Impact Plume Fractionation as Indicated by Size and Mineral Diversity in Archean Spherules

Introduction: Hydrocode modeling of impacts provides most of our information as to how impact plumes form. These models focus largely on modeling the K/T boundary event [e.g. 1,2,3,4]. Models of impact plumes have calculated the amount of target rock that would be evaporated [2,5], the size of spherules produced by the condensation and crystallization of molten and vaporized rock [3], composition of spherules [1], and the effects of impact angle [4]. Unfortunately, all models to date have focused on the first few seconds after impact. Little is known about how the plume evolves temporally and spatially. In addition, field data to constrain these models is sparse because there are not abundant spherule layers in the geologic record and those that are present are often thin, incomplete, or bioturbated. The spherule beds in the Barberton greenstone belt (BGB) provide an ideal case to study and understand the evolution of an impact plume in time and space. The S3 spherule bed in the BGB is a particularly good candidate, because it is very thick, produced by a bolide approximately 30 km in diameter [6], it has not been bioturbated, primary spinel and some primary textures are preserved, and at least two outcrops preserve a fall sequence [7]. This study focuses on the total number and size distribution of spherules preserved in a full fall sequence from the Barite Syncline locality. Thin sections from the full stratigraphic height of the section were examined, and 1445 spherules were identified, classified, and measured to determine the volumetric abundance, size distribution and overall composition of the deposit. Spherule Types: Five primary types of spherules are contained within the S3 spherule bed [7]. Each of the five types of spherules is observed in every section of S3 and represents the altered remnants of a diverse population of impact-produced spherules. The only primary minerals preserved are Ni-rich chromites [6,8,9]. Type 1 spherules are now composed entirely or largely of microcrystalline and megacrystalline quartz. They lack crystallites, and the largest of these spherules have a devitrification texture. Rims surrounding the spherules contain variable amounts of patchy phyllosilicate material, opaque oxides, and carbonaceous material. Type 2 spherules are characterized by abundant patchy phyllosilicates, primarily sericite, that are well distributed within the spherules. The abundance of phyllosilicates in this population clearly sets them apart the large quartz spherules in which minor amounts of phyllosilicates occur either along the margins or as small, isolated patches within spherules. These spherules also commonly have rims similar to the Type 1 spherules. Type 3 spherules are composed almost entirely of fine-grained phyllosilicates. These spherules also commonly have thin rims with abundant opaque material, including carbonaceous material and fine-grained Feand Ti-oxides. Type 4 spherules are composed of multiple, compositionally distinct concentric layers. Most of these layered spherules have two or three distinct layers, excluding dark rims and diagenetic coatings. The most common layered spherules have a layer of fine-grained phyllosilicates surrounding a core of microcrystalline or coarsely crystalline quartz. Approximately 40% of the quartz-phyllosilicate layered spherules have relict fibroradial textures in the outer phyllosilicate layer. Other layered spherules are comprised of two layers of phyllosilicates, either with or without a central quartz core. Both quartz-phyllosilicate and phyllosilicatephyllosilicate layered spherules commonly have a thin boundary layer between the outer layer and the core that is composed predominantly of fine-grained Tioxides, including both rutile and anatase. Fe-oxide and Ti-oxide-rich rims are also present along the outer margins of many of these spherules. Type 5 spherules are now composed of quartz and have a relict barred texture or a microporphyritic needle-like texture, inferred to have been originally olivine [8]. Relative proportions of spherule types: Images of all thin sections were captured. Using image processing software, individual spherules were outlined, classified, and measured. The total volume for each group was estimated using the histogram tool. The results of these calculations are given in Table 1 and Figure 1.