DIAGENESIS OF THE 3.24 GA SPHERULES IN THE BARBERTON GREENSTONE BELT, SOUTH AFRICA

Spherules in the S3 bed of the Barberton greenstone belt, South Africa are distal debris formed after a large impact event at 3.24 Ga. Since that time, diagenesis and lower greenschist grade metamorphism has resulted in the alteration of spherule mineralogy to quartz, various phyllosilicates, Ti-oxides, some feldspars and sulfides, while maintaining primary Ni-rich chromites. Initially, silica cementation of the spherules began during low-temperature interaction with seawater, and glassy spherules were palagonitized and devitrified. Further alteration by Si- and K-rich fluids resulted in a mineralogical assemblage of quartz, feldspar, and clays. Crystalline minerals such as barred olivine were replaced by dissolution-precipitation processes, preserving relict textures. Further silica cementation resulted in complete lithification of the bed. Most of this alteration occurred at the seafloor and during shallow burial as indicated by the pristine preservation of spherule shape and texture. During shallow burial, amorphous silica recrystallized to microcrystalline quartz. Later recrystallization of clays to micas and sericite occurred during regional metamorphism at peak temperatures of 300-320°C. Late-stage shearing and mineralization preferentially affected the northern region of the belt. Samples from different sections record highly variable local conditions. Water depth, the amount of pre-depositional transport, location within the belt, and proximity to igneous dikes all affect the diagenesis of the S3 spherules. Element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the BGB. The elements Al, Zr, Ti and Sc are the most immobile in the diagenetic environment, are consistent in all sections of the S3 spherule bed, and can be used to infer original composition of the spherules. Most of the rare earth elements are largely immobile, but are affected by carbonate diagenesis with the exception of Ce and Eu, which are significantly mobilized during diagenesis and low-grade metamorphism. The composition of these spherules vary significantly, and most likely represent primary fractionation within an impact plume.