arXiv subjects

Christoph Burkhardt

Publications and source records attributed to Christoph Burkhardt.

18 recordsLinked to original sources

arXiv2026

Origin of nucleosynthetic isotope variability in the NC reservoir: Evidence from Ti, Cr, and Mo isotopes

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.

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arXiv2025

Infall and Disk Processes - the Message from Meteorites

How do planetary systems, in general, and our own Solar System (SS), in particular, form? In conjunction, Astronomy and Isotope Cosmochemistry provide us with powerful tools to answer this age-old question. In this contribution, we review recent advances in our understanding of circumstellar disk evolution, including infall and disk processes, as explored through astrophysical models and nucleosynthetic isotope anomalies of SS materials. Astronomically, filamentary structures and anisotropy are observed across the dynamic range of star formation and disk substructures are found to be ubiquitous, highlighting how star- and planet-forming environments are far more complex and dynamic than previously thought. Isotopically, two decades of investigation of nucleosynthetic anomalies in bulk meteorites and refractory inclusions have produced a rich dataset, revealing the existence of pervasive heterogeneity in the early SS, both at the large- (i.e., NC-CC dichotomy) and fine-scale (i.e., trends within the NC group). Using an updated data compilation, we review the systematics and emerging structures of these anomalies as a function of their nucleosynthetic origin. We present the two main families of models - inheritance vs. unmixing - that have been proposed to explain the origin of the observed isotope heterogeneities, and discuss their respective implications for cloud infall and thermal processing in the disk. We also discuss how the extension of nucleosynthetic anomaly analyses to other chondritic components (Ameboid Olivine Aggregates, chondrules, matrix) has started to yield insights into transport, processing and mixing of dust in the disk. Limitations, open questions and key avenues for future work are presented in closing.

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arXiv2025

Origin of moderately volatile elements in Earth inferred from mass-dependent Ge isotope variations among chondrites

The bulk silicate Earth (BSE) is depleted in moderately volatile elements, indicating Earth formed from a mixture of volatile-rich and -poor materials. To better constrain the origin and nature of Earth's volatile-rich building blocks, we determined the mass-dependent isotope compositions of Ge in carbonaceous (CC) and enstatite chondrites. We find that, similar to other moderately volatile elements, the Ge isotope variations among the chondrites reflect mixing between volatile-rich, isotopically heavy matrix and volatile-poor, isotopically light chondrules. The Ge isotope composition of the BSE is within the chondritic range and can be accounted for as a ~2:1 mixture of CI and enstatite chondrite-derived Ge. This mixing ratio appears to be distinct from the ~1:2 ratio inferred for Zn, reflecting the different geochemical behavior of Ge (siderophile) and Zn (lithophile), and suggesting the late-stage addition of volatile-rich CC materials to Earth. On dynamical grounds it has been argued that Earth accreted CC material through a few Moon-sized embryos, in which case the Ge isotope results imply that these objects were volatile-rich, presumably because they were either undifferentiated or accreted volatile-rich objects themselves before being accreted by Earth.

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arXiv2025

Comparison of the earliest NC and CC planetesimals: Evidence from ungrouped iron meteorites

Isotope anomalies in meteorites reveal a fundamental dichotomy between non-carbonaceous- (NC) and carbonaceous-type (CC) planetary bodies. Until now, this dichotomy is established for the major meteorite groups, representing about 36 distinct parent bodies. Ungrouped meteorites represent an even larger number of additional and so far mostly unexplored parent bodies. Here, the genetics and chronology of 26 ungrouped iron meteorites are investigated. The ungrouped irons confirm the NC-CC dichotomy for Mo, where NC and CC meteorites define two distinct, subparallel s-process mixing lines. All ungrouped NC irons fall on the NC-line, which is now precisely defined for 41 distinct parent bodies. The ungrouped CC irons show scatter around the CC-line indicative of small r-process Mo heterogeneities among these samples. These r-process Mo isotope variations correlate with O isotope anomalies, most likely reflecting mixing of CI chondrite-like matrix, chondrule precursors and Ca-Al-rich inclusions. This implies that CC iron meteorite parent bodies accreted the same nebular components as the later-formed carbonaceous chondrites. The Hf-W model ages of core formation for the ungrouped irons reveal a narrow age peak at ~3.3 Ma after Ca-Al-rich inclusions for the CC irons. By contrast, the NC irons display more variable ages, including younger ages indicative of impact-induced melting events, which seem absent among the CC irons. This is attributed to the more fragile and porous nature of the CC bodies, making impact-induced melting on their surfaces difficult. The chemical characteristics of all iron meteorites together reveal slightly more oxidizing conditions during core formation for CC compared to NC irons. More strikingly, strong depletions in moderately volatile elements, typical of many iron meteorite parent bodies, predominantly occur among CC irons.

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arXiv2024

The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous chondrites

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu and CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites and Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

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arXiv2024

Condensate evolution in the solar nebula inferred from combined Cr, Ti, and O isotope analyses of amoeboid olivine aggregates

Refractory inclusions in chondritic meteorites, namely amoeboid olivine aggregates (AOAs) and Ca-Al-rich inclusions (CAIs), are among the first solids to have formed in the solar system. The isotopic composition of CAIs is distinct from bulk meteorites, which either results from extreme processing of presolar carriers in the CAI-forming region, or reflects an inherited heterogeneity from the Sun's parental molecular cloud. Amoeboid olivine aggregates are less refractory than CAIs and provide a record of how the isotopic composition of solid material in the disk may have changed in time and space. However, the isotopic composition of AOAs and how this composition relates to that of CAIs and later-formed solids is unknown. Here, using new O, Ti, and Cr isotopic data for eight AOAs from the Allende CV3 chondrite, we show that CAIs and AOAs share a common isotopic composition, indicating a close genetic link and formation from the same isotopic reservoir. Because AOAs are less refractory than CAIs, this observation is difficult to reconcile with a thermal processing origin of the isotope anomalies. Instead, the common isotopic composition of CAIs and AOAs is readily accounted for in a model in which the isotopic composition of infalling material from the Sun's parental molecular cloud changed over time. In this model, CAIs and AOAs record the isotopic composition of the early infall, while later-formed solids contain a larger fraction of the later, isotopically distinct infall. This model implies that CAIs and AOAs record the isotopic composition of the Sun and suggests that the nucleosynthetic isotope heterogeneity of the solar system is predominantly produced by mixing of solar nebula condensates, which acquired their distinct isotopic compositions as a result of time-varied infall from the protosolar cloud.

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arXiv2023

Distribution of s-, r-, and p-process nuclides in the early Solar System inferred from Sr isotope anomalies in meteorites

Nucleosynthetic isotope anomalies in meteorites allow distinguishing between the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs and show that correlated isotope anomalies exist in both reservoirs. It is debated, however, whether these anomalies reflect thermal processing of presolar dust in the disk or are primordial heterogeneities inherited from the Solar System's parental molecular cloud. Here, using new high-precision 84Sr isotope data, we show that NC meteorites, Mars, and the Earth and Moon are characterized by the same 84Sr isotopic composition. This 84Sr homogeneity of the inner Solar System contrasts with the well-resolved and correlated isotope anomalies among NC meteorites observed for other elements, and most likely reflects correlated s- and (r-, p-)-process heterogeneities leading to 84Sr excess and deficits of similar magnitude which cancel each other. For the same reason there is no clearly resolved 84Sr difference between NC and CC meteorites, because in some carbonaceous chondrites the characteristic 84Sr excess of the CC reservoir is counterbalanced by an 84Sr deficit resulting from s-process variations. Nevertheless, most carbonaceous chondrites exhibit 84Sr excesses, which reflect admixture of refractory inclusions and more pronounced s-process heterogeneities in these samples. Together, the correlated variations of s-, (r-, p-)-process nuclides revealed by the 84Sr data of this study refute an origin of these isotope anomalies solely by processing of presolar dust grains, but points to primordial mixing of isotopically distinct dust reservoirs as the dominant process producing the isotopic heterogeneity of the Solar System.

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arXiv2023

Origin of isotopic diversity among carbonaceous chondrites

Carbonaceous chondrites are some of the most primitive meteorites and derive from planetesimals that formed a few million years after the beginning of the solar system. Here, using new and previously published Cr, Ti, and Te isotopic data, we show that carbonaceous chondrites exhibit correlated isotopic variations that can be accounted for by mixing among three major constituents having distinct isotopic compositions, namely refractory inclusions, chondrules, and CI chondrite-like matrix. The abundances of refractory inclusions and chondrules are coupled and systematically decrease with increasing amount of matrix. We propose that these correlated abundance variations reflect trapping of chondrule precursors, including refractory inclusions, in a pressure maximum in the disk, which is likely related to the water ice line and the ultimate formation location of Jupiter. The variable abundance of refractory inclusions/chondrules relative to matrix is the result of their distinct aerodynamical properties resulting in differential delivery rates and their preferential incorporation into chondrite parent bodies during the streaming instability, consistent with the early formation of matrix-poor and the later accretion of matrix-rich carbonaceous chondrites. Our results suggest that chondrules formed locally from isotopically heterogeneous dust aggregates which themselves derive from a wide area of the disk, implying that dust enrichment in a pressure trap was an important step to facilitate the accretion of carbonaceous chondrite parent bodies or, more generally, planetesimals in the outer solar system.

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arXiv2022

Planetary Genealogy

The detection of exoplanets and accretion disks around newborn stars has spawned new ideas and models of how our Solar System formed and evolved. Meteorites as probes of geologic deep time can provide ground truth to these models. In particular, stable isotope anomalies in meteorites have recently emerged as key tracers of material flow in the early Solar System, allowing cosmochemists to establish a "planetary isotopic genealogy". Although not complete, this concept substantially advanced our understanding of Solar System evolution, from the collapse of the Sun's parental molecular cloud to the accretion of the planets.

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arXiv2022

Terrestrial planet formation from lost inner solar system material

Two fundamentally different processes of rocky planet formation exist, but it is unclear which one built the terrestrial planets of the solar system. Either they formed by collisions among planetary embryos from the inner solar system, or by accreting sunward-drifting millimeter-sized 'pebbles' from the outer solar system. We show that the isotopic compositions of Earth and Mars are governed by two-component mixing among inner solar system materials, including material from the innermost disk unsampled by meteorites, whereas the contribution of outer solar system material is limited to a few percent by mass. This refutes a pebble accretion origin of the terrestrial planets, but is consistent with collisional growth from inner solar system embryos. The low fraction of outer solar system material in Earth and Mars indicates the presence of a persistent dust-drift barrier in the disk, highlighting the specific pathway of rocky planet formation in the solar system.

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arXiv2021

Earth's accretion inferred from iron isotopic anomalies of supernova nuclear statistical equilibrium origin

Nucleosynthetic Fe isotopic anomalies in meteorites may be used to reconstruct the early dynamical evolution of the solar system and to identify the origin and nature of the material that built planets. Using high-precision iron isotopic data of 23 iron meteorites from nine major chemical groups we show that all iron meteorites show the same fundamental dichotomy between non-carbonaceous (NC) and a carbonaceous (CC) meteorites previously observed for other elements. The Fe isotopic anomalies are predominantly produced by variation in 54Fe, where all CC iron meteorites are characterized by an excess in 54Fe relative to NC iron meteorites. This excess in 54Fe is accompanied by an excess in 58Ni observed in the same CC meteorite groups. Together, these overabundances of 54Fe and 58Ni are produced by nuclear statistical equilibrium either in type Ia supernovae or in the Si/S shell of core-collapse supernovae. The new Fe isotopic data reveal that Earth's mantle plots on or close to correlations defined by Fe, Mo, and Ru isotopic anomalies in iron meteorites, indicating that throughout Earth's accretion, the isotopic composition of its building blocks did not drastically change. While Earth's mantle has a similar Fe isotopic composition to CI chondrites, the latter are clearly distinct from Earth's mantle for other elements (e.g., Cr and Ni) whose delivery to Earth coincided with Fe. The fact that CI chondrites exhibit large Cr and Ni isotopic anomalies relative to Earth's mantle, therefore, demonstrates that CI chondrites are unlikely to have contributed significant Fe to Earth.

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arXiv2021

Nucleosynthetic Pt isotope anomalies and the Hf-W chronology of core formation in inner and outer solar system planetesimals

The 182Hf-182W chronology of iron meteorites provides crucial information on the timescales of accretion and differentiation of some of the oldest planetesimals of the Solar System. Determining accurate Hf-W model ages of iron meteorites requires correction for cosmic ray expo-sure (CRE) induced modifications of W isotope compositions, which can be achieved using in-situ neutron dosimeters such as Pt isotopes. Until now it has been assumed that all Pt isotope variations in meteorites reflect CRE, but here we show that some ungrouped iron meteorites display small nucleosynthetic Pt isotope anomalies. These provide the most appropriate starting composition for the correction of CRE-induced W isotope variations in iron meteorites from all major chemical groups, which leads to a ~1 Ma upward revision of previously reported Hf-W model ages. The revised ages indicate that core formation in non-carbonaceous (NC) iron meteorite parent bodies occurred at ~1-2 Ma after CAI formation, whereas most carbonaceous (CC) iron meteorite parent bodies underwent core formation ~2 Ma later. We show that the younger CC cores have lower Fe/Ni ratios than the earlier-formed NC cores, indicating that core formation under more oxidizing conditions occurred over a more protracted timescale. Thermal modeling of planetesimals heated by 26Al-decay reveals that this protracted core formation timescale is consistent with a higher fraction of water ice in CC compared to NC planetesimals, implying that in spite of distinct core formation timescales, NC and CC iron meteorite parent bodies accreted about contemporaneously within ~1 Ma after CAI formation, but at different radial locations in the disk.

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arXiv2021

Collisional mixing between inner and outer solar system planetesimals inferred from the Nedagolla iron meteorite

The ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf-W chronology of Nedagolla indicates that this mixed NC-CC isotopic composition was established relatively late, more than 7 million years after Solar System formation. The mixed NC-CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re-equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit-and-run collision between two differentiated bodies, which also provides a possible pathway for Nedagolla's extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiter's growth.

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arXiv2020

Early evolution of the solar accretion disk inferred from Cr-Ti-O isotopes in individual chondrules

Isotopic anomalies in chondrules hold important clues about the dynamics of mixing and transport processes in the solar accretion disk. These anomalies have been interpreted to indicate either disk-wide transport of chondrules or local heterogeneities of chondrule precursors. However, all previous studies relied on isotopic data for a single element (either Cr, Ti, or O), which does not allow distinguishing between source and precursor signatures as the cause of the chondrules isotope anomalies. Here we obtained the first combined O, Ti, and Cr isotope data for individual chondrules from enstatite, ordinary, and carbonaceous chondrites. We find that chondrules from non-carbonaceous (NC) chondrites have relatively homogeneous Δ17O, ε50Ti, and ε54Cr, which are similar to the compositions of their host chondrites. By contrast, chondrules from carbonaceous chondrites (CC) have more variable compositions. Although the compositions of the analyzed CC and NC chondrules may overlap for either ε50Ti, ε54Cr, or Δ17O, in multi-isotope space none of the CC chondrules plot in the compositional field of NC chondrites, and no NC chondrule plots within the field of CC chondrites. As such, our data reveal a fundamental isotopic difference between NC and CC chondrules, which is inconsistent with a disk-wide transport of chondrules across and between the NC and CC reservoirs. Instead, the isotopic variations among CC chondrules reflect local precursor heterogeneities, which most likely result from mixing between NC-like dust and a chemically diverse dust component that was isotopically similar to CAIs and AOAs.The same mixing processes, but on a larger, disk-wide scale, were likely responsible for establishing the distinct isotopic compositions of the NC and CC reservoirs, which represent in inner and outer disk, respectively.

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arXiv2020

Isotopic evolution of the inner Solar System inferred from molybdenum isotopes in meteorites

The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca-Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun's parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals.

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arXiv2019

Elemental and isotopic variability in solar system materials by mixing and processing of primordial disk reservoirs

Isotope anomalies among planetary bodies provide key constraints on planetary genetics and the Solar System's dynamical evolution. However, to unlock the full potential of these anomalies for constraining the processing, mixing, and transport of material in the disk it is essential to identify the main components responsible for producing planetary-scale isotope variations, and to investigate how they relate to the isotopic heterogeneity inherited from the Solar System's parental molecular cloud. To address these issues we measured the Ti and Sr isotopic compositions of Ca,Al-rich inclusions (CAIs) from the Allende CV3 chondrite, as well as acid leachates and an insoluble residue from the Murchison CM2 chondrite, and combine these results with literature data for presolar grains, hibonites, chondrules, and bulk meteorites. Our analysis reveals that the mineral-scale nebular isotopic heterogeneity as sampled by leachates and presolar grains is largely decoupled from the planetary-scale isotope anomalies. Combining isotope anomaly data for a large number of elements reveals that the difference between non-carbonaceous (NC) and carbonaceous (CC) meteorites is the product of mixing between NC material and a nebular reservoir (termed IC) whose isotopic composition is similar to that of CAIs, but whose chemical composition is similar to bulk chondrites. In our preferred model, the distinct isotopic compositions of these two nebular reservoirs reflect an inherited heterogeneity of the solar system's parental molecular cloud core, which therefore has never been fully homogenized during collapse. Planetary-scale isotopic anomalies are thus caused by variable mixing of isotopically distinct primordial disk reservoirs, the selective processing of these reservoirs in different nebular environments, and the heterogeneous distribution of the thereby forming nebular products.

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arXiv2017

Mixing and transport of dust in the early solar nebula as inferred from titanium isotope variations among chondrules

Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here we show that chondrules from enstatite and ordinary chondrites show only small 50Ti variations and scatter closely around the 50Ti composition of their host chondrites. By contrast, chondrules from carbonaceous chondrites have highly variable 50Ti compositions, which, relative to the terrestrial standard, range from the small 50Ti deficits measured for enstatite and ordinary chondrite chondrules to the large 50Ti excesses known from Ca-Al-rich inclusions (CAIs). These 50Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous nuggets into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Lastly, the ubiquitous presence of 50Ti-enriched material in carbonaceous chondrites, and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other through the formation of Jupiter.

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arXiv2015

Planetary and meteoritic Mg/Si and d30Si variations inherited from solar nebula chemistry

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

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