Cr Isotope Fractionation Research Articles

Chromium cycle in Red Soil Critical Zone constrained by chromium isotopes

Chromium (Cr) isotopic system has been used to trace Cr pollution in the modern surface environment and redox change in the paleoenvironment. However, the transformation mechanism of Cr in soil and the accompanied Cr isotopic fractionation have not been clarified clearly. Here we measured Cr isotopic compositions (δ53Cr) of two paddy field profiles from the Red Soil Critical Zone Observatory in South China. The δ53Cr values of the young paddy fields, which have been cultivated for about 20 years, range from −0.34 ‰ to −0.22 ‰. The old paddy fields have been cultivated for >100 years and have more positive Cr isotopic compositions than the young paddy fields, from −0.20 ‰ to −0.06 ‰. The results of three-step leaching experiments show that iron and manganese oxides are enriched in heavy Cr isotopes, while organic matters have much lower Cr isotopic compositions, likely resulting from back reduction of Cr(VI). Our results suggest that Cr isotopic fractionation during the oxidation of Cr(III) is not the only reason for the depletion of heavy isotopes during oxidative weathering, and the partial back-reduction of generated Cr(VI) by organic matter plays an important role in Cr isotopic fractionation during weathering. Comparison between the old and young paddy fields further indicates that cultivation can significantly affect the Cr cycle in red soils. Paddy fields could be a potential sink for the Cr(VI) contaminant, and soils with a long history of cultivation would be more susceptible.

Chromium Isotope Behavior During Serpentinite Dehydration in Oceanic Subduction Zones

AbstractFluids released through the dehydration of serpentinite can be rich in Cl−, which enables the significant mobility of Cr in subduction zones. However, the Cr isotope behavior accompanying the mobility of Cr during serpentinite dehydration is still poorly constrained. Here, we report high‐precision Cr isotope data for a unique suite of serpentinites that represent metamorphic products at different depths in oceanic subduction zones. Low‐grade serpentinites affected by significant Cr loss during serpentinization exhibit remarkably higher δ53Cr, while samples with Cr contents >∼1,800 ppm typically preserve mantle‐like δ53Cr. Antigorite serpentinites have an average δ53Cr value of −0.17‰ ± 0.19‰ (n = 12, 2SD), which is statistically lower than those of low‐grade serpentinite (−0.05‰ ± 0.30‰, n = 80, 2SD) and higher‐grade chlorite harzburgite (−0.10‰ ± 0.27‰, n = 22, 2SD). This suggests that resolvable Cr isotope fractionation occurs during serpentinite dehydration, which is explained by the variability of Cr isotope behavior in the presence of Cl‐bearing fluids at different dehydration stages. No obvious Cr isotope fractionation was found during chlorite harzburgite dehydration, probably related to the limited Cr mobility in a Cl‐poor fluid. Other processes, such as melt extraction, external fluid influx and retrograde metamorphism, have negligible effects on the Cr isotope systematics of meta‐serpentinites. Fluids released by serpentinite dehydration may have a great effect on the Cr isotope heterogeneity of mantle wedge peridotites and arc magmas.

Chromium isotope variations in garnet-facies mantle rocks and their minerals: Implications for Cr isotope behavior in high-temperature processes

Previous work on Cr isotope composition of the lithospheric mantle focused on shallow spinel-facies peridotites and invoked the effects of partial melting and metasomatism. However, little is known on the Cr isotope composition and behavior in the deeper and more voluminous garnet-facies mantle, notably during generation and passage of magmas and fluids. Here, we present Cr isotope data for 26 garnet and ten spinel whole-rock peridotite xenoliths (mainly harzburgites) from the Udachnaya kimberlite on the Siberian craton and minerals from five of them, as well as for minerals of 14 eclogites and four garnet clinopyroxenites from the Dabie orogen in eastern China. Garnet (Gt) and clinopyroxene (Cpx) are major Cr hosts in the mantle. Our ionic modeling suggests the following order of 53Cr/52Cr values in coexisting minerals: garnet ≈ spinel > Cpx > olivine. Cr isotopes are equally partitioned between garnet and Cpx in four peridotites, while garnet is enriched in heavy Cr in one peridotite and all the eclogites (Δ53CrGt-Cpx 0.01‰ to 0.27‰, av. 0.10‰), but depleted in heavy Cr in the pyroxenites (Δ53CrGt-Cpx −0.03‰ to −0.16‰, av. −0.10‰). The Δ53CrGt-Cpx variations are attributed to effects of chemical composition, equilibration temperature and oxygen fugacity on equilibrium Cr isotope fractionation. Further, the anomalous Δ53CrGt-Cpx values in the Dabie pyroxenites and one peridotite may indicate disequilibrium inter-mineral isotope fractionation via kinetic diffusion during phase transitions as a result of pressure-temperature (P-T) changes and metasomatism.The whole-rock δ53Cr range in the garnet peridotites (−0.27 to 0.13‰; average −0.08 ± 0.18‰, 2 SD, n = 25, excluding an outlier with −0.86‰) overlaps bulk silicate Earth estimates whereas the spinel peridotites tend to have higher δ53Cr (−0.12 to 0.16‰; av. 0.02 ± 0.20‰, 2 SD, n = 10). While these rocks are residues of high degrees (30–40%) of melt extraction, their range contrasts with limited equilibrium Cr isotope fractionation we calculated for melting residues of both spinel and garnet peridotites (Δ53Crresidue-source < 0.03‰). The δ53Cr values in the spinel peridotites show correlations with modal abundances of Cpx, garnet and spinel, as well as major and trace element composition. We posit that the Cr isotope variations in these peridotites are somehow linked to solid–melt and inter-mineral diffusive exchanges during P-T changes, deformation and ingress of metasomatic media previously documented in the Udachnaya peridotites. The average δ53Cr value (−0.09 ± 0.16‰, 2 SD, n = 31) for garnet peridotites (excluding U9) in this and previous studies is similar to those previously obtained for (mainly off-craton) spinel peridotites and komatiites, and appears to be representative of the Cr isotope composition for bulk mantle. Numerical modeling for mantle magmas derived at different depths and by different melting degrees of garnet and spinel peridotites and eclogites yields a narrow δ53Cr range, possibly affected by oxygen fugacity.

Chromium isotope system of intraplate basaltic lavas: Implication for recycling materials into mantle

To explore the mantle source properties and the magmatic evolution process for intraplate basaltic lavas, as well as to investigate the potential of Cr isotopes to trace recycled crustal materials, we present Cr isotope compositions for 72 well-characterized samples of Cenozoic intraplate basaltic lavas from eastern China. These basalts display a resolvable δ53Cr range of −0.24‰ to −0.03‰ (n = 72), comparable to MORB and OIB. With respect to the evolved lavas, their δ53Cr scatter can be interpreted as a result of fractional crystallization of predominant olivine (or/and clinopyroxene) with 1% to 3% spinel from heterogeneous magma sources under different oxygen fugacity conditions. Combined with Sr-Nd-Zn-Fe isotope data, the least evolved basalts with MgO of ∼10% display a large δ53Cr range, indicating the presence of different magma component end-members, including one low-Si and two high-Si components. Two high-Si enriched components have δ53Cr values lower than those of primitive N-MORB and OIB magmas, consistent with their derivations most likely from relatively enriched mantle pyroxenites. Notably, the low-Si nephelinitic component could not be a simple end-member as proposed before. Correlations of δ53Cr with εNd(t)-87Sr/86Sr(i)-Fe/Mn-Ti/Ti*-Hf/Hf*-δ66Zn-δ56Fe [Ti/Ti* = TiN/(NdN-0.055 × SmN0.333 × GdN0.722), Hf/Hf* = HfN/(SmN × NdN)0.5. N represents the primitive mantle-normalized values] for nephelinites from both the North China Block and the South China Block suggest mixing between partial melts of carbonated peridotite and pyroxenite sources. The carbonated peridotite derived melt is characterized by depleted SrNd isotopes, low Fe/Mn, Ti/Ti* and Hf/Hf* ratios, and heavier than BSE's Cr-Fe-Zn isotope compositions. The highly oxidized environment induced by carbonatization effectively reduces Cr isotope fractionation between melts and residues during partial melting of the carbonated peridotite, resulting in the heavy Cr isotope feature of melts being inherited from the carbonated mantle source, consistent with the heavy Cr isotope compositions of carbonated mantle xenoliths obtained previously. Thus, this work highlights that Cr isotopes might act as a potential tracer for subducted sedimentary carbonates.

Chromium isotopes in an acidic fluvial system: Implications for modern and ancient Cr isotope records

Chromium is an important redox-active element, with significant Cr isotope fractionations driven by redox transformations in the natural environment and quantified through laboratory experiments. Recent work has demonstrated that non-redox-driven transformations can also produce small variations in Cr isotopes, but the role of these processes in controlling environmental Cr isotope variations remains unclear. The highly acidic but biologically diverse acid rock drainage system in Río Tinto, Spain (and affiliated rivers) provides a unique opportunity to examine how non-redox-related processes can substantially affect the isotopic composition of Cr in a natural system. Our results suggest that under conditions where Cr redox transformations are obviated, Cr cycling is largely controlled by formation of Fe-oxyhydroxides and Fe-hydroxysulfates, Cr adsorption, and Cr speciation, with little apparent Cr isotope fractionation between source materials, waters, and sediments. Moderate Cr isotope shifts, however, may be the result of non-redox processes, including adsorption to organic ligands and potentially biological uptake. These results add to a growing body of evidence for the importance of non-redox transformations in contributing to environmental Cr isotope variations, with implications for the use of chromium isotopes in studies of sedimentary Cr isotope records. In particular, this study is a validation of the potential for sulfide weathering in the Proterozoic to deliver relatively unfractionated Cr isotopes to the ocean, despite rising and sustained high oxygen levels.

Light stable Cr isotope compositions of mid-ocean ridge basalts: Implications for mantle source composition

Stable isotopes of chromium (Cr) and iron (Fe), both multivalent elements in silicate melts, are valuable proxies of redox conditions in magmatic systems. Partial melting and fractional crystallization have been identified as the dominant fractionation processes for Cr and Fe isotopes in high-temperature reservoirs that can lead to small but detectable isotope variations in igneous rocks. More recently, however, the source mineralogy has been suggested to play an important role, which can ultimately affect the magnitude of Cr and Fe isotopic fractionation during partial melting and thus the Cr and Fe isotopic composition of the originating melt.With the aim to investigate the role of magmatic processes and the source Cr and Fe isotope signatures on the isotope composition of these transition metals in normal mid-ocean ridge basalts (N-MORB), we analyzed 21 well-characterized MORB glasses from the southern East-Pacific Rise, Pacific-Antarctic Ridge and Mid-Atlantic Ridge. The Cr isotope compositions of N-MORB show a narrow range from −0.278 to −0.186 ‰ in δ53/52Cr (i.e., difference of a sample’s 53Cr/52Cr ratio relative to the international reference material NIST SRM979), with an average value of −0.237 ± 0.050 ‰ (2SD; n = 19). As such, the average N-MORB δ53/52Cr value is significantly lower than that of Bulk Silicate Earth of −0.12 ± 0.06 ‰ and the Cr isotope compositions reported for most ocean island basalts ranging from ∼ −0.23 to 0.00 ‰. Iron isotopic compositions of these MORB range from +0.032 to +0.137 ‰ in δ56/54Fe (i.e., difference of a sample’s 56Fe/54Fe ratio relative to the international reference material IRMM014) and are within the range of previously published MORB data. The lack of correlations between N-MORB Cr isotope signatures and indices of magmatic differentiation suggests a negligible control of this process on Cr stable isotopes. Partial melting modeling using pMELTS provides evidence that the significant offset towards lower δ53/52Cr values of N-MORB cannot be produced by decompression partial melting of a lherzolithic source such as the depleted upper mantle alone. Instead, we suggest that the lower δ53/52Cr values of MORB could be linked to intrinsic small-scale 53Cr-depleted pyroxene-rich domains in the upper mantle that influence the melt’s Cr isotopic budget during partial melting.

Evaluation of the Residual Mass Fractionation in High‐Precision Cr Isotopic Analysis with TIMS

The Cr isotope ratios of terrestrial and extra‐terrestrial materials are emerging as one of the most important tracers in geosciences. Previous studies on Cr isotopic measurements using TIMS have found that there is residual Cr isotopic fractionation between the mass‐fractionation‐corrected 53Cr/52Cr and 54Cr/52Cr ratios, which may cause an offset of obtained ratios from the reference values. The residual fractionation was thought to be caused by the evaporation of Cr‐oxide species during thermal ionisation, but the mechanism by which this residual fractionation could be reduced remained unclear. Here we revisit the issue of residual fractionation and propose that this problem can be alleviated by utilising W filaments instead of conventionally used Re filaments for Cr ionisation. Using W filaments, the formation of CrO+ was suppressed during heating as the filament temperature was ~ 100 °C lower than when Re filaments were used. In repeated measurement of a carbonaceous chondrite, the intermediate precisions of 53Cr/52Cr and 54Cr/52Cr ratios in the W filament runs were two to three times better than those of the Re filament runs. Therefore, the new finding of this study will be of key importance for future studies of Cr isotopes for terrestrial and extra‐terrestrial materials.

The stable chromium isotope composition of different mantle reservoirs

The stable chromium isotope composition of Earth’s mantle has been suggested to be heterogeneous, with mantle-derived chromitites slightly enriched in heavier Cr isotopes than mantle-derived silicate rocks. To better understand such heterogeneity and to search for a potentially complementary light Cr isotope reservoir in the mantle, we analyzed the stable Cr isotope compositions (δ53Cr expressed as deviation of sample 53Cr/52Cr from NIST SRM 979) of whole-rock pyroxenite and peridotite xenoliths from the North China Craton using a double spike technique. The δ53Cr values of the peridotites vary from −0.21‰ to −0.07‰ with an average value of −0.14 ± 0.10‰ (2SD, N = 5), which overlaps with the range of the Bulk Silicate Earth (−0.12 ± 0.10‰). In contrast, pyroxenites in this study yield lower δ53Cr values (−0.30‰ to −0.14‰ with an average value of −0.21 ± 0.08‰, 2SD, N = 20). The variation in the δ53Cr values of the pyroxenites can be explained by a Rayleigh distillation model during partial melting of the primitive mantle. The model provides minimum Cr isotope fractionation factors during partial melting ranging from 0.01‰ to 0.12‰, with light isotopes enriched in the melt. Combined with literature data, our results suggest that both peridotites and basalts are indistinguishable from the Bulk Silicate Earth in terms of δ53Cr. However, pyroxenites and chromitites are isotopically lighter and heavier than the Bulk Silicate Earth, respectively. Future work is needed to investigate the petrogenetic relationship between chromitites and pyroxenites in order to understand why they seem to complement each other in δ53Cr.

Cr stable isotope fractionation by evaporation from silicate melts

We present new experimental results of Cr isotope fractionation during degassing of silicate melts. Our experiments at oxidizing conditions (in air) at 1 bar total pressure show that evaporative loss of Cr is substantial and depends on run duration and on temperature (between 1350 and 1500 °C). The stable Cr (δ53Cr) isotope compositions of the evaporation residues were analyzed by thermal ionization mass spectrometry using the double spike method. Our results indicate that during degassing the lighter isotope 52Cr is enriched in the gas phase and consequently the residual melt becomes enriched in the heavier 53Cr isotope in a mass-dependent fashion. The kinetic isotopic fractionation factor, α, derived from our experimental data is 0.9995 ± 0.0001, which is significantly different from the fractionation factor predicted for an ideal gas composed of CrO3, αkin = 0.995. Our experimentally-determined value of α is consistent with diffusive transport of CrO3 (g) away from the melt surface, proportional to the inverse cube-root of the reduced masses of the two isotopologues. We hypothesize that Cr loss by evaporation may be recorded by Cr isotopes in terrestrial magmas with sufficiently high oxygen fugacities erupting near the surface, which are plausibly found in lava lakes or erupting arc magmas.

High P-T experimental perspective on Cr isotopic fractionation during planetary core formation

Core formation may modify the stable isotopic signatures for both the mantles and cores of differentiated planetary bodies. We performed high P-T experiments with a piston-cylinder apparatus at 1 GPa and 1873-2073 K to determine the Cr isotopic fractionation factor during metal-silicate segregation. Experimental results consistently indicate that the metal phase is isotopically heavier than the coexisting silicate phase, with Δ53Crmetal-silicate up to 0.3‰ at the investigated experimental conditions. Oxygen fugacity, silicate composition, and S content in the metal phase do not have significant effects on the Cr isotopic fractionation factor. By contrast, increasing Ni content in the metal increases the Δ53Crmetal-silicate value, implying that the Ni content of the core could influence planetary isotopic signatures. We conclude that heavier Cr isotopes enter the core preferentially during planetary core formation. The δ53Cr value of the terrestrial mantle could be lowered by up to ∼0.02‰ by core formation, despite that this is within current analytical uncertainty of chondritic Cr isotopic composition. For smaller bodies such as the Moon, Mars, and Vesta, the lower core formation temperatures could potentially generate a resolvable core-mantle Cr isotopic fractionation. However, the Moon's small core size would limit the change in the Cr isotopic composition of the lunar mantle compared to chondritic. For Vesta and Mars, core formation could lower the δ53Cr values of their mantles by ∼0.01-0.02‰, which is trivial relative to the analytical uncertainty. On the other hand, core formation could increase the δ53Cr values of the cores of the parent bodies of iron meteorites by up to ∼0.2‰ at 1873 K. Therefore, the significantly heavy Cr isotopic composition (up to 2.85‰) of iron meteorites cannot be explained by equilibrium fractionation between the core and the mantle of the parent bodies of iron meteorites.

Mesoproterozoic oxygenation event: From shallow marine to atmosphere

AbstractOxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1%–1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 &amp;gt; 0.1%–1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ~20‰ to ~50‰ and a subsequent decline to ~10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca + Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.

Chromium isotope systematics in three mantle-derived domains of Central European Variscides: Relationship between δ53Cr values and progressive weathering of serpentinized ultramafic rocks

Most chromium (Cr) entering the ocean originates from continental weathering of ultramafic rocks. Recent data indicate that (i) mantle-derived rocks are characterized by homogeneous δ53Cr values of approximately −0.12‰; (ii) some serpentinized peridotites, serpentinites and weathered serpentinites contain isotopically heavier Cr than mantle protoliths; and (iii) Cr transported by rivers is isotopically variable, but positive δ53Cr values predominate. There is a need to identify solid materials that, in sufficient volumes, store the complementary isotopically light Cr. We studied δ53Cr systematics in three mantle domains belonging to the Paleozoic Central European orogenic belt. These domains are characterized by contrasting P–T histories. At two of six study sites, a 30-m deep drilling was carried out to assess spatial gradients in Cr concentrations ([Cr]) and δ53Cr values toward the weathering front. The studied rocks, comprising serpentinized peridotites with preserved mantle signature, serpentinites, carbonatized serpentinites, and hornblendites, were characterized by a variable degree of weathering. Spinel in these rock types contained up to 33 wt% Cr but represented less than 1% of the rock volume; chromite was not detected in any of the samples. Silicates, such as pyroxene, serpentine, hornblende and chlorite, with an average [Cr] of 0.24 wt.% were likely the predominating potential Cr source for the environment. Despite the known higher weathering rates of Cr-silicates compared to Cr-spinelides, no systematic trends toward lower [Cr] and higher δ53Cr values were detected upcore. The overall range of whole-rock δ53Cr values was from −0.25 to 0.14‰ (mean of −0.09 ± 0.01‰). There was no relationship between whole-rock δ53Cr values and indices of alteration/weathering, such as water content, loss on ignition, and leaching of alkalies. We did not observe progressive removal of isotopically light Cr during continental weathering of serpentinized mantle-derived rocks that had been previously reported from altered and hydrated ultramafic rocks at some other sites. Our data suggest that such Cr isotope fractionation, if present, may be associated with earlier hydrothermal alteration rather than with weathering. Mean δ53Cr values of soils were indistinguishable from the underlying ultramafic rocks, indicating only small sensitivity of Cr isotope composition to pedogenesis.

Chromium isotope fractionation during adsorption of chromium(III) by soils and river sediments

Chromium (Cr) isotope compositions of sedimentary rocks have been widely used to unravel fluctuations in atmospheric oxygen levels during geologic history. A fundamental framework of this application is that any Cr isotope fractionation in natural environments should be related to the redox transformation of Cr species [Cr(VI) and Cr(III)]. However, the behavior of Cr isotopes during non-redox Cr cycling is not yet well understood. Here, we present laboratory experimental results which show that redox-independent adsorption of Cr(III) by natural river sediments and soils can be accompanied by obvious Cr isotope fractionation. The observed Cr isotope fractionation factors (−0.06‰ – −0.95‰, expressed as 10&lt;sup&gt;3&lt;/sup&gt;ln&lt;i&gt;α&lt;/i&gt;) are much smaller than those caused by redox processes. Combined with previous studies on redox-independent Cr isotope fractionation induced by ligand-promoted dissolution, we suggest that the systematic shift to highly fractionated Cr isotope compositions of sedimentary rocks is likely to represent atmospheric oxygenation, but muted signals observed in some geologic periods may be attributed to non-redox Cr cycling and should be interpreted with caution.

A transient oxygen increase in the Mesoproterozoic ocean at ∼1.44 Ga: Geochemical evidence from the Tieling Formation, North China Platform

Oxygen availability is crucial for the evolution of eukaryotes in geological history, yet detailed Mesoproterozoic oceanic-atmospheric redox conditions remain enigmatic. In contrast to the generally accepted hypothesis of an anoxic mid-Proterozoic ocean and atmosphere, several transient oxygenation events may occur at the Earth’s surface during the Mesoproterozoic, especially for the period around 1.4 Ga. The North China Platform develops one of the most complete and continuous Mesoproterozoic stratigraphic successions globally, preserving key information on the redox state of the surface ocean–atmosphere system during the mid-Proterozoic. In this study, I/(Ca + Mg), rare earth elements (REE) plus Y (REY), and paired stable isotopes (δ34SCAS and δ13Ccarb) data are reported for the carbonate rocks of ∼1.44 Ga Tieling Formation in the Jixian section, North China Platform. The lower Tieling Formation (i.e., the Daizhuangzi Member) exhibits middle-REE-enriched patterns with several relatively high I/(Ca + Mg) values (0.56–0.83 μmol/mol) suggesting a possible oxic or suboxic seawater condition. The upper Tieling Formation (i.e., the Laohuding Member) is characterized by modern seawater-like REY patterns with negative Ce anomalies (0.67–0.91) and variable I/(Ca + Mg) values (0.02–1.77 μmol/mol). The peak I/(Ca + Mg) ratios (up to 1.77 μmol/mol), negative shifts in Ce/Ce* and δ13Ccarb, and previously reported positive Cr isotope fractionation in the middle Laohuding Member indicate a significant oxygen increase in both ocean and atmosphere. A long-term increasing trend in δ34SCAS below the middle Laohuding Member outlines roles of pyrite burial in contributions of rising oxygen levels during this period.

Cr Isotopes and the Engineered Attenuation of Cr(VI)-Rich Runoff.

The leaching of lateritic soils can result in drainage waters with high concentrations of Cr(VI). Such Cr(VI)-rich waters have developed in streams that drain lateritic soils in Central Sulawesi Island, Indonesia. Chromium in this lateritic drainage system is removed by reduction of Cr(VI) to Cr(III) through two faucets delivering an FeSO4 solution to the drainage waters. Cr stable isotope compositions from both water and sediment samples along the drainage path were used to evaluate the efficacy of this remediation strategy. Overall, dissolved [Cr(VI)] decreased moving downstream, but there was an increase in [Cr(VI)] after the first faucet that was effectively removed at the second faucet. This intermittent increase in [Cr(VI)] was the likely result of oxidative remobilization of sediment Cr(III) through reaction with Mn oxides. Cr isotope distributions reflect near quantitative reduction associated with the FeSO4 faucets but also reveal that Cr isotope fractionation is imparted due to Cr redox cycling, downstream. During this redox cycling, fractionation appeared to accompany oxidation, with the product Cr(VI) becoming enriched in 53Cr relative to the reactant Cr(III) with an apparent fractionation factor of 0.7 ± 0.3‰. This study suggests that while FeSO4 effectively removes Cr(VI) from the drainage, the presence of Mn oxides can confound attenuation and improvements to Cr(VI) remediation should consider means of preventing the back reaction of Cr(III) with Mn oxides.

Chromium stable isotope geochemistry in the Mobile Bay Estuary

The chromium (Cr) isotope system is an important redox proxy for understanding Earth's oxygenation history. However, quantitative application of this proxy is hindered by a lack of clear understanding of the global Cr isotope mass balance. Estuarine environments potentially play important roles in influencing the global oceanic Cr mass balance because riverine Cr derived from continental weathering must first travel through estuaries before entering the open ocean. We collected water, suspended particulate matter, and sediment samples across a salinity gradient in the Mobile Bay Estuary located in the Northern Gulf of Mexico, with a purpose of evaluating whether Cr behaves conservatively in estuarine environments. A simple conservative mixing model between the fresh Mobile River water and Gulf of Mexico seawater alone was insufficient to explain the Cr concentration and isotope data, suggesting that Cr behaves non-conservatively in this estuary system. A model incorporating conservative mixing, partial Cr(VI) reduction and partial scavenging of the produced Cr(III) is developed to reproduce the observed data. Our data and modeling exercise suggest that up to 60% of Cr(VI) is reduced to Cr(III) in the Mobile Bay Estuary with a Cr isotope fractionation factor of −1.2‰, and 40% of the produced Cr(III) is removed from solution. Although these parameters may not be the sole solution to the system of equations involved, they are consistent with previous estimates. Future redox-specific Cr isotope data in estuarine environments are needed to place finer constraints on these parameters in estuaries, which is critical for estimating global marine Cr isotope mass balance using riverine Cr concentration and isotope data.

The coordination of Cr2+ in silicate glasses and implications for mineral-melt fractionation of Cr isotopes

The Cr2+-O and Cr3+-O bond lengths in CaO-MgO-Al2O3-SiO2 glasses containing ~0.3 wt% Cr with Cr2+/ΣCr = ~ 1 or 0 (where ΣCr = Cr2+ + Cr3+) were determined by extended X-ray absorption fine structure (EXAFS) spectroscopy to be 2.034(5) and 1.967(7) Å, respectively. The Cr2+-O bond length is consistent with square planar coordination and the Cr3+-O bond length with octahedral coordination. These bond lengths were used to calculate force constants of 1206(9) Nm−1 for Cr2+-O and 2001(21) Nm−1 for Cr3+-O in silicate melts. The value for Cr3+-O is similar to previous estimates but that for Cr2+-O is almost 20% lower. The force constants were used to calculate the difference in Cr isotopic composition (Δ53Cr) between both olivine and melt and spinel and melt for MORB with equal amounts of Cr2+ and Cr3+ at 1150 °C. The resulting values, Δ53Crol-mlt = −0.050(4) and Δ53Crspl-mlt = 0.087(4), are larger by ~0.03 than those obtained using Cr-O bond lengths estimated from ionic radii. For Δ53Crspl-mlt this difference is equivalent to a change in temperature of over 300 °C at constant Cr2+/ΣCr. The fractionation of Cr isotopes in ocean island basalts should be larger for fractional crystallisation in the crust than partial melting in the mantle due to the strong negative effect of pressure on Cr2+/ΣCr. The new bond length data for Cr2+-O provide an additional constraint for modelling and interpreting Cr isotope fractionation during igneous petrogenesis.

Geochemical and isotopic constraints on the dynamics of the Earth: a Special Issue dedicated to Jan Dirk Kramers and his legacy to geochemistry

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.

Tracing serpentinite dehydration in a subduction channel: Chromium element and isotope evidence from subducted oceanic crust

Serpentinite is one of the most important sources of volatiles and fluid-mobile elements in subduction zones. Attempts have been made to discriminate serpentinite-derived fluid from other sources (e.g., sediments, altered oceanic crusts) using a variety of geochemical tools, but such differentiation has proven to be changeling because fluids from different lithologies are dynamically mixed in the subduction channel. Serpentinites are essentially distinguished by high Cr contents and variable degrees of 53Cr excess. Given high Cr mobility in Cl-rich metamorphic fluids based on experimental determination, Cr elemental and isotope compositions could be potential markers for fluid-mediated mass transfer from serpentinites. Here, we test this hypothesis by analyzing Cr concentrations and isotope compositions of meta-basalts, as well as country mica schists, marbles and serpentinites from the southwestern Tianshan Orogen. The meta-basalts have highly variable Cr concentrations (30.0–625 ppm) and resolvable δ53Cr variations (−0.25‰ to −0.05‰), while mica schists and marbles have similar isotope compositions and low Cr contents (δ53Cr of −0.15‰ to −0.14‰ with Cr contents of 89.5–110 ppm for mica schists, δ53Cr of −0.21‰ to −0.17‰ with Cr contents of 4.1–8.73 ppm for marbles, respectively). Most of the investigated meta-basalts were carbonated. The positive correlations of loss on ignition (LOI), CO2, Sb and Sb/Ce with Cr contents, as well as with δ53Cr for these carbonated meta-basalts indicate that the metamorphic fluids are at least partially sourced from serpentinites and sedimentary carbonates. High pressure serpentinites display similar δ53Cr variations from −0.19‰ to −0.02‰, but high Cr contents ranging from 934 to 4920 ppm. Chromium isotope variations of these serpentinites could not be generated by serpentinization, due to opposite trends of Cr isotopes and serpentinization indexes compared with previous observations. Instead, dehydration of serpentinites during subduction/exhumation accounts for Cr isotope variations, generating fluids with high Cr concentrations and relatively lighter isotope compositions. According to a Rayleigh dehydration model, the estimated Cr isotope fractionation factor (αfluid-residue) between fluids and residual serpentinites is approximately 0.99995 –0.99975, implying that Cr species in the subducted serpentine derived fluid is probably as complexes of Cr3+-Cl−H2O(OH–). The mixing model reveals that investigated carbonated meta-basalts were sourced from initial evolved oceanic basalts metamorphosed by fluids from serpentinite dehydrations, with contributions from sedimentary carbonates. This work emphasizes that coupled Cr element and isotope systems present the potential ability to discriminate the complex fluid sources in subduction zones, particularly serpentinites.

The chromium isotope fractionation factor in seawater

The chromium (Cr) stable isotope system is an increasingly used redox proxy for studying the oxygenation history of Earth's ocean-atmosphere system, but understanding of Cr isotope fractionation in modern seawater is still nascent. A linear relationship has been found between the isotope composition and the logarithmic concentration of total dissolved Cr in the global ocean. Such relationship was used to extract an ‘effective’ Cr isotope fractionation factor of ca. 0.7‰ associated with the removal of Cr from seawater. However, there is a statistically significant proportion of data that deviate from the global linear array, and the proportion of data plotting below the array is higher than that plotting above the array. Such scattering and skewing patterns are present even after excluding data that may have been caused by freshwater influences and potential inconsistency in methods used to purify Cr from seawater. A simple numerical exercise was conducted to demonstrate that the observed data scattering and skewing could be explained by parallel removal pathways with ‘intrinsic’ Cr isotope fractionation factors ranging from −0.8‰ to −1.6‰ combined with the presence of variable amount of dissolved Cr(III) in seawater. These results based on global seawater Cr isotope data are consistent with previous conclusions based on individual oceanographic sites.