Chromium isotopic fractionation during Cr(VI) reduction by Bacillus sp. under aerobic conditions

This study investigated the fractionation of chromium isotopes during chromium reduction by Bacillus sp. under aerobic condition, variable carbon source (glucose) concentration (0, 0.1, 1, 2.5 and 10mM), and incubation temperatures (4, 15, 25 and 37°C). The results revealed that the δ53Cr values in the residual Cr(VI) increased with the degree of Cr reduction, and followed a Rayleigh fractionation model. The addition of glucose only slightly affected cell-specific Cr(VI) reduction rates (cSRR). However, the value of ε (2.00±0.21‰) in the experiments with different concentrations of glucose (0.1, 1, 2.5 and 10mM) was smaller than that from the experiment without glucose (3.74±0.16‰). The results indicated that the cell-specific reduction rate is not the sole control on the degree of isotopic fractionation, and different metabolic pathways would result in differing degrees of Cr isotopic fractionation. The cSRR decreased with decreasing temperature, showing that the values of ε were 7.62±0.36‰, 4.59±0.28‰, 3.09±0.16‰ and 1.99±0.23‰ at temperatures of 4, 15, 25 and 37°C, respectively. It shown that increasing cSRR linked to decreasing fractionations has been associated with increasing temperatures. Overall, our results revealed that temperature is a primary factor affecting Cr isotopic fractionation under microbial actions.

The isotopic composition of authigenic chromium in anoxic marine sediments: A case study from the Cariaco Basin

Chromium (Cr) isotopes are an emerging proxy for tracking redox processes at the Earth's surface. However, there has been limited exploration of the Cr isotope record of modern and recent marine sediments. The basic inorganic chemistry of Cr suggests that anoxic marine basins should factor prominently in the global Cr cycle and that sediments deposited within anoxic basins may offer a valuable Cr isotope archive throughout Earth's history. Here, we present δ53Cr data from sediments of the Cariaco Basin, Venezuela—a ‘type’ environment for large, perennially anoxic basins with a relatively strong hydrological connection to the global oceans. We document a marked positive shift in bulk δ53Cr values following the termination of the Last Glacial Maximum, followed by relative stasis. Based on a suite of independent redox proxies, this transition marks a switch from oxic to persistently anoxic and sulfidic (euxinic) depositional conditions within the basin. We find good agreement between two independent approaches toward estimating the δ53Cr composition of authigenic Cr in euxinic Cariaco Basin sediments and that these estimates are very similar to the δ53Cr composition of modern open Atlantic Ocean seawater. These data, together with considerations of reaction kinetics and mass balance within the Cariaco Basin, are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first-order trends in seawater δ53Cr composition. Additionally, the Cariaco Basin data suggest that there has been secular stability in the average δ53Cr value of Atlantic seawater over the last ∼15 kyr.

Examination of Cr(VI) treatment by zero-valent iron using in situ, real-time X-ray absorption spectroscopy and Cr isotope measurements

A series of replicate flow-through cell experiments was conducted to characterize Cr isotope fractionation during Cr(VI) treatment by granular zero-valent iron (ZVI). Synthetic groundwater containing 50mgL−1 Cr(VI) was pumped upward through a custom-made cell packed with ZVI under anaerobic conditions. The geochemical evolution of the system was monitored using pH and redox measurements, while aqueous effluent samples were retained for analysis of cations and Cr isotopes. Real-time, in situ X-ray absorption near edge structure (XANES) spectroscopy collected via a Kapton® window in the cell provided additional information on the speciation of the reaction products. Increases in δ53Cr values corresponding to decreases in Cr(VI) concentration suggested the occurrence of redox processes. Spectroscopic results correlated well with the isotope data, indicating reduction of Cr(VI) to Cr(III). The isotope data did not appear to follow a single trend. A two-stage system was proposed to explain the complex isotope trend, where the rapid Cr removal was associated with very little fractionation (ε=−0.2‰), whereas slower removal was associated with a greater degree of fractionation (ε=−1.2‰ to −1.5‰). Reactive transport modeling was used to quantify distinct isotope fractionation values (ε), differentiated by a significant change in the Cr removal rate.

Application of chromium stable isotopes to the evaluation of Cr(VI) contamination in groundwater and rock leachates from central Euboea and the Assopos basin (Greece)

Major and trace elements (a) in groundwater, ultramafic rocks from natural outcrops and soil samples from cultivated sites of Central Euboea and Assopos basin, and (b) in experimentally produced laboratory water leachates of rocks and soils were investigated by SEM/EDS, XRD and ICP/MS. In addition, stable chromium isotopes (expressed as δ53Cr values) were measured in groundwater and leachates in order to identify potential sources for Cr-contamination.The higher Cr(VI) concentrations in soil leachates compared to those in the rock pulp leachates potentially can be explained by the presence of larger amounts of Fe (Fe(II)) and Mn (Mn-oxides acting as oxidizing catalysts). Assuming that redox processes produce significant Cr isotope fractionation (groundwater δ53Cr values range between 0.8 and 1.98‰), the compilation of the obtained analytical data suggests that the dominant cause of Cr isotope fractionation is post-mobilization reduction of Cr(VI). However, the lack of a very good negative relationship between Cr(VI) concentrations and δ53Cr values may reflect that sorption, precipitation and biological processes (fractionation during uptake by plants) complicate the interpretation of the Cr isotope signatures.The variation in δ53Cr values (0.84 to 1.98‰ in groundwater from Euboea, and from 0.98 to 1.03‰ in samples from the Assopos basin) imply initial oxidative mobilization of Cr(VI) from the ultramafic host rocks, followed by reductive processes that lead to immobilization of portions of Cr(III). Using a Rayleigh distillation model and different fractionation factors of Cr(VI) reduction valid for aqueous Fe(II) and Fe(II)-bearing minerals, we calculate that more than ~53%, but maximum ~94%, of the originally mobile Cr(VI) pool was reduced to immobile Cr(III) in the waters investigated. This indicates that efficient processes in the aquifers may facilitate natural attenuation of the toxic Cr(VI) to less harmful Cr(III).

Weathering on land and transport of chromium to the ocean in a subtropical region (Misiones, NW Argentina): A chromium stable isotope perspective

We have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of South America (Misiones Province, Argentina), in an attempt to link Cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and, ultimately, with Cr isotope effects observed during oxidative surface weathering in subtropical red soils. Cr concentrations and stable Cr isotopic compositions (expressed as δ53Cr ‰ values) in two typical and representative surface profiles of weathered basalt show significant depletion of Cr in the soils of up to 50%, together with pronounced negatively fractionated δ53Cr values which are indicative of oxidative mobilization of heavy Cr(VI) into the run-off. The behavior of Cr in the studied weathering profiles is not correlated with that of other redox sensitive elements, such as Ce and U; this is essentially due to the affinity of REE and U, but not Cr with secondary phosphates which form during weathering processes.Smaller tributaries in NW Argentina to the Paraná River (second largest river in South America) carry dissolved Cr in the order of 0.7–1.4ppb (13–27nM) with δ53Cr values of +0.2 to +0.4‰, balancing the negatively fractionated weathering products. The isotope composition and concentration of dissolved Cr in the ca. 1200km long Paraná River from Misiones to its estuary and discharge area into the South Atlantic Ocean remains relatively constant with an average Cr concentration around 2.4ppb (46nM) and an average δ53Cr value of +0.32‰. The Cr concentration in the estuary itself drops by ca. 50% but with only minor change in its Cr isotope composition. Results from the Paraná estuary are identical with recently analyzed surface seawater from the Argentine Basin with Cr contents of ~0.3ppb (~6nM) and δ53Cr values ~+0.4‰ (Bonnand et al., 2013), and indicate that there is only a minimal Cr isotopic variability during riverine transport, even during long transport distances as shown in our example of the Paraná River. Simple Cr input flux calculations reveal that the Paraná River accounts for ~5% of the total yearly Cr flux to the world's oceans today and that its isotopic signature seems to be, at least locally, imparted to the surface seawater of the Argentine Basin. Whether or not this Cr isotope signature is generally exhibited by the world's oceans needs further investigations, particularly the characterization of seawater around the globe.

The fate of chromium during tropical weathering: A laterite profile from Central Madagascar

We performed a mineral, geochemical and Cr–Sr–Pb isotope study on a laterite profile developed on ca. 540Ma old tonalitic bedrock in Madagascar with special emphasis on the behavior of chromium during tropical weathering. The observed strong depletions of Ca, Si, and P, and enrichment of Fe and Al, in the soil, relative to bedrock and underlying saprolite, are the characteristic features pertinent to laterites. The enrichment of Fe in topsoil horizon can be correlated with enrichment of P, and the redox sensitive elements Mn and Cr, and indicates redistribution of these elements related to oxidation–reduction processes. The slight scatter of 87Sr/86Sr, and 206,207,208Pb/204Pb values in the profile is one consequence of such redistribution processes. Our results are compatible with a two stage process during alteration: (1) an incipient alteration stage characterized mainly by a pervasive, effective and complete loss of sulfides accompanied by patchy alteration constrained along fissures and cracks, and (2) a main stage of soil formation (actual laterization) with loss of Na, K, Ca, and Si, and accumulation of Fe and Mn relative to the unaltered bedrock. This two stage evolution is also depicted by Cr concentrations and δ53Cr values along the studied profile, where the first incipient alteration stage caused a pronounced depletion of Cr particularly hosted by sulfides, and where the second alteration (laterization) led to redistribution and a small increase in Cr concentration in the uppermost portions of the soil profile relative to stage one altered saprolite. This gain in Cr is accompanied by decreasing δ53Cr values and can be explained by partial immobilization (possibly by adsorption/coprecipitation on/with Fe-oxy-hydroxides) of mobile Cr(III) during upward transport in the weathering profile. The negatively fractionated δ53Cr values measured in the weathering profile relative to the unaltered tonalitic bedrock characterized by a high temperature magmatic inventory Cr isotope signature are consistent with loss of a positively fractionated Cr(VI) pool formed during weathering. The predicted existence of a former, positively fractionated and mobile chromium pool has been experimentally constrained in circumneutral and basic leachates of powdered tonalite bedrock where δ53Cr of +0.21 to +0.48‰ was measured. Our results show that mobilization of chromium is effective under highly oxidative conditions, which in well drained sulfide-bearing parent bedrocks potentially lead to both, acid dissolution of sulfide-hosted Cr and redox-promoted mobilization of Cr(VI) from silicates during later stages of weathering under basic pH conditions.

Chromium isotope fractionation during oxidative weathering—Implications from the study of a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada

This study reports detailed stable Cr, Sm–Nd and Rb–Sr isotope data for a ca. 1.9Ga old subaerial weathering profile at Schreiber Beach, Ontario, Canada, from which detailed major and trace element signatures and δ18O values were previously reported. The weathering profile developed on Neoarchean (∼2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (∼1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation.The aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough (in a period following the Great Oxygenation Event (GOE) ∼2.45Ga ago and prior to the deposition of the Gunflint cherts at ∼1.82Ga). These iron-rich cherts belong to one of the last regionally extensive banded iron formations (BIFs) deposited before the ∼1 billion year-long Mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of BIFs.The gradual textural, mineralogical, and geochemical changes from unweathered basalts to strongly weathered hematite-bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions. The δ53Cr value of unweathered pillow basalt cores (−0.19±0.02‰, 2σ) is within the range of mantle inventory values, whereas weathered brown to green basalts (soils), exhibiting up to 30% lower Cr concentrations compared to unweathered pillow cores, are isotopically lighter (δ53Cr=−0.35±0.11‰). In contrast, red, hematite-rich basalts and hyaloclastites underlying the brown to green basaltic soils, with the highest δ18O enrichment in the profile, are isotopically heavier (δ53Cr=+0.05±0.15‰). Rb–Sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574±24Ma (MSWD=13) which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence/burial of the Gunflint basin. The non-correlation of K enrichment factors with δ53Cr, δ18O and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering. Instead, correlations between δ53Cr and δ18O, and between δ53Cr, Ce/Ce*, U and V, indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re-deposition at depth from high δ18O freshwaters. Partial re-precipitation of heavy Cr at depth can be explained on the basis of the weak positive correlation between δ53Cr and Fe enrichment factors; it is seen as a consequence of the reduction of mobile Cr(VI) and co-precipitation as mixed Fe(III)–Cr(III) oxhydroxides during contemporaneous oxidation of the Fe2+-bearing groundwaters at depth, a process similar to the one governing the deposition of most Precambrian BIFs. Redistribution of certain elements during the weathering process, in this case REEs, is furthermore indicated by a correlation line in a Sm–Nd isochron diagram defined by the Schreiber Beach data with a slope corresponding to an age of 1.93±0.19Ga (MSWD=26), an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality.An oxidative atmosphere at ∼1.9Ga, as implied by the results from the Schreiber profile, is furthermore supported by positively fractioned Cr isotopes (δ53Cr from +0.06 to +0.39‰) recorded in the iron-rich Gunflint Cherts directly above the palaeo-weathered horizons at Schreiber Beach. These values are interpreted to stem from a positively fractionated shallow seawater chromium composition at ∼1.88Ga, potentially reflecting a continental run-off characterized by positive δ53Cr, and are in accordance with Cr isotope signatures in worldwide BIFs (including the Gunflint Iron Formation).Our results show the potential of Cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes, making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in Earth's history.

Accurate and precise determination of stable Cr isotope compositions in carbonates by double spike MC-ICP-MS

Techniques for the separation of small quantities of Cr from carbonate material and for the analysis of stable Cr isotopes in carbonates by MC-ICP-MS are presented in this study. In comparison with previously published methods, we have developed a one-step Cr separation procedure that is relatively simple, and has a low blank (0.12–0.20 ng). Moreover, careful optimisation of the desolvating sample introduction system allows a significant increase in the sensitivity of our MC-ICP-MS technique compared to previous studies. Instrumental mass bias effects and fractionation of Cr isotopes during Cr separation are corrected using a carefully optimised 50Cr–54Cr double-spike method. Novel numerical simulations demonstrate that the effects of potential isobaric interferences from Ti, Fe and V are negligible, even if they are isotopically fractionated. Small deviations in the δ53Cr value of the NBS 979 standard between different analytical sessions are due to small deviations from exponential mass fractionation behaviour. The long-term reproducibility of δ53Cr for a spiked NBS 979 Cr isotope reference material is ±0.031‰ (2 S.D., n = 147). Analyses of carbonates reveal that they have δ53Cr values of 0.747 to 1.994‰, distinctly heavier than continental crust and the terrestrial mantle. The carbonates record Cr isotopic fractionation that may be used to understand redox reactions in the oceans. Although this study focuses on carbonate samples, our mass spectrometry technique can be applied to the analysis of any samples with low levels of Cr, including river waters and seawater.

Analytical method of Cr stable isotope and its application to water pollution survey

High Cr (VI) concentration is a potential serious risk to environment and ecologic system. Thus, it is essential to effectively monitor Cr contamination in water systems and scientifically evaluate its evolution trend. A new method of in-situ tracer technique by Cr isotopic analysis for quantitatively estimating Cr-contamination and its regional evolution in natural waters as well as a case application are reported in this paper. Based on documented literatures, an experimental procedure for chemical separation and purification of Cr in water samples and its analytical method for Cr isotopic measurement by thermal ionization mass spectrometry (TIMS) are established. Eleven samples collected around a chemical factory in Hubei Province were analyzed by this technique. These samples show δ53Cr values ranging from −1.7‰ to 7.3‰ and an elevating trend with an increasing in distance from the sampling site to pollution source. A functional correlation following Rayleigh fractionation law was established between sample’s δ53Cr value and its extent of Cr contamination. This study suggests that Cr isotopic analysis is an effective method to reveal the spatial distribution and its evolution trend of Cr contamination in the regional water system. It is thus helpful to assessing the self-purifying ability of local water system according to Cr isotopic fractionation.

Mn−Cr ages of Fe-rich olivine in two Rumuruti (R) chondrites

Abstract Mn−Cr systematics in olivine of two Rumuruti (R) chondrites was investigated. Mn/52Cr ratios up to 1800 and 1300, and δ53Cr of up to 25‰ and 7‰ were observed for NWA 753 and Sahara 99531, respectively. All data points of NWA 753 show a linear correlation between δ53Cr values and Mn/52Cr ratios on the isochron diagram. The inferred initial 53Mn/55Mn ratio for NWA 753 is (1.84 ± 0.42(2σ)) × 10−6. In the case of Sahara 99531, a positive correlation interpreted as an isochron for 53Mn/55Mn = 2.75 ± 1.55(2σ) × 10−6 was obtained for only one chondrule. Data from other chondrules in Sahara 99531 give an upper limit of 53Mn/55Mn = 0.49 × 10−6. The Mn−Cr ages of NWA 753 and a chondrule in Sahara 99531 are slightly older than that of the angrite LEW 86010 (Lugmair and Shukolyukov, 1998). Other chondrules in Sahara 99531 are at least 5 Ma younger than the LEW 86010. The Mn-Cr ages of olivine in R chondrites correspond to the time when olivine became a closed system either during slow cooling from the peak metamorphic temperature or during rapid cooling by impact excavation. In either case the olivine closure occurred earlier than the final assembly of the brecciated chondrites.