Sedimentologic and geochemical constraints on impact processes in the Archean and implications for the early Earth: Studies in the 3.5--3.2 Ga Barberton greenstone belt, South Africa

My dissertation reports the findings of a study of Archean meteoritic material, which are used to provide independent evidence to constrain the evolution of impact vapor plumes, the Archean atmosphere and ocean, and the flux of cosmic material to the early Earth. Spherules, preserved in the Barberton greenstone belt in South Africa, are small spherical particles formed by the crystallization of plume material formed after vaporization of bolide and target rock immediately following a meteorite impact. The first three chapters of my thesis focus on spherules from the 3.24 billion-year-old S3 spherule bed, which are well-preserved in multiple locations throughout the belt and were deposited in a diverse array of depositional environments. The first chapter characterizes the stratigraphy of pure fall-deposited sections of S3 in order to constrain the evolution of impact plumes. Petrographic and geochemical results demonstrate significant compositional grading in the impact deposit, suggesting a systematic condensation sequence predicted for a fractionation-modified mixing of chondritic and basaltic materials. The second chapter uses impact spinel composition from S3 to infer the abundance of oxygen in the Archean atmosphere. Results of this study have shown the Archean atmosphere had oxygen levels below 10-4 PAL. The third chapter explores the diagenesis and metamorphism of the spherules in the S3 section. Results of this study trace the diagenetic pathways of the original minerals through diagenesis and metamorphism, and demonstrate that nearly all of the primary mineralogy was altered before significant burial of the spherule beds. The last chapter is a study of the flux of cosmic material in the Archean using empirical data from iridium analyses of a condensed section of core. This study demonstrates that the flux of cosmic dust to the surface of the Earth is not significantly different from today. Because of the lack of microfossils and datable material in the Archean rock record, Ir abundance is therefore an important tool for calculating sedimentation rate in the Archean.