Cr Isotopic Compositions Research Articles

Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes

Enstatite achondrites (including aubrites) are the only differentiated meteorites that have similar isotope compositions to the Earth-Moon system for most of the elements. However, the origin and differentiation of enstatite achondrites and their parent bodies remain poorly understood. Here, we report high-precision mass-independent and mass-dependent Cr isotope data for 10 enstatite achondrites, including eight aubrites, Itqiy and one enstatite-rich clast in Almahatta Sitta, to further constrain the origin and evolution of their parent bodies. The ε54Cr (per 10,000 deviation of the mass bias corrected 54Cr/52Cr ratio from a terrestrial standard) systematics define three groups: main-group aubrites with ε54Cr = 0.06 ± 0.12 (2SD, N = 7) that is similar to the enstatite chondrites and the Earth-Moon system, Shallowater aubrite with ε54Cr = −0.12 ± 0.04 and Itqiy-type meteorites with ε54Cr = −0.26 ± 0.03 (2SD, N = 2). This shows that there were at least three enstatite achondrite parent bodies in the Solar System. This is confirmed by their distinguished mass-dependent Cr isotope compositions (δ53Cr values): 0.24 ± 0.03‰, 0.10 ± 0.03‰ and −0.03 ± 0.03‰ for main-group, Shallowater and Itqiy parent bodies, respectively. Aubrites are isotopically heavier than chondrites (δ53Cr = −0.12 ± 0.04‰), which likely results from the formation of an isotopically light sulfur-rich core. We also obtained the abundance of the radiogenic 53Cr (produced by the radioactive decay of 53Mn, T1/2 = 3.7 million years). The radiogenic ε53Cr excesses correlate with the 55Mn/52Cr ratios for aubrites (except Shallowater and Bustee) and also the Cr stable isotope compositions (δ53Cr values). We show that these correlations represent mixing lines that also hold chronological significance since they are controlled by the crystallization of sulfides and silicates, which mostly reflect the main-group aubrite parent body differentiation at 4562.5 ± 1.1 Ma (i.e., 4.8 ± 1.1 Ma after Solar System formation). Furthermore, the intercept of these lines with the ordinate axis which represent the initial ε53Cr value of main-group aubrites (0.50 ± 0.16, 2σ) is much higher than the average ε53Cr value of enstatite chondrites (0.15 ± 0.10, 2SD), suggesting an early sulfur-rich core formation that effectively increased the Mn/Cr ratio of the silicate fraction of the main-group aubrite parent body.

Chromium isotope fractionation during black shale weathering and its environmental implications

Black shale contains abundant pyrite and organic matter that are susceptible to weathering when exposed to the air. In the process of weathering, acid mine drainage can be produced, and a range of toxic trace elements including Cr can be released and transported into rivers, groundwater and soils, potentially leading to severe environmental pollution. In order to study Cr migration and Cr isotopic fractionation during black shale weathering, we sampled metalliferous black shales and cherts from two weathering profiles at Shadi and Yutangba from the Permian Maokou Formation in Enshi Prefecture. The unweathered samples in Shadi and Yutangba have high Cr contents (1562 μg/g and 643 μg/g, respectively), and highly fractionated Cr isotopic compositions (2.04 ± 0.11‰ and 1.91 ± 0.09‰, respectively, expressed as δ53Cr). The narrow range of authigenic δ53Cr in these unweathered shales suggests that the δ53Cr value of the seawater was relatively stable during the period of deposition.Strongly weathered shales in Shadi and Yutangba both display significant Cr losses compared to the unweathered counterparts. Their average δ53Cr values are 1.75 ± 0.12‰ and 1.85 ± 0.39‰ for Shadi and Yutangba, respectively, which are isotopically lighter than fresh samples. This indicates that heavier Cr isotopes are preferentially leached into fluids, leaving the residues enriched in lighter isotopes during black shale weathering. However, the δ53Cr values of the samples close to the water table are higher than those of the unaltered ones, which can be explained by adsorption or quantitative reduction of Cr(VI) near the water table. The fact that Cr isotopes are fractionated during black shale weathering may complicate the application of δ53Cr in polluted samples to identify the Cr sources in areas with exposed black shales. The δ53Cr of seepage water can be measured and treated as a more realistic source signal.

Chromium isotope heterogeneity on a modern carbonate platform

Chromium isotope (δ53Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in δ53Cr values within a carbonate platform and slope system and assess the preservation potential of marine δ53Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of δ53Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new δ53Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy δ53Cr values from +0.77 to +1.07‰ while drill core samples record a larger range of δ53Cr values from +0.51 to +0.97‰. Down-core variability in δ53Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive δ53Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of δ53Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the δ53Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of δ53Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.

Hybrid Accretion of Carbonaceous Chondrites by Radial Transport across the Jupiter Barrier

Abstract Understanding the origin of chondritic components and their accretion pathways is critical to unraveling the magnitude of mass transport in the protoplanetary disk, as well as the accretionary history of the terrestrial planet region and, by extension, its prebiotic inventory. Here we trace the heritage of pristine components from the relatively unaltered CV chondrite Leoville through their mass-independent Cr and mass-dependent Zn isotope compositions. Investigating these chondritic fractions in such detail reveals an onion-shell structure of chondrules, which is characterized by 54Cr- and 66Zn-poor cores surrounded by increasingly 54Cr- and 66Zn-rich igneous rims and an outer coating of fine-grained dust. This is interpreted as a progressive addition of 54Cr- and 66Zn-rich, CI-like material to the accretion region of these carbonaceous chondrites. Our findings show that the observed Cr isotopic range in chondrules from more altered CV chondrites is the result of chemical equilibration between the chondrules and matrix during secondary alteration. The 54Cr-poor nature of the cores of Leoville chondrules implies formation in the inner solar system and subsequent massive outward chondrule transport past the Jupiter barrier. At the same time, CI-like dust is transferred inward. We propose that the accreting Earth acquired CI-like dust through this mechanism within the lifetime of the disk. This radial mixing of the chondrules and matrix shows the limited capacity of Jupiter to act as an efficient barrier and maintain the proposed noncarbonaceous and carbonaceous chondrite dichotomy over time. Finally, also considering current astrophysical models, we explore both inner and outer solar system origins for the CV chondrite parent body.

Chromium isotopic insights into the origin of chondrite parent bodies and the early terrestrial volatile depletion

Chondrites are meteorites from undifferentiated parent bodies that provide fundamental information about early Solar System evolution and planet formation. The element Cr is highly suitable for deciphering both the timing of formation and the origin of planetary building blocks because it records both radiogenic contributions from 53Mn-53Cr decay and variable nucleosynthetic contributions from the stable 54Cr nuclide. Here, we report high-precision measurements of the mass-independent Cr isotope compositions (ε53Cr and ε54Cr) of chondrites (including all carbonaceous chondrites groups) and terrestrial samples using for the first time a multi-collection inductively-coupled-plasma mass-spectrometer to better understand the formation histories and genetic relationships between chondrite parent bodies. With our comprehensive dataset, the order of decreasing ε54Cr (per ten thousand deviation of the 54Cr/52Cr ratio relative to a terrestrial standard) values amongst the carbonaceous chondrites is updated to CI = CH ≥ CB ≥ CR ≥ CM ≈ CV ≈ CO ≥ CK > EC > OC. Chondrites from CO, CV, CR, CM and CB groups show intra-group ε54Cr heterogeneities that may result from sample heterogeneity and/or heterogeneous accretion of their parent bodies. Resolvable ε54Cr (with 2SE uncertainty) differences between CV and CK chondrites rule out an origin from a common parent body or reservoir as has previously been suggested. The CM and CO chondrites share common ε54Cr characteristics, which suggests their parent bodies may have accreted their components in similar proportions. The CB and CH chondrites have low-Mn/Cr ratios and similar ε53Cr values to the CI chondrites, invalidating them as anchors for a bulk 53Mn-53Cr isochron for carbonaceous chondrites. Bulk Earth has a ε53Cr value that is lower than the average of chondrites, including enstatite chondrites. This depletion may constrain the timing of volatile loss from the Earth or its precursors to be within the first million years of Solar System formation and is incompatible with Earth’s accretion via any of the known chondrite groups as main contributors, including enstatite chondrites.

Chromium isotope systematics and the diagenesis of marine carbonates

Stable chromium (Cr) isotopes have emerged as a new tool for tracking broad-scale changes in Earth's surface oxygen levels. Carbonates are one proposed sedimentary Cr isotope archive. In order to contribute to the development of a robust framework for interpreting carbonate Cr isotopic compositions and evaluating their ability to record the global redox state, we explored Cr isotope systematics of modern and Archean carbonate successions—representing end member oxic and anoxic Earth system states. We generated new data from carbonate platform sediments from the Great Bahama Bank with variable post-depositional histories to better understand the effects of diagenetic alteration on carbonate-bound Cr and its isotopic composition in a modern well-oxygenated ocean-atmosphere system. More specifically, we investigated the effects of dolomitization and aragonite-to-calcite neomorphism in marine and meteoric fluids. We also present δ53Cr values from three carbonate successions, the ∼3.0 Ga Chobeni Formation (South Africa), ∼2.8 Ga Mosher Carbonate Formation (Canada), and ∼2.65 Ga Cheshire Formation (Zimbabwe), that were deposited under an anoxic atmosphere. We find that modern Bahamian carbonates have a large range of almost exclusively positive δ53Cr values (from −0.04 to 2.88‰). The δ53Cr values appear to be altered during both meteoric diagenesis and dolomitization but there may also be instances of rock-buffered conditions that may best preserve depositional δ53Cr values. These observations provide a baseline for carbonate Cr isotope behavior in a well-oxygenated ocean-atmosphere system. Our Archean carbonate successions contain both positively fractionated and crustal δ53Cr values, ranging between −0.37 and 0.89‰. The Cr isotope fractionation observed in Archean strata is most plausibly linked to either non-redox dependent Cr isotope fractionation, local Cr redox cycling, late-stage diagenetic alteration, or some combination thereof. Given the significant effects of post-depositional alteration on the isotopic composition of carbonate-bound Cr, we suggest that Cr isotope values from ancient carbonate sediments should be generally interpreted with caution and cannot be straightforwardly linked to changes in atmospheric oxygen abundance.

Coupled stable chromium and iron isotopic fractionation tracing magmatic mineral crystallization in Archean komatiite-tholeiite suites

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.

Assessment of Cr Isotopic Heterogeneities of Volatile-rich Asteroids Based on Multiple Planet Formation Models

Abstract Describing the comprehensive evolutionary scenario for asteroids is key to explaining the various physical processes of the solar system. Bulk-scale carbonaceous chondrites (CCs) possibly record the primordial information associated with the formation processes of their parent bodies. In this study, we tried to estimate the relative formation region of volatile-rich asteroids by utilizing the nucleosynthetic Cr isotopic variation (54Cr/52Cr) in bulk-scale CCs. Numerical calculations were conducted to track the temporal evolution of isotopically different (solar and presolar) dust and 54Cr/52Cr values for mixed materials with disk radius. First, we found that isotopic heterogeneities in CC formation regions were preserved with a weak turbulence setting that increased the timescales of the advection and diffusion in the disk. Second, we assessed the effects of gaps formed by giant planets. Finally, the distance from the injected supernovae and Cr isotopic compositions of the presolar grains were investigated in terms of the estimated formation region of CCs. In our results, a plausible formation region of four types of CCs could be obtained with the supernova from approximately 2 pc and typical Cr isotopic compositions of presolar grains. Among the parent bodies of CCs (i.e., volatile-rich asteroids), B-type asteroids formed in the outermost region, which is inconsistent with the present population, showing that D-type asteroids are generally located beyond most of the C-complex asteroids. Both the initial and present orbits of asteroids might be explained by the scatter attributed to the inward-outward migration of Jupiter and Saturn.

Anoxic continental surface weathering recorded by the 2.95 Ga Denny Dalton Paleosol (Pongola Supergroup, South Africa)

Iron mobilization during continental weathering was pervasive before the Great Oxidation Event (GOE) that started at around 2.43 billion years (Ga) ago, due to the soluble nature of reduced iron. However, various geochemical proxies indicate transient oxygenation during deposition of the Mesoarchean (∼2.95 Ga) Pongola Supergroup, South Africa, which suggests that continental weathering could have also occurred under transiently oxic conditions before the GOE. We analyzed trace elemental and Fe, Ti, and Cr isotopic compositions of the ca. 2.95 Ga Denny Dalton paleosol in the Pongola Supergroup to better understand continental weathering and redox conditions in the ancient critical zone, and the nature of geochemical fluxes from the continents to the oceans and marine sediments. Iron isotope systematics are consistent with a model where Fe was released during intense leaching from the paleosol to concentrate in Fe-rich groundwaters in the deeper part of the soil horizon. We show for the first time that Fe isotopic fractionation during Mesoarchean continental weathering was limited, and Fe enrichments and depletions are coupled with those of divalent transition metals, Co, Ni, and particularly Zn. This suggests that Fe redox cycling was not involved in paleosol formation, and Fe2+ was mobilized under anoxic conditions. Chromium isotopes are also unfractionated relative to the parent igneous rock in this paleosol, which precludes removal of isotopically heavy Cr6+ and thus supports anoxic continental weathering. We show that previously reported Cr isotopic fractionation in another Denny Dalton paleosol profile does not follow a Cr6+ leaching trend, but instead scales with Cr enrichment and may reveal Cr enrichment from post-burial fluids. Thus, there is no clear evidence for an oxidative continental weathering during deposition of the Pongola Supergroup. Titanium isotopes are not significantly fractionated in the paleosol, suggesting that continental weathering and erosion in the Archean did not fractionate Ti isotopes. Similarly, Ni/Co and Th/Sc ratios are reasonably conserved, which validates their use as a robust proxy for upper continental crust composition in shales, whereas La/Sc, Cr/Zn and Cr/U ratios are highly variable relative to the provenance composition, which suggests that caution should be used when applying these ratios in shale studies of the ancient upper continental crust composition.

Chromium mobility in ultramafic areas affected by mining activities in Barro Alto massif, Brazil: An isotopic study

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.

The fall, recovery, classification, and initial characterization of the Hamburg, Michigan H4 chondrite.

The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic 3He, 21Ne, and 38Ar are ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic 10Be concentration point to a small 40–60 cm diameter meteoroid. An 40Ar‐39Ar age of 4532 ± 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA‐ICPMS give an average Pb‐Pb age of 4549 ± 36 Ma. This is in good agreement with the average SIMS Pb‐Pb phosphate age of 4535.3 ± 9.5 Ma and U‐Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.

Mass-independent and mass-dependent Cr isotopic composition of the Rumuruti (R) chondrites: Implications for their origin and planet formation

Chromium (Cr) isotopes play an important role in cosmochemistry and planetary science because they are powerful tools for dating (53Mn–53Cr short-lived chronometry), tracing (54Cr nucleosynthetic anomalies) the origins of the materials, and studying the processes involved in volatile element fractionation and planetary differentiation (Cr stable isotopic fractionation). To use Cr isotopes for these purposes, it is essential to precisely know the compositions of the various chondritic reservoirs. However, the Cr isotope composition of Rumuruti (R) chondrites remains unknown. Here, we report high-precision mass-independent (average 2SE uncertainty of ∼0.02 and ∼0.06 for ε53Cr and ε54Cr, respectively; ε indicates 10,000 deviation) and mass-dependent (uncertainty of average 0.03‰ for δ53Cr; δ indicates 1000 deviation) Cr isotope data for 12 bulk R chondrites of petrologic types 3–6 (including R chondrite breccias), and one R chondrite-like clast (MS-CH) in the Almahata Sitta polymict ureilite. All the R chondrites show homogeneous bulk ε54Cr values, −0.06 ± 0.08 (2SD), similar only to those of the Earth–Moon system and enstatite chondrites. This first ε54Cr dataset for R chondrites provides significant addition to the ε54Cr–Δ17O diagram, and positions them as a potential endmember for planetary precursors. The R chondrites possess a higher 55Mn/52Cr of 0.68 ± 0.04 relative to most of carbonaceous chondrites and higher ε53Cr values 0.23 ± 0.05 (2SD) relative to most of chondrite groups. This likely results from the lower chondrule abundance in R chondrites compared to that of ordinary and enstatite chondrites. The stable Cr isotope composition of R chondrites is homogeneous with a δ53Cr = −0.12 ± 0.03‰ (2SD). Combined with previous data of other groups of chondrites, we show that the stable Cr isotopic composition of all the chondrites is homogeneous with δ53Cr of −0.12 ± 0.04‰ (2SD, N = 42) and is independent of the petrologic type and redox conditions. The lack of mass-dependent fractionation between all groups of chondrites suggests that the average chondrite δ53Cr value is also representative for the initial composition of all differentiated planets in the Solar System. Finally, the MS–CH clast in Almahata Sitta has a Cr isotopic composition (ε53Cr = 0.18 ± 0.04, ε54Cr = −0.16 ± 0.07, and δ53Cr = −0.11 ± 0.05‰) that is consistent (within error) with it being an R chondrite-like clast.

Redox control on chromium isotope behaviour in silicate melts in contact with magnesiochromite

Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr2+, Cr3+ and Cr6+) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 < logfO2 < −4), this study presents the first experimental petro-isotopic investigation of Cr isotope fractionation and reveals clear systematics between Cr concentration, isotopic compositions and fO2. Two series of experiments were performed to study (a) the dissolution of a natural magnesiochromite into Cr-free silicate melts (series A) and (ii) the crystallisation of magnesiochromite from Cr-doped silicate melts (series B). In agreement with previous studies, the Cr solubility in the silicate melts at equilibrium with magnesiochromite is strongly controlled by oxygen fugacity. Melts produced at low fO2 are enriched in Cr compared to more oxidised melts. In series A experiments, the Cr isotopic composition of silicate melts are lighter than the initial chromite starting material. The experiments, performed at −12 < logfO2 < −6, reveal that Cr isotopic compositions of the silicate melts are correlated with fO2. This demonstrates that, as for the Cr solubility, Cr isotopes are sensitive to fO2 and could be used to track changes in redox conditions in high-temperature processes. Furthermore, the Cr isotopic compositions of silicate melts that are reacted under more oxidising conditions (logfO2 > −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions.

The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth

Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide information about core formation, partial melting and conditions of the Moon formation. In order to detect the small Cr isotopic differences between various reservoirs in the solar system, their compositions need to be precisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolve differences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceous δ53Cr = −0.12 ± 0.05‰; ordinary δ53Cr = −0.11 ± 0.04‰). The composition of the bulk silicate Earth (BSE) is often used as a reference point for comparisons to other planetary reservoirs. However, past attempts to estimate the Cr isotopic composition of the BSE have been unable to provide a well-constrained BSE value. Traditional methods, using mantle peridotites, are affected by the susceptibility of Cr isotopes to fractionation during metasomatism. More recently, the Cr isotope composition of the BSE has been calculated using komatiites, in addition to mantle peridotites, to produce a more precise value (Sossi et al., 2018). In order to constrain the BSE composition to a higher precision, the δ53Cr of remarkably fresh komatiite lava flows from three localities, ranging in age from 2.7 Ga to 89 Ma, have been investigated in detail. These included the Tony's Flow in the Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites from Gorgona Island in Colombia.In the komatiites studied, a range in Cr isotopic compositions was found, from δ53Cr = −0.16 ± 0.02 to −0.01 ± 0.02‰. We show that the high degrees of partial melting that produced the komatiites, did not result in Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopic fractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mg content and high Cr2+/ƩCrTOT (the molar ratio of Cr2+/(Cr2+ + Cr3+)), such as Tony's Flow and Gorgona, δ53Cr increases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion of light Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fraction of the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into account when calculating the Cr isotopic composition of the source of komatiite lavas.The weighted average of δ53Cr for the komatiite lavas analysed is −0.12 ± 0.04‰ (n = 5) and represents our best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providing an improvement to the uncertainty. There is no resolvable difference between this value and that of chondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least the Archean.

Source identification of chromium in the sediments of the Xiaoqing River and Laizhou Bay: A chromium stable isotope perspective

Hexavalent chromium, Cr(VI), is a heavy metal contaminant and the reduction of Cr(VI) is accompanied by large isotopic fractionation. In this study, the sources of Cr were explored using the Cr isotopic composition of sediments from the Xiaoqing River, a heavily polluted river located in the Shandong Province of China, which flows into Laizhou Bay. The results show that δ53Cr values of the sediments are the highest upstream near the pollution source, and gradually decrease along the river toward the range for igneous reservoirs observed near the estuary. Based on the calculation of authigenic Cr isotopic composition (δ53Crauth) using the detrital index and leaching experiments, we suggest that the authigenic Cr in the sample near the pollution source with the highest δ53Crauth value mainly comes from the reduction of Cr(VI) discharged by anthropogenic activity, and authigenic Cr in other samples in the midstream with δ53Crauth values slightly higher than the range of igneous reservoirs may come from natural oxidative Cr weathering products. By introducing a Rayleigh model, we calculate that at least 31%–55% of Cr(VI) in the river water had been reduced to Cr(III) near the pollution source. Due to the self-purification ability of the river, Cr(VI) was reduced; thus, there is no record of high δ53Crauth values in the downstream of the Xiaoqing River and Laizhou Bay, indicating no obvious Cr pollution in these locations. The limited variation of δ53Cr values for samples from a sediment core in Laizhou Bay is also indicative of no obvious Cr pollution in the history. The Cr isotopic compositions of the river sediments are useful for the identification of Cr sources and can be used to advise environmental remediation on Cr pollution.

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

The application of Cr isotope compositions to the investigation of magmatic and post-magmatic effects on chromitites is unexplored. This study presents and compiles the first Cr stable isotope data (δ53Cr values) with major and trace element, contents from the Balkan Peninsula, aiming to provide an overview of the compositional variations of δ53Cr values in ophiolite-hosted chromitites and to delineate geochemical constraints controlling the composition of chromitites. The studied chromitites exhibit δ53Cr values ranging from −0.184‰ to +0.159‰, falling in the range of so-called “igneous Earth” or “Earth’s mantle inventory” with values −0.12 ± 0.11‰ to 0.079 ± 0.129‰ (2sd). A characteristic feature is the slightly positively fractionated δ53Cr values of all chromitite samples from Othrys (+0.043 ± 0.03‰), and the occurrence of a wide range of δ53Cr values spanning from positively, slightly negatively to the most negatively fractionated signatures (Pindos, δ53Cr = −0.147 to +0.009‰; Skyros, δ53Cr = −0.078 to +0.159‰). The observed negative trend between δ53Cr values and Cr/(Cr + Al) ratios may reflect a decrease in the δ53Cr values of chromitites with increasing partial melting degree. Alternatively, it may point to processes related to magmatic differentiation, as can be seen in our data from Mikrokleisoura (Vourinos).

The influence of igneous processes on the chromium isotopic compositions of Ocean Island basalts

We present the first stable chromium isotopic data for a suite of ocean island basalts (OIB) in order to investigate the Cr isotope fractionation during major igneous processes such as partial melting and fractional crystallisation. Twenty-one basaltic samples from Fangataufa Island (Tuamotu Archipelago, Pacific Ocean) have been analysed for major- and trace-element concentrations, and Sr, Nd and Cr isotopic compositions. They define two distinct series: medium to high-K calc-alcaline and low to medium-K calc-alcaline. The variations in incompatible elements such as La and Yb mostly result from varying degrees of partial melting of a mixed mantle source composed of two lithologies: garnet bearing peridotite and a recycled “fertile” component. The recycled component is also identified with the Sr and Nd isotopic composition of Fangataufa basalts. In contrast, the variations in compatible element contents such as Cr and Ni are governed by fractional crystallisation of a mixture of olivine, clinopyroxene and spinel. The samples analysed in this study are also characterised by small Cr isotope variations from −0.24 to −0.17‰. The Cr-poor samples have on average lighter Cr isotopic compositions compared to Cr-rich ones. The observed variations in the low-K series can be modelled by a Rayleigh fractionation model with a fractionation factor (Δ53Crcrystals-melt) of −0.010 ± 0.005‰. The fractionation is more limited than that observed in lunar basalts and two hypotheses may explain this observation: a change in crystallising phases (cpx + spinel on Earth and spinel on the Moon) and/or the difference in temperature and oxygen fugacity between the crystallisation of lunar and terrestrial basalts. The more primitive basalts from Fangataufa have an average Cr isotopic composition of −0.18 ± 0.01‰, lighter than the Cr isotopic composition of pristine mantle xenoliths. Chromium isotopes are therefore slightly fractionated during partial melting with the melts depleted in heavy Cr isotopes. The difference between silicate melts and mantle xenoliths indicates that partial melting could produce small but resolvable shift in mantle xenoliths Cr isotopic composition of up to 0.05‰. The covariation between δ53Cr and ε143Nd values in Fangataufa basalts can be explained by two processes: (i) lower degrees of partial melting produce lighter Cr isotopic composition in basaltic liquids relative to their sources and (ii) melts produced by low degree of partial melting have also experienced more fractional crystallisation.

A Mg(OH)2 coprecipitation method for determining chromium speciation and isotopic composition in seawater

AbstractChromium (Cr) stable isotopes have emerged as a powerful tool for tracking environmental redox transformations. This is because Cr isotopes are fractionated during redox reactions between Cr(III) and Cr(VI). In order to fully exploit the information recorded within Cr isotope compositions, we must be able to track changes in Cr speciation throughout the environment and, in particular, the changes in speciation between input to the ocean and eventual deposition in sediments. We must also be able to access the isotope compositions of each Cr species, rather than only total dissolved Cr. We have thus developed a magnesium hydroxide coprecipitation method that meets these objectives. This method achieves complete recovery and has a typical precision on concentration measurements of ±8% (1σ). It was tested using seawater collected from Saanich Inlet, a persistently anoxic fjord on the Pacific coast of Canada. Chromium speciation profiles and proof‐of‐concept isotope ratio measurements on selected samples indicate that isotopically lighter Cr(III) can be isolated from coexisting isotopically heavier Cr(VI), effectively resolving species‐specific Cr isotope compositions. While the oxygenated surface waters of Saanich Inlet follow the generally observed correlation between seawater Cr concentration and its isotopic composition, seawater from anoxic depths diverges from this array, indicating that different processes are responsible for setting the isotope composition of these deeper waters. Broader application of Mg(OH)2 coprecipitation has strong potential to yield new insights into the fractionation of Cr isotopes in the oceans and the pathways that ultimately set the Cr isotopic composition of marine sediments and sedimentary archives.

Subtle Cr isotope signals track the variably anoxic Cryogenian interglacial period with voluminous manganese accumulation and decrease in biodiversity

Earth’s atmosphere experienced a step of profound oxygenation during the Neoproterozoic era, accompanied by diversification of animals. However, during the Cryogenian period (720–635 million years ago) Earth experienced its most severe glaciations which likely impacted marine ecosystems and multicellular life in the oceans. In particular, large volumes of Mn and Fe accumulated during the interglacial intervals of the Cryogenian glaciations, indicating large anoxic marine metal reservoirs. Here we present chromium isotope-, rare earth element-, and redox-sensitive trace element data of sedimentary rocks from the interglacial Datangpo Formation deposited between the Sturtian and Marinoan glaciations in South China, in an attempt to investigate the oxidation state of the oceans and atmosphere. Both the Cr isotope and trace element data indicate mainly anoxic water conditions with cryptic oxic surface water incursions after the Sturtian glaciation. Glacial-fed manganese precipitated as manganese carbonate in anoxic basins, and the non-fractionated δ53Cr record of −0.10 ± 0.06‰ identifies anoxic conditions with a cryptic component of slightly fractionated Cr isotope composition in manganese ore, in line with distinctly fractionated Mo isotope composition. Both the manganese carbonate ore and the black shales exhibit very low redox-sensitive element concentrations. Our study demonstrates that the oxygenation of the seawater, and inferably of the atmosphere, at the beginning of the Cryogenian interglacial interval was much subdued. The post-glacial rebound then allowed the Ediacaran biological diversity.

The First Samples from Almahata Sitta Showing Contacts Between Ureilitic and Chondritic Lithologies: Implications for the Structure and Composition of Asteroid 2008 TC3.

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70-80% are ureilites (achondrites) and 20-30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal-sulfide, as well as chondrules (~130-600 μm) and chondrule fragments. The C1 material consists of fine-grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28-42), an unidentified Ca-rich silicate phase, Fe,Ni sulfides, and minor Ca-phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC-like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75-88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal-sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal-sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04-0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH- in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F-type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A-like materials, with as much as 40-70% of the latter, and <10% of OC, EC and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g/cm3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7-2.2 g/cm3). Its porosity (36%) is near the low end of estimates for the asteroid (33-50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC-like body impacted into already well-gardened ureilitic + impactor-derived debris. AhS 91A bulk samples do not show a solar wind (SW) component, so they represent sub-surface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5-9 Ma) than previously studied AhS stones (11-22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.