Stardust in Primitive Astromaterials: Insights into the Building Blocks and Early History of the Solar System

Mixed within the fine-grained material of primitive astromaterials (e.g., meteorites, micrometeorites (MMs) and interplanetary dust particles (IDPs)) are tiny presolar grains, which are remnants of the original material from which the Solar System formed. These presolar grains are (sub-)micrometer-size carbonaceous (e.g., nanodiamonds, SiC, graphites) and oxygen-rich (e.g., oxides, silicates) condensates that formed in circumstellar envelopes or in the ejecta of stellar explosions. The study of presolar grains opened a new field in astronomy, astrophysics and cosmochemistry, allowing the direct in situ study of individual stars and providing ground-truth information on stellar evolution, nucleosynthesis of the elements, and grain condensation in circumstellar envelopes, as well as on the secondary processing of fine-grained material in the solar nebula and on their parent-body asteroids. In this dissertation, I focus on combining the identification and characterization of presolar grains in the most primitive carbonaceous chondrites and fine-grained Antarctic MMs with coordinated in-situ chemical and structural analyses of fine-grained materials in these same samples. In Chapter 1, I introduce the research objectives of the dissertation and provide some basic information on the classification, composition and formation model(s) of unequilibrated extraterrestrial material, in particular primitive meteorites and micrometeorites. I also briefly introduce each of the main types of presolar grains. While each chapter includes a sample and methods section, I also provide here a general description of all the samples and experimental methods used in this dissertation. Chapter 2 provides a detailed review of the isotopic and elemental compositions, mineralogies, and abundances of presolar silicate grains in meteorites, micrometeorites and interplanetary dust particles. In Chapters 3 and 4, I focus on the compositions of the building blocks of our Solar System and their origin(s) in presolar circumstellar environments or stellar ejecta. In Chapter 3, I report the discovery of two supernova silica (SiO2) grains in the carbonaceous chondrites LAP 031117 (CO3.0) and GRV 021710 (CR2). The two grains are characterized by moderate enrichments in 18O relative to solar, indicating that they originated in core collapse (Type II) supernovae, providing definitive evidence of the condensation of silica dust in supernova ejecta. In Chapter 4, I report on the identification of two unique presolar graphite grains. One of the grains (LAP-141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP-149) is extremely 13C-rich and 15N-poor (12C/13C = 1.41 0.01 and 14N/15N = 941 81). Comparison of its isotopic compositions (C, N, Si & S) with new stellar nucleosynthesis and dust condensation models indicates an origin in the ejecta of a 0.6M CO nova and it is the first putative nova grain that quantatively best matches the predictions from nova models, providing the first strong evidence that graphite grains condense in nova ejecta. In Chapters 5 and 6, I investigate the accretion and secondary processing of fine-grained material in the early Solar System history. In Chapter 5, I compare presolar grain abundances, elemental compositions, and mineralogies in the matrix and individual fine-grained chondrule rims (FGRs) of the unequilibrated (primitive) CO3.0 chondrites LAP 031117, ALHA77307 and DOM 08006. This study shows systematically lower abundances of presolar silicate-oxide grains in the FGRs than in the matrix of CO3.0 chondrites, likely reflecting pre-accretionary aqueous alteration of the rims in the solar nebula. In addition to the systematic differences in O-anomalous grain abundances, uniform porosity in both matrix and FGRs, and the identification of a composite FGR (a small rimmed chondrule within a larger FGR), provide strong evidence for a nebular origin of the FGRs in CO3 chondrites by dust accretion onto freely floating chondrules in the solar nebula, before assembly into their parent bodies. In Chapter 6, I examine the presolar grain abundances and distribution of supernova grains in fine-grained micrometeorites (MMs). This study shows that O- and C-anomalous grain abundances in MMs are similar to those in primitive meteorites, but are significantly lower than abundances observed in primitive IDPs. The data also show a higher proportion of 18O-rich oxide and silicate grains (Group 4) and extremely 13C-rich SiC grains (12C/13C 10) in micrometeorites (27 6% and 16 8%, respectively) than in chondrites (~10% and \u3c 5%). This heterogeneity between MMs and meteorites likely reflects formation of their respective parent-bodies in different places in the solar nebula with different abundances of supernova grains, or at different times during the formation of the Solar System. If most of these 18O-rich silicate-oxide grains and extremely 13C-rich SiC grains originated from a Type II supernova, our observation is also consistent with models of injection of short-lived radionuclides from a nearby supernova explosion in the solar nebula. In summary, the study of the isotopic, elemental and structural compositions and abundances of presolar grains in the most primitive astromaterials (e.g., CO3.0 chondrites and micrometeorites) provides information on the nature and origins of the building blocks of the Solar System, as well as its early formation history. The in-situ survey of fine-grained material in CO3.0 chondrite LAP 031117 has allowed the identification of new types of presolar grains and new stellar sources, such as supernova SiO2 grains, a presolar Fe-sulfide, and CO nova graphite. In addition, the comparison of presolar grain abundances with the m-scale mineralogies and elemental compositions of fine-grained material in CO3.0 chondrites and micrometeorites has provided constraints on dust accretion onto freely floating chondrules in the solar nebula and the model of injection of newly formed material from a nearby supernova explosion in the solar nebula