The onset of planet formation inferred from isotope anomalies in meteorites
by Fridolin Spitzer
Date of Examination:2024-11-22
Date of issue:2024-12-18
Advisor:Prof. Dr. Thorsten Kleine
Referee:Prof. Dr. Thorsten Kleine
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Francois Tissot
Persistent Address:
http://dx.doi.org/10.53846/goediss-10925
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Abstract
English
Variations in the isotopic compositions of extraterrestrial samples provide important insights into the formation of planetesimals—the fundamental building blocks of planets. Particularly, isotopic anomalies of nucleosynthetic origin are suitable for tracing the transport, mixing, and processing of materials from the collapse of the parental molecular cloud to the final assembly of the terrestrial planets. They have revealed a fundamental dichotomy between the non-carbonaceous (NC) and carbonaceous (CC) meteorite supergroups, whose parent bodies are thought to have formed in the inner and outer disk, respectively. Although this finding significantly enhanced our understanding of the Solar System’s early evolution, important questions regarding the timing, conditions, and mechanisms of planetesimal formation in different reservoirs of the solar accretion disk remain unresolved. This thesis addresses these questions through the analysis of Mo, Ni, W, and Pt isotopic compositions in a comprehensive suite of meteorites that represent the full spectrum of planetesimal formation in the early Solar System. By providing genetic and chronologic data for 26 ungrouped iron meteorites, this thesis effectively doubles the number of early-formed parent bodies for which such information is available. The Mo and Ni isotopic signatures of the ungrouped iron meteorites confirm the NC-CC dichotomy, overlap with the compositions of the 13 established iron meteorite groups, and reveal that approximately two-thirds of all iron meteorite parent bodies are of CC origin and one-third are of NC origin. The only exception is the meteorite Nedagolla, which displays both NC and CC isotopic characteristics, and together with its chemical and petrological characteristics points towards an impact origin between an NC and a CC body. This collision likely occurred more than 7 million years (Ma) after the start of the Solar System and was triggered by gravitational interactions with the gas giants. Overall, the results reveal an early and efficient separation of the NC and CC reservoirs, supporting the rapid growth of Jupiter’s core as the likely cause for preventing significant dust drift from the outer to the inner disk. This implies either that dust in the inner disk was stored for at least ∼2 Ma until the accretion of NC chondrite parent bodies, or that later-formed NC planetesimals predominantly accreted from secondary dust produced by collisions among pre-existing NC planetesimals. Regarding the conditions of planetesimal formation, three key observations emerged from the Mo and Ni isotope systematics of grouped and ungrouped iron meteorites: (1) a correlation in NC meteorites between s- and r-process nuclides suggests the former presence of an inner disk reservoir enriched in s-process nuclides, which contributed to Earth’s building blocks; (2) CC meteorites exhibit r-process heterogeneity decoupled from s-process variability, reflecting different proportions of isotopically distinct components in the dust mix during parent body accretion; (3) overlapping isotopic compositions of CC iron meteorites and chondrites suggest that the components found in chondrites today (refractory inclusions, chondrule precursors, FeNi metal, and CI-like dust) were already present in the outer disk less than 1 Ma after the start of the Solar System. Thus, despite recurrent CC planetesimal formation for ~3–4 Ma, CC chondrites and differentiated meteorites accreted from similar mixtures of the same dust components. This suggests the presence of dust drift barriers with varying dust-trapping efficiencies, enabling some regions to form planetesimals quickly while others retained dust over several million years. Determining the accretion timescales of iron meteorite parent bodies is crucial for understanding early Solar System dynamics. However, obtaining accurate ages remains challenging because Hf-W chronometry, which dates core formation and is linked to accretion ages via thermal models, is hampered by cosmic ray-exposure (CRE) effects. Significant progress was made by using Pt isotopes as a neutron dosimeter to correct such CRE effects. However, this thesis reports the discovery of nucleosynthetic Pt isotope variations. When taken into account, these anomalies result in revised ~1 Ma younger core formation ages for magmatic iron meteorites. Furthermore, the new Hf-W model ages of core formation for the ungrouped iron meteorites overlap with those of the iron meteorite groups and reveal a narrow age peak around 3.3 Ma after the start of the Solar System for most CC iron meteorites. In contrast, the NC iron meteorites display more variable ages, including evidence of younger ages, likely caused by impact-induced metal-silicate re-equilibration. These secondary melting events appear to be absent among CC irons, possibly due to the more fragile and porous nature of CC planetesimals, making impact-induced surface melting difficult. The delayed onset of core formation in most CC irons, previously attributed to differences in accretion timescales, is here proposed to reflect the accretion of water ice in CC bodies that formed beyond the water snow line. This is consistent with the chemical composition of CC iron meteorites, which is indicative of more oxidizing formation conditions. A new thermal model, which incorporates the effect of water ice on the time of core formation, supports the idea that both NC and CC planetesimals accreted rapidly and contemporaneously within ~1 Ma after the start of the Solar System, but in distinct regions of the protoplanetary disk. The late-stage formation of planetesimals in the outer disk is explored by measuring the Ni isotopic compositions of CC chondrites, as well as samples returned from asteroid Ryugu by JAXA's Hayabusa2 mission. The new Ni isotopic data reveal that Ryugu and CI chondrites share indistinguishable Ni isotope anomalies, differing from all other CC meteorites. This unique Ni (and Fe) isotopic signature likely reflects a more efficient incorporation of small FeNi metal grains, possibly because CI-like bodies formed at the end of the disk’s lifetime about 4 Ma after the birth of our Solar System, when planetesimal formation was driven by photoevaporation of the gas rather than dust accumulation in dust drift barriers. As such, isotopic variations among CC chondrites likely reflect the fractionation of distinct dust components from a common reservoir, implying that while CI chondrites could have formed at greater heliocentric distances, they may have also formed within the same reservoir as other CC chondrites. Finally, the investigation of samples from early-formed, differentiated CC bodies revealed no CI-like isotopic composition in the first generation of planetesimals, thus, supporting that CI chondrite formation was triggered by photoevaporation, whereas other CC bodies formed by trapping of dust in pressure maxima. Moreover, this analysis revealed that many of the analyzed ungrouped CC iron meteorites are isotopically akin to CR chondrites, which might have formed in a disk reservoir that was spatially separated from the other CC chondrites, possibly located beyond the orbit of Saturn. This highlights the recurrent formation of planetesimals in distinct outer disk reservoirs and, therefore, the need for trapping and storing of dust for several Ma of disk evolution.
Keywords: Core formation; Hf-W chronology; Iron meteorites; Chondrite components; Ryugu; Planetesimal formation; Carbonaceous chondrites; Protoplanetary disks; Nucleosynthetic isotope anomalies; Isotope cosmochemistry
