Mineralogy and diagenesis of 3.24 Ga meteorite impact spherules

Spherules in the S3 bed of the Barberton greenstone belt, South Africa are distal fallout formed within an impact plume after a large impact event at 3.24 Ga. Since that time, diagenesis and lower greenschist grade metamorphism of the spherules has changed the mineralogy, though shape and texture are largely preserved. Alteration of the S3 bed has resulted in spherules composed of quartz, phyllosilicates, Ti- and Fe-oxides, and some Ni-rich chromites. Initially, glassy spherules were palagonitized and silica cementation of the spherules began during low-temperature interaction with seawater. Further alteration by Si- and K-rich fluids resulted in a mineralogical assemblage of quartz, feldspar, and clays. Crystalline minerals 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. With continued burial, amorphous silica recrystallized to microcrystalline quartz. Later recrystallization of clays to micas 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. Silica and barite concentrations are lower, and carbonate concentrations are higher in the deep-water depositional environments. Element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the BGB. The most immobile elements are Al, Zr, Ti, Sc and the high field strength elements and present element ratios can be used to infer original composition of the spherules. The large ion lithophile elements are highly mobile, as are the light rare earth elements (REEs), which are particularly susceptible to mobilization during carbonate diagenesis and phosphate authigenesis. Of the REEs, Ce and Eu show the largest variability, suggesting significant mobilization during diagenesis and low-grade metamorphism. Consistent Cr/Ir ratios, particularly in the high concentrations suggest limited mobility, and segregation of platinum group elements into Ni-rich chromite-bearing spherules. Sulfide mineralization has not affected the Ir concentration in the S3 layer.

► Diagenetic alteration largely controlled by depositional environment.
► Spherules deposited in shallower depositional environments display the largest degree of silicification.
► Al, Ti, and the HFSEs are largely immobile during diagenesis.
► LREE enrichment and Eu anomaly caused by carbonate diagenesis.
► Original mineralogy included low- and high Si glasses, olivine, plagioclase, augite, and spinel.