Reverse Engineering the Thermochemistry of the Early Solar Nebula Through Transmission Electron Microscopy and Thermodynamic Modelling of Refractory Planetary Materials

Abstract

Calcium-aluminum-rich inclusions (CAIs) are mm- to cm-sized objects in chondritic meteorites that are widely accepted as the first formed solids in our solar system. Thermodynamic models of the solar nebula suggest that the major mineral phases present in CAIs formed at high temperatures, in excess of 900 K. Astrophysical models place their formation in the inner regions of the solar nebula, and further suggest their large-scale transport across the disk. The large-scale transport as evidenced by the presence of CAI-like fragments in cometary samples was vital to their storage and incorporation into the asteroid parent bodies. Some CAIs then experienced thermal and secondary alteration in the solar nebula and on their asteroid parent bodies. Analyses of them therefore provide insight into the thermochemical landscape and processes that occurred in the early stages of solar system formation. This dissertation is focused on studying various high-temperature mineral phases, viz. oxides, silicates, and refractory alloys, to probe their compositions and microstructures and gain insight into the T-P conditions and chemical pathways governing their formation and subsequent alteration. This work uses electron microprobe (EMP), scanning electron microscopy (SEM), and focused ion beam (FIB) techniques to inform and support transmission electron microscopy (TEM) studies. The TEM analyses focused on structural and chemical characterization of various refractory assemblages from CAIs. The data acquired from TEM analyses were used to guide thermodynamic modeling efforts to understand the T-P conditions and chemical pathways associated with the formation of CAIs. Chapter 1 of this dissertation provides an introduction to the solar protoplanetary disk (nebula) and associated dynamical processes, chondritic meteorites, CAIs, condensation thermodynamics, nebular and parent-body processes, and the material systems associated with the mineral phases. Chapter 2 describes the analytical methods used to complete the work including EMP, SEM, FIB-SEM and TEM. The chapter also provides an introduction into the theories behind thermodynamic modeling. The three subsequent chapters describe research aimed at filling knowledge gaps in our understanding of the origins and histories of CAIs by reverse engineering the thermochemistry that affected them in the early solar nebula and on their parent asteroids. Chapter 3 focuses on the analysis of a type of CAI referred to as compact- type A inclusion (CTA), identified in the NWA 5028 chondrite. CTA CAIs are considered to have undergone partial to complete melting and crystallization of the primary condensates. This study characterizes the overprinting of primary condensation signatures by thermal processing and secondary alteration. The analyses reveal the complex history of the inclusion, showing evidence for thermal processing and secondary alteration while retaining signatures of primary condensation. Chapter 4 focuses on the study of refractory-siderophile-rich metal grains in fluffy-type A inclusions (FTAs) from the Leoville and NWA 8323 CV3 chondrites. The work combines TEM analyses with thermodynamic modeling to understand the formation of refractory alloys. The analyses show that the metal grains have complex microstructures that include various alloys, oxides, and silicates. Thermodynamic models predict the condensation temperatures of these alloys as high as 1831 K, well above those of the oxides and silicates observed within their host CAIs. Comparison of the microstructural data to thermodynamic predictions and astrophysical models suggest that the gas-dust boundary in the solar protoplanetary disk was a closer to the early Sun than currently accepted. Chapter 5 focuses on the study of a spinel-rich inclusion from the Leoville CV3 chondrite. The inclusion is part of a compound CAI that also contains a fluffy-type A inclusion. Microstructural and textural data show that the spinel-rich inclusion is composed of three regions, that formed individually and coalesced to form the CAI. Thermodynamic modeling predicts the condensation of Ti-bearing pyroxenes as high as 1662 K, placing them above the adjacent and surrounding melilite and spinel. The data suggest that the inclusion formed as the result of gas-solid condensation, with one of the regions experiencing sub-solidus thermal processing prior to aggregation. Chapter 6 provides a summary of the work presented in the previous chapter, a discussion of the implications to the existing knowledge gaps and presents avenues for future work on CAIs. Chapter 3 was published in Meteoritics & Planetary Science (MAPS), Chapter 4 is currently being submitted to Geochemica et Cosmochemica Acta (GCA), and Chapter 5 is in preparation for submission to MAPS.