The Equatorial Pacific Ocean (EPO) is one of the most productive regions in the ocean with upwelling of nutrient-rich subsurface waters and strong outgassing of CO2, and thereby playing a prominent role in the global carbon cycle. We report distributions of dissolved (d) and particulate (p) trace metals (TM) including nickel (dNi), copper (dCu), zinc (dZn) and cadmium (dCd), along the GEOTRACES transect, GP11, in the EPO during a developing El Niño phase, associated with anomalous surface winds and currents. Using estimates of surface dTM fluxes and water mass mixing in deeper waters, we assessed biogeochemical controls on dTM distributions in the water column. Surface waters (depths < 100 m) showed dTMs minima and significant correlations between pTM and particulate phosphorus, indicating phytoplankton uptake of TMs and low atmospheric input. A large proportion (64–99%) of dNi, dZn and dCd in the surface mixed layer (24–51 m) was supplied by vertical upwelling, which was strongly influenced by the west-east shoaling of the thermocline and transport of sub-surface waters by the Equatorial Undercurrent. Moreover, spatial variability in vertical fluxes controlled biological Cd uptake in the EPO. In mesopelagic and deep waters (depths > 200 m), water mass mixing and organic matter (OM) remineralization largely governed the dCd distribution. The dNi, dCu and dZn distributions and their relationships with macronutrients were determined by particle scavenging of remineralized Ni, Cu and Zn and/or their retention (slower remineralization) to particles. Strong enrichments in dNi and dCu were observed in deep waters (depths > 2500 m) of the eastern and central EPO, which could not be explained by water mass mixing and OM remineralization. Particulate data indicated a limited influence of reversible scavenging and deep-water remineralization, with the supply from sediments likely supporting the dNi and dCu increases in deep waters. Our results underscore the need to better quantify benthic trace metal fluxes across Pacific abyssal sediments, as sedimentary inputs may substantially influence deep ocean micronutrient reservoirs and their eventual redistribution to the upper ocean.
Understanding how critical metals such as platinum (Pt) are enriched in marine environments is essential for advancing knowledge of their geochemical cycles and the formation of oceanic mineral resources. Pt occurs in seawater at picomolar concentrations, yet deep-sea ferromanganese (FeMn) deposits contain up to eight orders of magnitude more Pt. The mechanism behind this enrichment remains unresolved. Although FeMn deposits exhibit high Pt partitioning, their low Pt levels and the co-occurrence of multiple elements pose challenges to investigating Pt oxidation state and uptake processes using traditional spectroscopic methods. In this study, high-energy-resolution X-ray absorption spectroscopy (XAS) was used to elucidate how Pt is sequestered at the atomic scale in FeMn crusts and nodules from the Pacific, Atlantic, and Indian oceans. Pt is present in the tetravalent form, resulting from the oxidation of dissolved divalent Pt in seawater upon interaction with Mn oxides. Tetravalent Pt is precipitated as α-PtO2 nanoparticles with longer Pt-Pt distances than in well-crystallized α-PtO2. Density functional theory calculations indicate that the local structure of Pt is best described by α-PtO2 layers topotactically stacked on vernadite phyllomanganates, similar to (Co,Ni)-asbolane. The Pt content depth profiles are constant during spot analyses by laser ablation-inductively coupled plasma-mass spectrometry, suggesting that the Pt nanoparticles are uniformly dispersed within the ablated volume, rather than occurring as discrete metallic nuggets. These findings reveal a new pathway for Pt immobilization in marine environments and suggest that thermodynamically stable α-PtO2 serves as a long-term sink for oceanic Pt. The advanced capabilities of high-energy-resolution XAS offer new opportunities to deepen understanding of how trace metals are sequestered in structurally disordered and chemically complex natural materials, with potential applications in georesource exploration.
The India-Asia convergence generated a series of porphyry deposits formed in the post-collisional stage, yet the enrichment mechanisms of critical ore-forming elements such as chlorine (Cl) remain poorly understood. Multi-isotope systems, particularly Cl isotopes, offer a powerful tool to trace these material cycling processes. However, the limited understanding of the δ37Cl values of collisional zones has hindered our understanding of Cl cycling. To address this, we systematically collected ore-forming porphyries, granitoid batholiths, a deep crustal granulite xenolith, an ultramafic cumulate, and an ultrapotassic volcanic rock from the southern Tibetan Plateau. Apatite Cl and Nd isotopes, along with whole-rock boron (B) isotopes, were analyzed to constrain the Cl isotope compositions of various endmembers and to further trace the origin of Cl for porphyry ore-forming magmas. From the Late Cretaceous to the Oligocene, the Zedong granitoid batholiths exhibit progressively enriched apatite εNd(t) and decreasing apatite δ37Cl, indicating increasing involvement of a low-δ37Cl enriched crustal component. Two components are therefore recognized in the collisional crust: a juvenile, depleted crust with high δ37Cl (+0.62 ± 0.28 ‰), and a reworked, enriched crust with relatively low δ37Cl (−0.51 ‰). The post-collisional Qulong and Jiama porphyry deposits show significantly higher apatite δ37Cl values (Qulong: +0.88 ± 0.42 ‰; Jiama: +0.72 ± 0.61 ‰) compared to the Zedong granitoid batholiths (+0.11 ± 0.44 ‰), yet similar to those of ultrapotassic magmas (+0.85 ± 0.10 ‰) and the juvenile crust (+0.88 ± 0.46 ‰). Combined with Nd isotope evidence, we suggest that metasomatized lithospheric mantle (represented by the ultrapotassic magmas) and juvenile crustal components are the dominant sources of Cl in the collisional porphyry ore-forming magmas.
Concretions are lithified bodies found in sedimentary rocks often formed as a result of microbial decay of an organic carbon source thereby promoting carbonate authigenesis. Prior studies have demonstrated that this process can enhance the preservation of biological and geochemical signals relative to the unconsolidated host sediment. What remains uncertain is why certain locations or geologic time periods appear to be particularly conducive toward concretion formation. Here we present an integrated radiocarbon, carbonate clumped isotope, and stable carbon isotope analysis of concretions from three Late Quaternary-age deposits across West Greenland and Canada which have encapsulated fish remains. These concretions contain capelin, Mallotus villosus, a small forage fish whose spawning leads to mass mortalities of males. Microbial decay of the resultant carrion generates alkalinity that can stimulate concretion formation. However, despite the periodic nature of the capelin lifecycle, concretion assemblages do not occur after every post-spawning mortality event. Carbon isotope values (–19.8 to –14.0‰) indicate that alkalinity was derived from capelin carrion via bacterial sulfate reduction rather than methanogenesis. Radiocarbon ages of concretion carbonate date the fish-derived carbon and variable contributions of 14C-depleted (old) carbon from the marine reservoir and therefore provide maximum age constraints on formation timing. After applying appropriate site-specific marine reservoir corrections, ages subsequently converge with independent stratigraphic chronologies at each location. Clumped isotope analysis and reconstructed precipitation temperatures from West Greenland reflect regional oxygen isotope and temperature trends associated with cooling, increasing aridity/evaporation, and glacier stillstands during the 8.2-kyr climate anomaly. Results from Canada reveal a record of glacial freshening and retreat of the Champlain Sea coinciding with a ∼60 m decrease in sea level together with anomalously cool and arid periods. We propose a two-factor model in which cryogenic and/or evaporative concentration of dissolved inorganic carbon during cold-climate intervals, coupled with transient microbial alkalinity pulses from capelin mass mortalities, mediated rapid syn-sedimentary concretion formation. Our work further suggests that geochemical records preserved within carbonate concretions may be utilized to supplement and possibly reconstruct past climate events.
Thermodynamic constraints shape the degradation and persistence of dissolved organic matter (DOM) in aquatic environments. Predicting the Gibbs energy of the oxidation half reaction per mole of carbon (ΔG°(Cox)) is therefore central to assessing the thermodynamic potential of organic matter degradation. A widely used approach is the linear relationship between nominal oxidation state of carbon (NOSC) and ΔG°(Cox), introduced by LaRowe and Van Cappellen, which provides a convenient and versatile single-proxy estimate. However, NOSC is a stoichiometric descriptor and assigns identical ΔG°(Cox) values to formulae that reflect structures differing by more than 90 kJ molC-1. This underlying diversity of structures can be isomeric, but can also reflect variation in molecular size, heteroatom composition, hydrogen abundance, and level of unsaturation. Here we present a structure-informed Random Forest model for ΔG°(Cox) trained on a curated set of over 3,500 quantum-chemically calculated DOM-relevant molecules spanning diverse elemental compositions and structures. The model was trained using formula-derived descriptors, including elemental ratios, molecular mass (Mr), double bond equivalence (DBE), heteroatom content, NOSC and others. Predictions are made at the molecular level and represent an average across potential isomers of a formula. It relies on ΔG°(Cox) data derived with the Becke three-parameter Lee-Yang-Parr density functional (B3LYP) and captures thermodynamic variability that the NOSC-based linear relationship systematically misses. As proof of concept, the model was applied to ultra-high resolution mass spectrometry (UHR-MS) data for four benchmark DOM reference materials, yielding ΔG°(Cox) distributions that are broader and multimodal compared to the NOSC-based estimates. These results demonstrate that machine learning models grounded in quantum chemical data provide a more differentiated thermodynamic representation of DOM formula space and support improved prediction of organic matter oxidation potential.
Multicomponent diffusion in natural silicate melts controls many igneous processes such as magma mixing and crystal growth. In N-component silicate melts, multicomponent diffusion is characterized by an N-1×N-1 diffusion matrix [D], whose eigenvectors define N-1 eigen-components that diffuse independently. Each eigenvalue is the diffusion coefficient for the corresponding eigen-component. Previous studies demonstrated that diffusion eigenvectors in 8-component SiO2-TiO2-Al2O3-FeO-MgO-CaO-Na2O-K2O basaltic melts are roughly invariant with temperature. In this work, we extend this investigation to andesitic compositions to test the temperature independence of diffusion eigenvectors in more evolved melts. Thirteen diffusion couple experiments were carried out at 1250, 1350, and 1500 °C: twelve andesite-andesite couples and one basalt-andesite couple. Diffusion profiles from the twelve andesite-andesite diffusion couple experiments can be fit simultaneously with a single eigenvector matrix, supporting the hypothesis that the eigenvectors are temperature invariant in andesitic melts.The eigenvector matrix obtained from the andesitic melt is similar to that in basalt, indicating possible compositional invariance across the basaltic–andesitic range. To test compositional independence, we simultaneously fit andesitic diffusion couple profiles in this study and basaltic diffusion couple profiles from previous investigations using a single eigenvector matrix. All diffusion profiles can be well reproduced using a single eigenvector matrix, supporting the hypothesis that a common eigenvector/eigensubspace structure can characterize multicomponent diffusion in basaltic to andesitic melts over the investigated temperature and compositional range. The fitted eigenvector matrix was further tested using diffusion profiles from olivine dissolution experiments. The model reproduces the major features of the fitted profiles and can predict diffusion profiles at higher pressures that were not used in fitting. The results provide additional support for the broader applicability of the proposed multicomponent diffusion model. The extracted eigenvector matrix and eigenvalues are incorporated in an updated open-access diffusion calculator to compute multicomponent diffusion profiles in basaltic to andesitic melts.
Chassignite meteorites are martian achondrites that display a magmatic cumulate texture and are mainly composed of olivine with a small amount of poikilitic pyroxene. This study presents a detailed petrological and microstructural analysis using electron backscatter diffraction of all Chassignite meteorites documented up to now, Chassigny, NWA 2737 and NWA 8694, in order to decipher early magmatic, deformation and shock processes. The new results reveal that olivine crystals in Chassignite meteorites initially formed through synneusis, a magmatic process where individual crystals accumulate and aggregate together in specific orientations in the earlier stages of consolidation. Many olivine crystals stick together along (100) crystalline faces as evidenced by the significant concentration of misorientation [100] axes at grain boundaries for all three samples. Synneusis was probably interrupted and progressively replaced by crystal settling until complete crystallisation was achieved. Olivine crystallographic preferred orientations indicate slip on [100](010) and olivine subgrain boundaries reveal high densities of geometrically necessary dislocations of the [100](010) type. The [100](010) slip system is active at high temperatures and low stresses in the Earth’s upper mantle and is often observed in plastically deformed peridotites, though it is not necessarily dominant in terrestrial olivine cumulative rocks. Furthermore, pyroxene crystallographic preferred orientations are consistent with those of olivine ([100] olivine axes parallel to pyroxene [001] axes), supporting the hypothesis of minor plastic deformation of olivine and pyroxene at high temperatures and low stresses. These observations are consistent with possible crystallization and emplacement of the Chassignite magma in the shallow Martian crust. Finally the strong shock events recorded by these meteorites is supported by significant internal deformation of olivine grains, concentration of misorientation axes along their [100] axes, and the activation of the [001](010) slip system leading to formation of olivine subgrain boundaries. Altogether these results are good indicators to decipher primary planetary processes.
Subduction serves as the primary mechanism for recycling surface materials into the mantle. While the carbon cycle in oceanic subduction zones has been extensively studied, the migration, storage and speciation of carbon in continental subduction zones remain poorly understood. Here, we present an integrated study of whole-rock major and trace element compositions, Sr-Nd-Ca-Mg isotopes and in situ clinopyroxene major-trace elements and Sr isotopes in orogenic peridotites from the North Qaidam orogen, Tibetan Plateau. The results show that the garnet peridotites originated from highly depleted subcontinental lithospheric mantle and experienced variable degrees of metasomatism. Garnet dunites and garnet lherzolites exhibit higher δ44/40Ca values (0.96 ± 0.08‰ to 1.22 ± 0.13‰) than the normal mantle (0.94 ± 0.05‰), indicating modification by partial melting. From garnet dunites through garnet lherzolites to garnet pyroxenites, Ca content increases while δ44/40Ca decreases progressively, indicating metasomatism by a light-Ca-enriched agent. These ultramafic rocks have nearly uniform δ26Mg values (−0.37 ± 0.03‰ to −0.17 ± 0.05‰), slightly lower than or consistent with the normal mantle, suggesting metasomatism by a light-Mg-enriched agent. The combined Ca-Mg isotopic systematics point to metasomatism by carbonate-bearing fluids. High 87Sr/86Sr and negative εNd(t) values further indicate metasomatism by a silicate melt derived from subducted granitic gneiss. Clinopyroxenes show high Ca/Al, (La/Yb)N, and Nb/Yb ratios, consistent with carbonatitic melt metasomatism, however their covariation of Zr/Hf and Ti/Eu ratios deviate from typical carbonate metasomatism. Together with the high 87Sr/86Sr ratios, these features suggest that the metasomatic agent was not a pure carbonatitic melt but a carbonated silicate melt, generated by reaction between carbonate and silicate melt derived from subducted continental crust. However, the low MgO content of carbonated silicate melts is insufficient to explain the Mg isotopic fractionation observed in the Lüliangshan peridotites. Instead, the P-T conditions and zircon characteristics indicate incorporation of carbonate into refractory peridotites via supercritical fluids, which effectively induced the observed Mg isotopic fractionation. Geochronological constraints suggest that Mg-rich supercritical fluid metasomatism likely occurred during ultrahigh-pressure metamorphism, whereas carbonated silicate melt metasomatism took place during slab exhumation. Thus, continental subduction can transfer substantial surface carbon into the subcontinental lithospheric mantle, with potential later recycling to the surface via post-collisional magmatism
The chemistry of detrital apatite has increasingly been employed for provenance studies and to constrain the composition of the parent magma, yet this tool has to date rarely been applied to the topic of Archean crustal evolution. In this study, we present new halogen and trace element compositions analyzed via EPMA and LA-ICP-MS in detrital apatites collected from Paleoarchean metasediments in the Barberton Greenstone Belt (BGB), South Africa, and in magmatic apatites of the surrounding tonalite-trondhjemite-granodiorite (TTG) granitoid-gneisses. Detrital apatites from both the Hooggenoeg (deposited at ∼ 3.43 Ga) and Schapenburg (deposited at ∼ 3.24 Ga) (meta-)sedimentary rocks retained trace element chemistry consistent with a primary igneous origin. They show strong fractionation of their REE spectra (LaN/LuN = 29.4 and 5.43 respectively) and low Eu/Eu* (∼0.5 and 0.8 respectively). On the other hand, the Hooggenoeg apatites have high Sr (600–1200 ppm) and moderate Mn content (400–800 ppm), are F-rich (∼2.2 wt%) and have Cl content ∼ 0.4 wt%; whereas the Schapenburg apatites have uniformly low Sr (∼200 ppm), higher Mn (800–1800 ppm) together with higher F (∼3.2 wt%) and lower Cl (∼0.01 wt%) contents. Strikingly, the characteristics of neither of these populations match with those of apatites from the Barberton TTGs but rather overlap well with apatites crystallized in Neoarchean granitoids, notably sanukitoids, either low-SiO2 ones in the case of the Hooggenoeg apatites, or high-SiO2 for the Schapenburg ones. Therefore, rocks akin to the sanukitoid suite were already present and exposed to surface alteration in the Paleoarchean felsic crust at the time the sediments were deposited. This implies that the felsic crust around the BGB at 3.43–3.24 Ga was more compositionally diverse than the exposed rock record (dominated by TTGs) suggests. In turn, we propose that the onset of observed granitoid diversity in the Neoarchean is an artifact related to a preservation bias, i.e. the higher (upper crustal) emplacement level of K-rich granitoids compared with mid-crustal TTGs made them more prone to erosion before ca. 3 Ga, while the onset of collisional processes at 3.0–2.5 Ga allowed to “shield” these high-level intrusions in the newly amalgamated continental masses and preserve them to our time.
Nucleosynthetic isotope signatures indicate that the Earth is predominantly made from inner solar system non-carbonaceous (NC) materials. However, a major uncertainty in models of water addition to the proto-Earth is the extent to which H (calculated in this study as µg/g H2O, but present as H-bearing species) in nominally anhydrous minerals (NAMs) from NC materials could contribute to the bulk Earth water budget. The preserved water concentration of NC meteorite NAMs may also shape our understanding of processes occurring in the solar protoplanetary disk (e.g., implantation of H on the surfaces of NAM grains) and in planetesimals (e.g., metamorphism in chondrite parent bodies). Reported water contents for NAMs from the ordinary chondrites (OCs), the dominant NC material falling to Earth today, range between 100-104 µg/g H2O. In order to better constrain the potential contribution of ordinary chondrites to the Earth’s water budget we have measured water concentrations in NAMs from eight equilibrated OCs, six of which have not previously been investigated for water, and two of which (Chelyabinsk and Bensour) were previously measured in other laboratories. We find that olivine and low-Ca pyroxene from equilibrated OCs contain less than ∼ 10 µg/g H2O. Based on these measurements, the water content of the NAM fraction of equilibrated OCs is < 10 µg/g H2O (a factor of ∼ 60–120 lower than prior estimates). Combining these constraints of equilibrated OC NAM water contents with published measurements of NAMs and glassy mesostases from unequilibrated OCs, we estimate that NAMs and glass in OC parent bodies could have delivered no more than ∼ 0.2 ocean masses of water to Earth (∼1% of an assumed total water budget of 18 ocean masses). Additional water could have been delivered from phyllosilicates and organics in the most primitive OC material that is not considered in our modeling. The difference in NAM water concentrations obtained here relative to some prior studies may be rooted in analytical artifacts associated with their nanoscale secondary ion mass spectrometric measurements.
Rare earth elements (REEs) are widely used as tracers of chemical weathering, yet their redistribution in intensely weathered subtropical systems remains poorly constrained. We investigate REE redistribution in loess-like Quaternary Red Clay (QRC) from mid-subtropical China using geochemical, statistical, and clay mineralogical approaches. Across intensely weathered conditions (CIA > 80), segmented regression and generalized additive models (GAMs) reveal a reproducible non-monotonic ΣREE trajectory with statistically supported breakpoints near CIA ≈ 86 and ≈ 89. Rather than following a simple monotonic depletion trend expected from progressive leaching alone, ΣREE shows a relative stabilization or slight recovery within the CIA 86–89 interval, followed by pronounced depletion under more extreme weathering conditions. Clay mineralogical observations indicate progressive shifts in mineral assemblages, including decreasing proportions of primary minerals and increasing abundance of secondary minerals, providing a plausible context for observed REE fractionation. The reproducibility of this non-monotonic pattern across multiple sections suggests that progressive weathering dominates, whereas variations in sediment input modulate its local expression. These findings demonstrate that REE redistribution in deeply weathered subtropical systems reflects coupled geochemical, mineralogical, and open-system sedimentary controls.
Organic carbon stabilization in seasonally frozen soils has important implications for carbon cycling and climate feedback. However, the role of weathering-driven clay mineral evolution in regulating carbon partitioning remains unclear. We analyze clay mineral assemblages, major element geochemistry, soil organic matter (SOM) fractions, carbon and nitrogen contents, and microbial glycerol dialkyl glycerol tetraethers (GDGTs) along an altitudinal gradient on the Haizigou hillslope at the eastern margin of the Qinghai-Tibetan Plateau (QTP). Mineral-associated organic matter (MAOM) is the dominant SOM fraction, accounting for 70.52% on average. The chemical index of alteration (CIA) increases significantly with altitude (r = 0.98, p < 0.001), indicating enhanced chemical weathering at higher altitudes. MAOM also increases with altitude (r = 0.69, p < 0.01), suggesting a close association between weathering and organic carbon partitioning. The dominant factors controlling MAOM stabilization shift along the altitudinal gradient. At middle altitudes, MAOM formation is most closely associated with 2:1-type vermiculite and hydroxy-interlayered vermiculite-rich assemblages under conditions of active organic input and rapid biological turnover. At higher altitudes, the content of kaolinite is statistically correlated with MAOM. Intensified weathering, Ca2+, Na+, and K+ leaching, and relative Al enrichment favor the formation of 1:1-type kaolinite, which supplies more effective mineral interfaces for organic carbon stabilization. Lower temperatures at higher altitudes are associated with weaker microbial processing and a greater transfer of carbon from the rapidly cycling particulate organic matter pool to the more protected mineral-associated pool. These results indicate that weathering-driven clay mineral evolution was a primary control on SOM fractionation and carbon stabilization in mountain seasonally frozen soils. Altitudinal changes in the weathering regime, mineral assemblages, and biological processing reorganize carbon stabilization pathways from a relatively fast-turnover mode toward a more protection-dominated mode.
Biogenic apatite is a key geochemical archive for palaeoceanography, yet the interpretive basis for its trace-element proxies remains inadequately constrained. Here, we evaluate the robustness of the Sr–Ca system in conodont and ichthyolith apatite across a broad stratigraphic range to disentangle biological vital effects from thermodynamic and seawater signatures. We show that (Sr/Ca)PO4 is largely resistant to diagenetic overprinting. Biological and crystallographic controls exert a first-order influence on Sr partitioning, manifested by a reproducible non-linear relationship between (Sr/Ca)PO4 and specimen weight in conodonts. This is attributed to kinetic Sr2+ partitioning in response to physiologically controlled precipitation and challenges the trophic interpretation of conodont (Sr/Ca)PO4. The persistence of this relationship over ∼ 300 Myr suggests that conodont biomineralisation was governed by conserved developmental controls, allowing ontogenetic effects to be mathematically corrected in Sr-based proxiesA consistent negative correlation between (Sr/Ca)PO4 and δ18OPO4 in conodonts suggests that (Sr/Ca)PO4 has strong potential as a thermometer, provided that taxon-specific size standardisation is applied. Directional offsets between shallow- and deep-dwelling conodont taxa further support the thermal sensitivity of (Sr/Ca)PO4. Finally, we mathematically bridge (Sr/Ca)PO4 with paired δ18OPO4 measurements to quantitatively estimate past seawater Sr/Ca ratios, establishing a multi-purpose proxy from a single biogenic apatite substrate with sensitivity to temperature and changes in marine Ca cycling associated with seawater total alkalinity and glacioeustatic fluctuations.
Nucleosynthetic isotope anomalies allow distinguishing between non-carbonaceous (NC) and carbonaceous (CC) type meteorites, and have revealed correlated isotope variations especially among NC bodies. Understanding the origin of this NC trend is important for identifying the processes that produced the NC isotope heterogeneity, and for using these isotope anomalies to reconstruct the early evolution of the solar protoplanetary disk. We report mass-independent Ti, Cr, and Mo isotope compositions for a comprehensive set of previously not or only poorly investigated meteorites, as well as acid leachates obtained from the sequential digestion of primitive ordinary chondrites. Some of the samples investigated in this study fill previously identified apparent gaps in the NC trend, suggesting these gaps reflect unrepresentative sampling of a more continuous isotopic trend. Bulk meteorites and leachates exhibit distinct isotope systematics, indicating that the NC isotope variability does not reflect selective thermal processing of presolar carriers in the disk. The NC trend also cannot reflect the continuous addition of CC dust from the outer to the inner disk, because early- and late-formed NC meteorites display largely overlapping isotopic compositions. Instead, we find that the NC isotope heterogeneity is best accounted for by fractionation and mixing among chemically and isotopically distinct dust components, similar to the processes that produced the isotopic variability among carbonaceous chondrites. On this basis we argue for the presence of substructures in the inner disk, which facilitated fractionation and mixing among distinct dust components, and helped preserve a long-lived dust reservoir from which NC planetesimals accreted over an extended period of time.
