Cr Isotopes Research Articles

Radial transport and nebular thermal processing of millimeter‐sized solids in the Solar protoplanetary disk inferred from Cr‐Ti‐O isotope systematics of chondrules

AbstractUnderstanding the material transport and mixing processes in the Solar protoplanetary disk provides important constraints on the origin of chemical and isotopic diversities of our planets. The limited extent of radial transport and mixing between the inner and outer Solar System has been suggested based on a fundamental isotopic dichotomy between non‐carbonaceous (NC) and carbonaceous (CC) meteorite groups. The limited transport and mixing could be further tested by tracing the formation regions of individual meteoritic components, such as Ca‐Al‐rich inclusions (CAIs) and chondrules. Here, we show further evidence for the outward transport of CAIs and chondrules from the inner and subsequent thermal processing in the outer region of the protoplanetary disk based on the petrography and combined Cr‐Ti‐O isotope systematics of chondrules from the Vigarano‐like (CV) carbonaceous chondrite Allende. One chondrule studied consists of an olivine core that exhibits NC‐like Ti and O, but CC‐like Cr isotopic signatures, which is enclosed by a pyroxene igneous rim with CC‐like O isotope ratios. These observations indicate that the olivine core formed in the inner Solar System. The olivine core then migrated into the outer Solar System and experienced nebular thermal processing that generated the pyroxene igneous rim. The nebular thermal processing would result in Cr isotope exchange between the olivine core and CC‐like materials, but secondary alteration effects on the parent body are also responsible for the CC‐like Cr isotope signature. By combining previously reported Cr‐Ti‐O isotope systematics of CV chondrules, we show that some CV chondrules larger than ~1 mm would have formed in the inner Solar System. The accretion of the millimeter‐sized, inner Solar System solids onto the CV carbonaceous chondrite parent body would require their very early migration into the outer Solar System within the first 1 million years after the Solar System formation.

Impact of correlations on nuclear binding energies

A strong effort will be dedicated in the coming years to extend the reach of ab initio nuclear-structure calculations to heavy doubly open-shell nuclei. In order to do so, the most efficient strategies to incorporate dominant many-body correlations at play in such nuclei must be identified. With this motivation in mind, the present work analyses the step-by-step inclusion of many-body correlations and their impact on binding energies of Calcium and Chromium isotopes. Employing an empirically-optimal Hamiltonian built from chiral effective field theory, binding energies along both isotopic chains are studied via a hierarchy of approximations based on polynomially-scaling expansion many-body methods. More specifically, calculations are performed based on (i) the spherical Hartree–Fock–Bogoliubov mean-field approximation plus correlations from second-order Bogoliubov many-body perturbation theory or Bogoliubov coupled cluster with singles and doubles on top of it, along with (ii) the axially-deformed Hartree–Fock–Bogoliubov mean-field approximation plus correlations from second-order Bogoliubov many-body perturbation theory built on it. The corresponding results are compared to experimental data and to those obtained via valence-space in-medium similarity renormalization group calculations at the normal-ordered two-body level that act as a reference in the present study. The spherical mean-field approximation is shown to display specific shortcomings in Ca isotopes that can be understood analytically and that are efficiently corrected via the consistent addition of low-order dynamical correlations on top of it. While the same setting cannot appropriately reproduce binding energies in doubly open-shell Cr isotopes, allowing the unperturbed mean-field state to break rotational symmetry permits to efficiently capture the static correlations responsible for the phenomenological differences observed between the two isotopic chains. Eventually, the present work demonstrates that polynomially-scaling expansion methods based on unperturbed states that possibly break (and restore) symmetries constitute an optimal route to extend ab initio calculations to heavy closed- and open-shell nuclei.

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.

Shelf-to-basin shuttle of highly fractionated chromium isotopes in the Arctic Ocean

The oceanic chromium (Cr) cycle is mainly governed by the interconversion and the distribution of Cr(VI) and Cr(III) species and their stable isotopic ratios (δ53Cr) in the water column. As a result, the Cr cycle generates a strong correlation between the natural logarithm of its dissolved concentration and δ53Cr regardless of the location of sampling. A few studies have reported the Cr composition of certain regions falling off the global Cr array, highlighting the local prevalence of underlying mechanisms participating in the Cr cycle in the oceans. In an effort to better constrain the global Cr array, this study presents an extensive dataset for total dissolved Cr concentration ([Cr]T) and δ53Cr in the Arctic Ocean in regions meeting the environmental conditions where Cr was observed to fall off the global Cr array (e.g. continental shelves, restricted water circulation, sea ice melting). We find that more than 70% of the Arctic seawater collected plot below the global Cr array due to a small addition of highly fractionated Cr (−2.8 ‰ to −1.1 ‰) and its transport across all the Arctic regions sampled. We identify the Chukchi Shelf as the region where highly fractionated Cr is produced, from where a Cr shuttle could work in tandem with the Arctic Fe and Mn shuttles to explain the production and widespread export of isotopically light Cr in the Arctic waters. We identify a second non-reductive release of highly fractionated Cr in the Canadian Arctic Archipelago, Baffin Bay and potentially the Labrador Sea via sediment resuspension, alongside addition of isotopically light Cr originating from crustal rocks. These findings demonstrate that the Arctic-modified outflow signature of Cr isotopes modify the Cr isotopic signature of the North Atlantic waters, and that the North Atlantic waters may deviate from the global Cr array depending on whether the isotopically light Cr added to the Arctic Ocean is Cr(III) that has not been scavenged or Cr(III) that has oxidized to Cr(VI).

Oxidative weathering on the continent and seawater upwelling along the passive continental margin promoted widespread phosphorite formation at the Neoproterozoic-Cambrian boundary in South China

The latest Proterozoic to early Cambrian is characterized by major global phosphogenic events and the diversification of complex multicellular life. However, the feedback mechanism of atmospheric oxidation and upwelling processes to form voluminous phosphorite deposits is unclear. South China is a key example of the sedimentary spectrum related to the Neoproterozoic to Early Cambrian bloom of oxygenic photosynthesis, with alginite deposits (sapropelite; “stone coal”), barite, polymetallic black shales and phosphorites along the passive continental paleo-margin of the Yangtze craton. Here, we present a comprehensive geochemical analysis of drill core samples from the Zhijin phosphorite deposit, including Cr isotope, Fe speciation, and bulk elemental composition of dolostone, phosphorite and black shale from the Ediacaran to Early Cambrian sequence. The positively fractionated δ53Cr values of 0.11 to 1.81 ‰ of leachates from Ediacaran dolostone and Terreneuvian phosphorite indicate intensive oxidative terrestrial weathering. The phosphorite samples exhibit REE distribution patterns similar to modern seawater and strongly negative Ce anomalies (0.47–0.61), suggesting an oxic, open ocean environment. In contrast, the absence of Ce anomaly, high FeHR/FeT, and Fepy/FeHR ratios, enhanced redox-sensitive element concentrations, high Mo/TOC and TS/TOC, and high molar Corg:P ratios suggest euxinic to anoxic conditions for the overlying black shale sedimentation in a restricted sedimentary marine environment. Bulk-rock shale samples retain the positively fractionated Cr isotope pattern. The dramatic change from oxic to anoxic conditions aligns with the fossil record, which shows a proliferation of small shelly fossils in the phosphorite and their extinction in the overlying black shales, likely by hydrogen sulfide poisoning. We infer that nutrient-rich, oxidized upwelling seawater transported phosphorus and other nutrients to the photic zone, from where organic matter settled down to the seafloor and formed thick phosphorite beds at the oxic-anoxic boundary at and below the seafloor. Our study highlights that atmospheric and seawater oxygenation accelerated the Early Cambrian bio-diversification, and the subsequent restricted basin situation is responsible for the extinction at the Neoproterozoic-Cambrian boundary.

Isotopic evidence for a common parent body of IIG and IIAB iron meteorites

Magmatic iron meteorites are thought to sample the metallic cores of differentiated planetesimals and are subdivided into several chemical groups, each representing a distinct parent body. The only exceptions are the groups IIAB and IIG, which have been proposed to sample two immiscible melts from the same core. To test this model, we report the first Fe, Ni, O, and Cr isotope data for IIG iron meteorites and the first high-precision O isotope data for IIAB iron meteorites. The new data demonstrate that IIG iron meteorites belong to the non-carbonaceous (NC) meteorites. This is evident from the isotope anomaly of each of the four elements investigated, where the IIG irons always overlap with the compositions of NC meteorites but are distinct from those of carbonaceous (CC) meteorites. Moreover, among the NC meteorites and in particular, the NC irons, the isotopic composition of the IIG irons overlaps only with that of the IIAB irons. The combined Fe-Ni-O-Cr isotope data for IIAB and IIG iron meteorites, therefore, reveal formation from a single isotopic reservoir, indicating a strong genetic link between the two groups. The indistinguishable isotopic composition of the IIAB and IIG irons, combined with chemical evidence for the formation of IIG irons as late-stage liquids of the IIAB core, strongly suggests that both groups originate from the same core. The results underscore the strength of utilizing multiple elements and their isotopic compositions to establish genetic links among meteorites, rather than using a single element. They also highlight the significance of integrating multiple geochemical tracers and petrologic observations to accurately determine genetic relationships and the formation of meteorites within the same parent body.

Lithium isotopic records of anthropogenic activity in the Xiaoqing River basin, eastern China

The environmental impact of the discharge of lithium (Li) by anthropogenic activity has been overlooked. By analyzing Li concentrations and isotope compositions (δ7Li) of water and sediment samples, this study evaluates the influence of anthropogenic Li discharge on the Xiaoqing River and Laizhou Bay, which are heavily polluted areas in China. High Li concentrations of the river water (up to 7.8 μmol/L) should be linked to anthropogenic Li discharge. However, no profound δ7Li anomalies were observed, preventing identification of the exact discharge sources. In the river sediments, Li concentrations (19.0–45.0 μg/g) were weakly correlated with Zn, Cu, and Cr concentrations, whereas δ7Li values ranged from 0.6 ‰ to 13.9 ‰ with high values being accompanied by high contents of total organic carbon and heavy Cr isotope compositions (δ53Cr). All these point to significant influence of anthropogenic activity on the Li budget of river sediments. A simple mass balance calculation indicates that smelters, Li-bearing therapeutic drugs, and secondary Li-ion batteries are the main anthropogenic Li sources. In contrast to river sediments, marine sediments in the Laizhou Bay were broadly homogeneous at both spatial and temporal scales, indicating no significant influence of anthropogenic Li discharge. Overall, our data indicate that Li isotope systematics in river sediments, especially sediments near intense anthropogenic activity, are effective at tracing potential Li pollution and can help obtain accurate results for environmental inspection.

Impactor identification with spallogenic Cr isotopes: The Wabar impact craters (Saudi Arabia)

AbstractPrecise measurements of Cr isotopic composition of terrestrial impactites have successfully provided evidence for the presence of extraterrestrial material and have, in some cases, allowed the identification of the type of impactor responsible for the formation of the impact structure. The high Cr abundance in most meteorite groups aids in detecting extraterrestrial contamination while their distinct isotopic compositions can help with the identification of the nature of the projectile. However, this common approach of detection and identification of extraterrestrial contamination using mass‐independent 53Cr and 54Cr variations fails when the impactor type is an iron meteorite because of their low Cr abundances (which are in a similar range to terrestrial rocks). The present study demonstrates the viability of a spallogenic Cr contribution in iron meteorites (resulting from their long cosmic ray exposure times), which compensates for their low Cr abundances and facilitates the identification of iron‐meteoritic contamination in terrestrial impactites. Thus, it broadens the scope of impactors (and impactites) that can be investigated using mass‐independent Cr isotopes from solely chondrites and primitive achondrites to include iron meteorites. The Wabar impact craters are an optimal candidate for this study, characterized by low weathering, diverse impactites, partial meteorite survival, substantial impactor material contamination, and a felsic target lithology with low background Cr concentration. The Cr isotopic composition of the Wabar background sand, which represents the target lithology, is indistinguishable from the terrestrial Cr isotopic composition range, whereas the Wabar iron meteorites show coupled spallogenic excesses in ε53Cr and ε54Cr. The Cr isotopic compositions of Wabar impactites show resolved deviations from the terrestrial Cr isotopic composition, thereby indicating the presence of Wabar meteoritic contamination. Moreover, the study demonstrates that even an impactor with a non‐carbonaceous chondritic origin, such as a IIIAB iron meteorite, can have a carbonaceous chondrite‐like signature in ε54Cr anomalies due to spallogenic Cr contamination. The study advocates for a comprehensive investigation combining platinum group elements and Cr (and/or Ni, Ru) isotopes to accurately characterize impactor types.

Identification of Earth's late accretion by large impactors through mass independent Cr isotopes

The late addition of extra-terrestrial material may represent an important source of Earth's volatiles. The composition of impactors can be reconstructed using 54Cr abundances in impact related rocks preserved in the terrestrial record. The average ε53Cr and ε54Cr of Earth's mantle determined from mantle rocks of 3.8 Ga to present are 0.03 ± 0.02 and 0.08 ± 0.04, respectively. Impact melt rocks and spherule beds linked to impact structures larger than 50 km that formed between 3.4 Ga and 66 Ma have ε53Cr ranging from −0.04 to 0.17, and ε54Cr ranging from −0.64 to 1.41. A carbonaceous chondrite-like impactor contribution dominated the Meso- to Paleoarchean spherule layers (> 3.0 Ga), whereas a mixed chondrite flux composed of carbonaceous and non‑carbonaceous chondrites, with a possible contribution of differentiated meteorites is observed in the younger spherule layers (2.5 Ga to 66 Ma). This likely reflects the break-up of distinct asteroid families through time. Although available impact materials are limited, the Cr isotope signatures of materials related to large impacts suggest a change in the composition of crater-forming impactors on Earth, from predominantly carbonaceous chondrite-like more oxidized material during the Archean to predominantly non‑carbonaceous -like more reduced and volatile poor material in recent times. Chromium isotopes suggest that not more than 0.01 wt% of CC-like material added to the Earth's mantle after the Archean. Thus, it is inferred that the mass accreted after 3.0 Ga contributed insignificantly to the water and other volatile element budget of the Earth.

The anomalous polymict ordinary chondrite breccia of Elmshorn (H3‐6)—Late reaccretion after collision between two ordinary chondrite parent bodies, complete disruption, and mixing possibly about 2.8 Gyr ago

AbstractElmshorn fell April 25, 2023, about 30 km northwest of the city of Hamburg (Germany). Shortly after the fall, 21 pieces were recovered totaling a mass of 4277 g. Elmshorn is a polymict and anomalous H3‐6 chondritic, fragmental breccia. The rock is a mixture of typical H chondrite lithologies and clasts of intermediate H/L (or L, based on magnetic properties) chondrite origin. In some of the 21 pieces, the H chondrite lithologies dominate, while in others the H/L (or L) chondrite components are prevalent. The H/L chondrite assignment of these components is based on the mean composition of their olivines in equilibrated type 4 fragments (~Fa21–22). The physical properties like density (3.34 g cm−3) and magnetic susceptibility (logχ <5.0, with χ in 10−9 m3 kg−1) are typical for L chondrites, which is inconsistent with the oxygen isotope compositions: all eight O isotope analyses from two different fragments clearly fall into the H chondrite field. Thus, the fragments found in the strewn field vary in mineralogy, mineral chemistry, and physical properties but not in O isotope characteristics. The sample most intensively studied belongs to the stones dominated by H chondrite lithologies. The chemical composition and nucleosynthetic Cr and Ti isotope data are typical for ordinary chondrites. The noble gases in Elmshorn represent a mixture between cosmogenic, radiogenic, and primordially trapped noble gases, while a solar wind component can be excluded. Because the chondritic rock of Elmshorn contains (a) H chondrite parent body interior materials (of types 5 and 6), (b) chondrite parent body near‐surface materials (of types 3 and 4), (c) fragments of an H/L chondrite (dominant in many stones), (d) shock‐darkened fragments, and (e) clasts of various types of impact melts but no solar wind‐implanted noble gases, the different components cannot have been part of a parent body regolith. The most straightforward explanation is that the fragmental breccia of Elmshorn represents a reaccreted rock after a catastrophic collision between an H chondrite parent body and another body with H/L (or L) chondrite characteristics but with deviating O isotope values (i.e. that of H chondrites), complete disruption of the bodies, mixing, and reassembly. This is the only straightforward way that the implantation of solar wind gases could have been avoided in this kind of complex breccia. The gas retention ages of about 2.8 Gyr possibly indicate the closure time after the catastrophic collision between H and H/L (or L) chondrite parent bodies, while the cosmic ray exposure age for Elmshorn, which had a preatmospheric radius of 25–40 cm, is ~17–20 Myr.

Surface water oxygenation and low bioproductivity during deposition of iron formation of the Jacadigo Group (Brazil): Insights from combined cadmium – Chromium isotopes

The Banda Alta Formation (Urucum district, Mato Grosso do Sul, Brazil) comprises ∼600 Ma Fe and Mn deposits, which are among the world's youngest and largest Neoproterozoic sedimentary Fe and Mn formations (IF; MnF). These have been deposited in a redox-stratified, marine sub-basin (Jacadigo Basin), which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling of metal-enriched deep anoxic seawater. Cr and Cd isotopes measured on meticulously separated hematite mesobands from drill core samples are relatively homogenous throughout the ca. 325 m thick sequence sampled in the Banda Alta Fm., with average authigenic δ53Cr values of +0.93 ± 0.24 ‰ (2σ; n = 23) and δ114Cd values of −0.14 ± 0.14 ‰ (2σ; n = 15). The significant enrichment of Cr, in parallel with the strong enrichments of other redox sensitive elements (U, Mo), attests for effective and efficient reduction removal processes in the surface waters during cycles where upwelling Fe2+-rich waters reached the oxygenated surface layer exposed to the atmosphere during episodic glacier retreat stages. Assuming a similar quantitative and efficient removal pathway of dissolved Cd by iron oxyhydroxides, the so-inferred average δ114Cd signature of −0.14 ± 0.14 ‰ in the Jacadigo Basin surface water is significantly lower than signatures of modern ocean surface waters with a range of δ114Cd of ca. ∼0.4 to ∼1 ‰ and even lower than the signature of modern ocean deep waters with δ114Cd of ∼0.3 ‰. It possibly attests to reduced primary production levels and lower nutrient utilization rates during deposition of the Late Neoproterozoic Jacadigo Group, compared to today. This despite the inferred oxidized surface water layer that must have prevailed during this time, as implied by the strongly positively fractionated Cr isotope signatures and pronouncedly negative Ce-anomalies recorded in the seawater-like, shale-normalized Rare Earth Element and Yttrium (REY) patterns exhibited by the hematite mesobands. Data presented herein speak for: (1) a stable, isotopically heavy Cr input to the Jacadigo Basin at the time of deposition, implying high atmospheric O2 levels in the Late Neoproterozoic (2) likely quantitative, reductive incorporation / adsorption processes of dissolved Cr and Cd, respectively, into/onto precipitating iron oxyhydroxides, and (3) the prevalence of low nutrient concentrations and utilization rates in the Jacadigo Basin during glacier retreat cycles. Banded iron formations are considered suitable archives for reconstructing redox and bioproductivity levels in past marine depositional basin, including those prevalent in Neoproterozoic glacial conditions, via employing the CrCd isotope double tracer to iron-rich mesobands.

Unraveling the Cr Isotopes of Ryugu: An Accurate Aqueous Alteration Age and the Least Thermally Processed Solar System Material

The analysis of samples returned from the C-type asteroid Ryugu has drastically advanced our knowledge of the evolution of early solar system materials. However, no consensus has been obtained on the chronological data, which is important for understanding the evolution of the asteroid Ryugu. Here, the aqueous alteration age of Ryugu particles was determined by the Mn–Cr method using bulk samples, yielding an age of 4.13 + 0.62/−0.55 Myr after the formation of Ca–Al-rich inclusions (CAI). The age corresponds to 4563.17 + 0.60/−0.67 Myr ago. The higher 55Mn/52Cr, ε 54Cr, and initial ε 53Cr values of the Ryugu samples relative to any carbonaceous chondrite samples implies that its progenitor body formed from the least thermally processed precursors in the outermost region of the protoplanetary disk. Despite accreting at different distances from the Sun, the hydrous asteroids (Ryugu and the parent bodies of CI, CM, CR, and ungrouped C2 meteorites) underwent aqueous alteration during a period of limited duration (3.8 ± 1.8 Myr after CAI). These ages are identical to the crystallization age of the carbonaceous achondirtes NWA 6704/6693 within the error. The ε 54Cr and initial ε 53Cr values of Ryugu and NWA 6704/6693 are also identical, while they show distinct Δ'17O values. This suggests that the precursors that formed the progenitor bodies of Ryugu and NWA 6703/6693 were formed in close proximity and experienced a similar degree of thermal processing in the protosolar nebula. However, the progenitor body of Ryugu was formed by a higher ice/dust ratio, than NWA6703/6693, in the outer region of the protoplanetary disk.

Combined cadmium and chromium isotopes record a collapse of bioproductivity across the Cretaceous—Paleogene boundary in the Danish basin

Cadmium (Cd) isotope signatures (δ114Cd) deduced from modern and ancient marine deposits are often used as a proxy for bioproductivity and for micronutrient cycling, whereas chromium (Cr) isotope signatures (δ53Cr) are used as a potential proxy for oxygenation. The Cretaceous—Paleogene (K/Pg) boundary records one of Earth's five big mass extinctions, which contains geochemical fingerprints of the Chixculub asteroid impact and is coeval with the Deccan Trap volcanism. Here we present for the first time a combined record of Cd and Cr isotopes from the Danish Basin to interrogate the effects on bioproductivity across the K/Pg boundary. The δ114Cd values range between −0.28‰ and 0.27‰ defining an average of 0.05‰ ± 0.24 (2σ, n = 87). Authigenic δ53Cr values range between 0.06‰ and 1.06‰ and define an average of 0.59‰ ± 0.51 (2σ, n = 81). An abrupt jump, across the K/Pg boundary layer, from consistently positively fractionated Cd and Cr isotope signatures recorded in Late Maastrichtian white chalks towards less positively fractionated δ114Cd values and heterogeneous δ53Cr values in the Early Danian carbonates, likely indicates a decrease in surface water bioproductivity. This decrease is accompanied by redox fluctuations in the water column as a consequence of environmental changes either related to the Chixculub impact event and/or to the Deccan volcanic eruptions coeval with this boundary. Assuming an efficient removal of Cd and Cr through consumption and/or adsorption by primary producers, or by incorporation into the skeleton/shells of calcifiers, and using the fractionation factor of Cd into carbonates, we reconstruct surface water conditions during the Late Masstrichtian—Early Danian that are compatible with those of modern oceans, with δ114Cd varying from 0.17 to 0.72‰. Overall, the combined Cr-Cd isotope fluctuations over the K/Pg boundary are consistent with a scenario characterized by climate-induced changes of nutrient availability and concomitant responses of primary production levels, ultimately related to the asteroid impact and to the proceeding volcanic activities that influenced the ocean's chemistry. We anticipate our results to point for further studies of other stratigraphic sections worldwide to reveal an extensive Cd-Cr record for the Cretaceous—Paleogene boundary and its corresponding collapse of bioproductivity and decreasing ocean oxygenation.

Stable Chromium Isotope Fractionation During the Alteration of Abyssal Peridotite: Implications for Marine Chromium Isotope Mass Balance

AbstractThe stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater‐peridotite reaction on the global marine Cr isotope mass balance, we report high‐precision Cr isotope data (δ53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a δ53Cr value of −0.08 ± 0.06‰ (2SD, n = 4) for the oceanic mantle which is consistent with the established δ53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive δ53Cr values (up to 0.04‰). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of ∼3.5 × 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108–109 mol/yr. Furthermore, the MC results suggest that the peridotite‐sourced net Cr flux has a negative δ53Cr signature (−0.33 ± 0.21‰, 2SD). Thus, seawater‐peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance.

LATE ARCHEAN SHELF-TO-BASIN IRON SHUTTLE CONTRIBUTES TO THE FORMATION OF THE WORLD-CLASS DATAIGOU BANDED IRON FORMATION

Abstract Banded iron formations (BIFs) are among the few chemical sedimentary archives that capture the biogeo-chemical evolution of Fe cycling and the redox evolution of the early Earth. Although biologically recycled continental Fe has been previously proposed to be a significant source of Fe in BIFs deposited from a stratified ocean at the onset of the Great Oxidation Event (GOE; ~2.5–2.2 Ga), constraining Fe sources and pathways in Archean BIFs remains challenging. Here we present major and trace element and Fe-Nd-Cr isotope data for the largest BIF (i.e., Dataigou) in China to test whether a benthic Fe shuttle was operative during deposition of pre-GOE BIFs. The absence of true, shale-normalized Ce anomalies, coupled with unfractionated Cr and positive Fe isotope compositions, suggests that BIF deposition occurred in an anoxic water column under reducing atmospheric conditions, whereas positive Eu anomalies indicate a significant input from a high-temperature hydrothermal source. Based on a significant correlation between initial Nd and Fe isotope data, we suggest that two Fe sources were periodically mixed and resulted in deposition of the Dataigou BIF. Here, we suggest the following sources: (1) hydrothermal fluids from sea-floor systems (low εNd(t) and high δ56Fe), derived from the interaction of fluids with underlying, older continental crust, and (2) a benthic Fe flux (high εNd(t) and low δ56Fe), generated by microbial Fe(III) reduction in coastal sediments during weathering of a nearby depleted landmass. Results presented here confirm, for the first time, that a microbially driven Fe shuttle was operational and supplied Fe on a basin-wide scale in the absence of atmospheric oxygen.

Modeling of Cosmogenic Cr Isotopes Produced in Lunar Rocks Compared With Existing Calculations and Measurements

AbstractThe nucleosynthetic Cr isotope anomalies provides useful information to trace the source and origin of extraterrestrial samples, but it is usually influenced by high‐energy cosmic rays, and evaluating such effect of cosmic rays in lunar samples is especially important. Those cosmic radiation particles (primary particles) can react with lunar materials, creating many secondary particles. Both primary and secondary particles can produce cosmogenic nuclides on the Moon. Radiation Environment and Dose at the Moon (REDMoon) is a novel GEANT4 Monte‐Carlo model built to simulate the interactions of space particles with the lunar surface and subsurface content. Using this model, we simulate the production of cosmogenic Cr isotopes (50Cr, 52Cr, 53Cr, 54Cr) at different depths of lunar surface, and compare the contribution of different reactions generating these nuclides. The results suggest that spallation reactions are the most important process producing cosmogenic Cr isotopes. We also analyze the relationship between 53Cr/52Cr and 54Cr/52Cr predicted by our model and compare it with different Apollo samples. As previously studied, we also find an approximate linear relationship between ɛ53Cr and ɛ54Cr, where ɛ53Cr (or ɛ54Cr) is the relative deviation from the standard 53Cr/52Cr ratio (or 54Cr/52Cr ratio), normalized to 1/10,000. Furthermore, we reveal a change of this linear relationship in different depths of lunar surface. Besides, we investigate how the slopes can be influenced by exposure age and the Fe/Cr ratio. With these additional factors carefully considered, the comparison between our modeled results and the measurements is better than previous studies.

Effects of Faraday cup deterioration on Sr and Cr isotope analyses by thermal ionization mass spectrometry.

By comparing data from an extensive set of Sr and Cr isotope measurements performed on two different thermal ionization mass spectrometers (TIMS), using three sets of Faraday cups with different usage histories, we assess the effects of Faraday cup deterioration on high-precision isotope measurements by TIMS. We find that dynamic 84Sr/86Sr and 87Sr/86Sr measurements provide stable and reproducible results over the entire 56 months of this study, regardless of which set of Faraday cups is used. By contrast, static 84Sr/86Sr and 87Sr/86Sr measurements lead to deviant results, drifts over time, and in general exhibit larger scatter. For the most part, these differences can be attributed to changing Faraday cup efficiencies. For the instruments of this study we find that the center cup is most affected, consistent with this cup often receiving the highest ion beam intensities during measurements conducted in our laboratory. For Cr isotopes, we find that the correlation between mass fractionation-corrected 53Cr/52Cr and 54Cr/52Cr ratios observed for static measurements in several prior studies also reflects different Faraday cup efficiencies. Again, the changing efficiency of predominantly the center cup can account for the observed drift and correlation in 53Cr/52Cr and 54Cr/52Cr. Multi-static Cr isotope measurements reduce this drift, but still result in a residual correlation between the two ratios, suggesting this correlation in part also reflects unaccounted mass fractionation effects.

Role of redox conditions and thermal metamorphism in the preservation of Cr isotopic anomalies in components of non-carbonaceous chondrites

Mass-independent isotopic anomalies in meteorites are probes to the dynamics and evolution of the protoplanetary disc and the reservoirs from which planetary bodies accreted. Variable Cr nucleosynthetic compositions between meteorite groups are observed but the cause is not well understood. Investigations on presolar carriers in unequilibrated chondrites are thus required to establish these relationships. For that purpose, we analysed the Cr isotope compositions of leachates (stepwise dissolution solutions) of one ordinary chondrite and two enstatite chondrites of EL and EH subgroups previously measured for Nd and Sm isotopes. The leachates of the ordinary chondrite display large variations of their Cr isotope compositions, whereas the leachates of the enstatite chondrites show very limited variations of their nucleosynthetic compositions, confirming other studies. The Cr isotope compositions of leachates of unequilibrated chondrites are significantly modified by thermal processing and aqueous alteration under various conditions. The leachates of the enstatite chondrites are relatively homogeneous for Cr isotope composition, suggesting that presolar carriers of Cr are not stable under very reduced conditions and are likely oxidised phases. After careful examination of the role of parent body processing, we conclude that there is one predominant presolar carrier enriched in 54Cr that produced most of the nucleosynthetic variations observed in leachates. This carrier is likely depleted in the carbonaceous reservoir relative to the non-carbonaceous reservoir. The drastically different isotope compositions of Cr and heavy refractory lithophile elements such as Nd of leachates of chondrites reflect relative preservation biases of their respective presolar carriers, namely a still poorly characterised presolar oxide and silicon carbide (SiC), respectively.

Fractionation of iron and chromium isotopes in hydrothermal plumes from the northern Mid-Atlantic Ridge

Hydrothermal venting impacts the global-scale biogeochemical cycles of many trace metals and their isotopes. Processes in hydrothermal plumes regulate the dispersal of vent-derived metals and may vary in response to differences in the geologic setting of vent fields and/or the geochemistry of the overlying ocean water. Here we present results of analyses of dissolved Fe and Cr concentrations, and dissolved Fe isotope (δ56Fe) and Cr isotope (δ53Cr) distributions, in seawater samples collected from above TAG and Rainbow vent sites on the Mid-Atlantic Ridge during the GEOTRACES GA13 cruise. We show that profiles of dissolved Fe and Cr isotopes through the near-field hydrothermal plumes are the mirror image of each other. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low δ56Fe values of dissolved Fe, as low as −1.83 ‰ at TAG and −6.94 ‰ at Rainbow. Plume samples with low δ56Fe values are associated with elevated δ53Cr values of dissolved Cr compared to background seawater (by up to +0.14 ‰ and +0.69 ‰ at TAG and Rainbow, respectively), while particulate Cr is characterised by relatively low δ53Cr values (−1.02 to −1.22 ‰). This striking result suggests that seawater Cr(VI) is reduced to Cr(III) and precipitates on the surface of Fe(III) particles within the hydrothermal plume. Reduction of Cr(VI) and scavenging of Cr(III) by plume Fe-(oxyhydr)oxide particles mean that high-temperature hydrothermal systems are likely a net sink for seawater Cr at Rainbow (and also at TAG). As the removal flux of Cr is related to the flux of hydrothermal Fe(II) and the rate of Fe(II) oxidation in the hydrothermal plume, it may (i) vary across vent sites at a global scale and (ii) change over glacial-interglacial cycles.

Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample

Studies of material returned from Cb asteroid Ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. Isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. Here, we show that isotopic anomalies for mildly soluble Cr are highly variable in Ryugu and CI chondrites, whereas those of Ti are relatively uniform. This variation in Cr isotope ratios is most likely due to physicochemical fractionation between 54 Cr-rich presolar nanoparticles and Cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred 5.2 − 1.4 + 1.8 Ma after Solar System birth. In contrast, Ti isotopes were marginally affected by this process. Our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water.