Precise measurement of chromium isotopes by MC-ICPMS

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 54Cr-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 [Formula: see text] 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.

Determination of mass-dependent chromium isotopic compositions in geological samples by double spike-total evaporation-thermal ionization mass spectrometry (DS-TE-TIMS)

BACKGROUND

Chromium isotopes have been used to trace geochemical and cosmochemical processes in the past. However, the presence of multivalent Cr species has made it difficult to isolate Cr from geological samples, particularly for samples with a low Cr mass fraction.

RESULTS

Here, a simple three-step ion exchange chromatography procedure is presented to separate Cr from various sample matrices, ranging from ultramafic to felsic rocks. Throughout each of the column chromatography step, 1 mL of cation exchange resin AG50W-X8 (200-400 mesh) was used as the stationary phase and oxalic acid as a chelating agent, was used in addition to the inorganic acids. This method yielded high recoveries of Cr [93 ± 8% (2SD, N = 7)] regardless of the lithology. The total procedural blank of Cr was <0.5 ng. We also developed a double spike-total evaporation-thermal ionization mass spectrometry (DS-TE-TIMS) technique that significantly reduced sample consumption to ∼20 ng of Cr per each measurement of mass-dependent 53Cr/52Cr.

SIGNIFICANCE

This study achieved a 2SD external precision of 0.02‰ for the analysis of NIST NBS3112a and of 0.01-0.07‰ for the geological samples. This study enabled high-precision Cr isotope analysis in geological samples with various matrix and Cr compositions using relatively small sample volumes.

Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae

Meteorites record processes that occurred before and during the formation of the Solar System in the form of nucleosynthetic anomalies: isotopic compositions that differ from the Solar System patterns. Nucleosynthetic anomalies are rarely seen in volatile elements such as potassium at bulk meteorite scale. We measured potassium isotope ratios in 32 meteorites and identified nucleosynthetic anomalies in the isotope potassium-40. The anomalies are larger and more variable in carbonaceous chondrite (CC) meteorites than in noncarbonaceous (NC) meteorites, indicating that CCs inherited more material produced in supernova nucleosynthesis. The potassium-40 anomaly of Earth is close to that of the NCs, implying that Earth's potassium was mostly delivered by NCs.

The Winchcombe meteorite, a unique and pristine witness from the outer solar system

Direct links between carbonaceous chondrites and their parent bodies in the solar system are rare. The Winchcombe meteorite is the most accurately recorded carbonaceous chondrite fall. Its pre-atmospheric orbit and cosmic-ray exposure age confirm that it arrived on Earth shortly after ejection from a primitive asteroid. Recovered only hours after falling, the composition of the Winchcombe meteorite is largely unmodified by the terrestrial environment. It contains abundant hydrated silicates formed during fluid-rock reactions, and carbon- and nitrogen-bearing organic matter including soluble protein amino acids. The near-pristine hydrogen isotopic composition of the Winchcombe meteorite is comparable to the terrestrial hydrosphere, providing further evidence that volatile-rich carbonaceous asteroids played an important role in the origin of Earth's water.

Ryugu's nucleosynthetic heritage from the outskirts of the Solar System

Little is known about the origin of the spectral diversity of asteroids and what it says about conditions in the protoplanetary disk. Here, we show that samples returned from Cb-type asteroid Ryugu have Fe isotopic anomalies indistinguishable from Ivuna-type (CI) chondrites, which are distinct from all other carbonaceous chondrites. Iron isotopes, therefore, demonstrate that Ryugu and CI chondrites formed in a reservoir that was different from the source regions of other carbonaceous asteroids. Growth and migration of the giant planets destabilized nearby planetesimals and ejected some inward to be implanted into the Main Belt. In this framework, most carbonaceous chondrites may have originated from regions around the birthplaces of Jupiter and Saturn, while the distinct isotopic composition of CI chondrites and Ryugu may reflect their formation further away in the disk, owing their presence in the inner Solar System to excitation by Uranus and Neptune.

Late delivery of exotic chromium to the crust of Mars by water-rich carbonaceous asteroids

The terrestrial planets endured a phase of bombardment following their accretion, but the nature of this late accreted material is debated, preventing a full understanding of the origin of inner solar system volatiles. We report the discovery of nucleosynthetic chromium isotope variability (μ⁵⁴Cr) in Martian meteorites that represent mantle-derived magmas intruded in the Martian crust. The μ⁵⁴Cr variability, ranging from -33.1 ± 5.4 to +6.8 ± 1.5 parts per million, correlates with magma chemistry such that samples having assimilated crustal material define a positive μ⁵⁴Cr endmember. This compositional endmember represents the primordial crust modified by impacting outer solar system bodies of carbonaceous composition. Late delivery of this volatile-rich material to Mars provided an exotic water inventory corresponding to a global water layer >300 meters deep, in addition to the primordial water reservoir from mantle outgassing. This carbonaceous material may also have delivered a source of biologically relevant molecules to early Mars.

Mechanisms of chromium isotope fractionation and the applications in the environment

Chromium (Cr) is a toxic heavy metal that gives rise to environmental pollution and human risk. Chromium stable isotopes have a wide range of applications in both environmental field and earth science field. In this contribution, we focus on the application of the Cr isotope in both tracing pollution sources and monitoring Cr(Ⅵ) pollution. Meanwhile, we also provide a description of the main influencing factors controlling Cr isotope fractionation, chromium isotope analytical methods, and terrestrial Cr release. Chromium isotope tracing of contaminant sources is a new application method, it has a tremendous advantage in searching for the source of Cr pollution, which has not been covered in previous reviews. At the end of the article, the current status of Cr isotope applications in the paleo-environment is explained. Although there are still some uncertainties in practical applications, chromium isotope system shows great promise in the environmental aspects.

The relationship between CM and CO chondrites: Insights from combined analyses of titanium, chromium, and oxygen isotopes in CM, CO, and ungrouped chondrites

A close relationship between CM and CO chondrites has been suggested by previous petrologic and isotopic studies, leading to the suggestion that they may originate from similar precursor materials or even a common parent body. In this study, we evaluate the genetic relationship between CM and CO chondrites using Ti, Cr, and O isotopes. We first provide additional constraints on the ranges of ε⁵⁰Ti and ε⁵⁴Cr values of bulk CM and CO chondrites by reporting the isotopic compositions of CM2 chondrites Murchison, Murray, and Aguas Zarcas and the CO3.8 chondrite Isna. We then report the ε⁵⁰Ti and ε⁵⁴Cr values for several ungrouped and anomalous carbonaceous chondrites that have been previously reported to exhibit similarities to the CM and/or CO chondrite groups, including Elephant Moraine (EET) 83226, EET 83355, Grosvenor Mountains (GRO) 95566, MacAlpine Hills (MAC) 87300, MAC 87301, MAC 88107, and Northwest Africa (NWA) 5958, and the O-isotope compositions of a subset of these samples. We additionally report the Ti, Cr, and O isotopic compositions of additional ungrouped chondrites LaPaz Ice Field (LAP) 04757, LAP 04773, Lewis Cliff (LEW) 85332, and Coolidge to assess their potential relationships with known carbonaceous and ordinary chondrite groups. LAP 04757 and LAP 04773 exhibit isotopic compositions indicating they are low-FeO ordinary chondrites. The isotopic compositions of Murchison, Murray, Aguas Zarcas, and Isna extend the compositional ranges defined by the CM and CO chondrites in ε⁵⁰Ti versus ε⁵⁴Cr space. The majority of the ungrouped carbonaceous chondrites with documented similarities to the CM and/or CO chondrites plot outside the CM and CO group fields in plots of ε⁵⁰Ti versus ε⁵⁴Cr, Δ¹⁷O versus ε⁵⁰Ti, and Δ¹⁷O versus ε⁵⁴Cr. Therefore, based on differences in their Ti, Cr, and O isotopic compositions, we conclude that the CM, CO, and ungrouped carbonaceous chondrites likely represent samples of multiple distinct parent bodies. We also infer that these parent bodies formed from precursor materials that shared similar isotopic compositions, which may indicate formation in regions of the protoplanetary disk that were in close proximity to each other.

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 massindependent 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.

Potassium isotope anomalies in meteorites inherited from the protosolar molecular cloud

Potassium (K) and other moderately volatile elements are depleted in many solar system bodies relative to CI chondrites, which closely match the composition of the Sun. These depletions and associated isotopic fractionations were initially believed to result from thermal processing in the protoplanetary disk, but so far, no correlation between the K depletion and its isotopic composition has been found. Our new high-precision K isotope data correlate with other neutron-rich nuclides (e.g., ⁶⁴Ni and ⁵⁴Cr) and suggest that the observed ⁴¹K variations have a nucleosynthetic origin. We propose that K isotope anomalies are inherited from an isotopically heterogeneous protosolar molecular cloud, and were preserved in bulk primitive meteorites. Thus, the heterogeneous distribution of both refractory and moderately volatile elements in chondritic meteorites points to a limited radial mixing in the protoplanetary disk.

The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal-rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC-like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth

Nucleosynthetic isotope variability among solar system objects provides insights into the accretion history of terrestrial planets. We report on the nucleosynthetic Fe isotope composition (μ⁵⁴Fe) of various meteorites and show that the only material matching the terrestrial composition is CI (Ivuna-type) carbonaceous chondrites, which represent the bulk solar system composition. All other meteorites, including carbonaceous, ordinary, and enstatite chondrites, record excesses in μ⁵⁴Fe. This observation is inconsistent with protracted growth of Earth by stochastic collisional accretion, which predicts a μ⁵⁴Fe value reflecting a mixture of the various meteorite parent bodies. Instead, our results suggest a rapid accretion and differentiation of Earth during the ~5-million year disk lifetime, when the volatile-rich CI-like material is accreted to the proto-Sun via the inner disk.

The origin of the unique achondrite Northwest Africa 6704: Constraints from petrology, chemistry and Re-Os, O and Ti isotope systematics

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and ¹⁸⁷Re-¹⁸⁷Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En_55-57Wo_3-4Fs_40-42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa_50-53; Fe/Mn = 1.1-2.1), chromite (Cr# ~ 0.94), awaruite, sulfides, plagioclase (Ab₉₂An₅Or₃) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1-10² °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high supersaturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re-Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O-Ti isotope systematics (Δ¹⁷O = -1.052 ± 0.004, 2 SD; ε⁵⁰Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = -2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Ultra-high-precision Nd-isotope measurements of geological materials by MC-ICPMS

We report novel techniques allowing the measurement of Nd-isotope ratios with unprecedented accuracy and precision by multi-collector inductively coupled plasma mass spectrometry. Using the new protocol, we have measured the Nd-isotopic composition of rock and synthetic Nd standards as well as that of the Allende carbonaceous chondrite. Analyses of BCR-2, BHVO-2 and GSP-2 rock standards yield mass-independent compositions identical to the JNdi-1 Nd-reference standard, with an external reproducibility of 2.4, 1.6, 1.6 and 3.5 ppm respectively, on μ142Nd, μ145Nd, μ146Nd and μ150Nd (μ representing the ppm-deviation of the ratios from JNdi-1) using 148Nd/144Nd for internal normalization. This represents an improvement in precision by a factor of 2, 7 and 9 respectively for μ142Nd, μ145Nd and μ150Nd. Near-quantitative recovery from purification chemistry and sample-standard bracketing allow for the determination of mass-dependent Nd-isotopic composition of samples. Synthetic standards, namely La Jolla and AMES, record mass-dependent variability of up to 1.2 ε per atomic mass unit and mass-independent compositions resolvable by up to 3 ppm for μ142Nd and 8 ppm for μ150Nd, relative to JNdi-1. The mass-independent compositions are consistent with equilibrium mass fractionation during purification. The terrestrial rock standards define a uniform stable ε145Nd of -0.24 ± 0.19 (2SD) relative to JNdi-1, indistinguishable from the mean Allende ε145Nd of -0.19 ± 0.09. We consider this value to represent the mass-dependent Nd-isotope composition of Bulk Silicate Earth (BSE). The modest mass-dependent fractionation of JNdi-1 relative to BSE results in potential effects on mass-independent composition that cannot be resolved within the reproducibility of our analyses when correcting for natural and instrumental mass fractionation by kinetic law, making it a suitable reference standard for analysis of unknowns. Analysis of Allende (CV3) carbonaceous chondrite returns an average μ142Nd deficit of -30.1 ± 3.7 ppm in agreement with previous studies. The apparent deficit is, however, lowered to -23.8 ± 4.0 ppm while normalizing to 148Nd/144Nd instead of 146Nd/144Nd. We interpret this as the effect of a possible nucleosynthetic anomaly of -6.3 ± 0.5 ppm in μ146Nd. As 142Nd and 146Nd are both s-process-dominated nuclides, this hints at the possibility that terrestrial μ142Nd excess may not reflect 146Sm decay as widely accepted.

Chromatographic speciation of Cr(III)-species, inter-species equilibrium isotope fractionation and improved chemical purification strategies for high-precision isotope analysis

Chromatographic purification of chromium (Cr), which is required for high-precision isotope analysis, is complicated by the presence of multiple Cr-species with different effective charges in the acid digested sample aliquots. The differing ion exchange selectivity and sluggish reaction rates of these species can result in incomplete Cr recovery during chromatographic purification. Because of large mass-dependent inter-species isotope fractionation, incomplete recovery can affect the accuracy of high-precision Cr isotope analysis. Here, we demonstrate widely differing cation distribution coefficients of Cr(III)-species (Cr(3+), CrCl(2+) and CrCl2(+)) with equilibrium mass-dependent isotope fractionation spanning a range of ∼1‰/amu and consistent with theory. The heaviest isotopes partition into Cr(3+), intermediates in CrCl(2+) and the lightest in CrCl2(+)/CrCl3°. Thus, for a typical reported loss of ∼25% Cr (in the form of Cr(3+)) through chromatographic purification, this translates into 185 ppm/amu offset in the stable Cr isotope ratio of the residual sample. Depending on the validity of the mass-bias correction during isotope analysis, this further results in artificial mass-independent effects in the mass-bias corrected (53)Cr/(52)Cr (μ(53)Cr* of 5.2 ppm) and (54)Cr/(52)Cr (μ(54)Cr* of 13.5 ppm) components used to infer chronometric and nucleosynthetic information in meteorites. To mitigate these fractionation effects, we developed strategic chemical sample pre-treatment procedures that ensure high and reproducible Cr recovery. This is achieved either through 1) effective promotion of Cr(3+) by >5 days exposure to HNO3H2O2 solutions at room temperature, resulting in >∼98% Cr recovery for most types of sample matrices tested using a cationic chromatographic retention strategy, or 2) formation of Cr(III)-Cl complexes through exposure to concentrated HCl at high temperature (>120 °C) for several hours, resulting in >97.5% Cr recovery using a chromatographic elution strategy that takes advantage of the slow reaction kinetics of de-chlorination of Cr in dilute HCl at room temperature. These procedures significantly improve cation chromatographic purification of Cr over previous methods and allow for high-purity Cr isotope analysis with a total recovery of >95%.

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

The short-lived (26)Al radionuclide is thought to have been admixed into the initially (26)Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent (54)Cr and (26)Mg*, the decay product of (26)Al. This correlation is interpreted as reflecting progressive thermal processing of in-falling (26)Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived (26)Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a (26)Mg*-depleted and (54)Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived (26)Al. The (26)Mg* and (54)Cr compositions of bulk metal-rich chondrites require significant amounts (25-50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants.