Petrologic and impact size controls on Precambrian spherule diameters

Spherules from three impact layers ranging in age from 3.24 Ga to 2.49 Ga display petrologic and size diversity. All of these layers represent the distal debris from asteroid impacts in the Archean and early Proterozoic. We examine the petrologic control on the size of these spherules. Though all three spherule layers have different diagenetic histories, some textural properties are consistent in all of the layers and can be used to infer original mineralogy. Spherules that are uniform in composition, with the exception of fine-grained minerals around the rims, are inferred to have been altered from an original glassy composition and are the largest spherules. Spherules that are heterogeneous and contain either pseudomorphs of olivine or contain Ni-chromite are the smallest spherules in all sections. Spherules with plagioclase pseudomorphs tend to be intermediate in size. The larger the impact, the more pronounced is the size segregation of these spherule types. Spherules in the older S3 layer from the Barberton greenstone belt are significantly larger and display a wider range of sizes, and these differences in sizes are related to their petrologic type compared to the younger Dales Gorge and Paraburdoo layers from the Hamersley Basin. The S3 layer also tends to have the largest aggregate bed thickness and Ir content, consistent with this bed resulting from a larger impactor. The Dales Gorge spherule layer contains nonspherical particles, indicative of a ballistic melt, and it is therefore used as a point of comparison with the vapor plume condensates of the S3 and Paraburdoo layers. The Dales Gorge spherules are similar in size to those of the Paraburdoo layer; however, they have a significantly higher percentage of largely glassy spherules, and only approximately 21% have crystalline pseudomorphs.