THE VARYING QUENCH TEXTURES OF IMPACT SPHERULES: A STRATIGRAPHIC TOOL?

Impact spherules are perfectly round, tiny pieces of impact melt. Our spherules are between 30 and 200 microns in diameter. They are from abyssal cores that are Eocene and younger. All of the surface morphologies of our impact spherules are quench textures. We find four common types of quench textures: 1) glass, 2) columnar jointing, 3) spinifex, and 4) crystalline. The glassy spherules have extremely smooth surfaces and high silica contents. Some are nearly 100% SiO2. The remaining three types of spherules contain between 1% and 3% K, which precludes an origin as cosmic spherules. Some appear glassy in visible light, but SEM photos show rough hexagons on their surface that resemble columnar joints. The hexagons are not perfect, as might be expected in molds of microfossils. The spherules with columnar jointing are usually yellow in color and contain more iron than the other spherule types. Some spherules contain radiating aggregates of crystals that strongly resemble the spinifex texture found in Archean komatiites. Some crystalline spherules have crystals that appear to radiate out from the center of the spherule. Still other crystalline spherules have surfaces that are covered with crystals that are oriented horizontally to the surface of the spherule. Some of the crystalline spherules are hollow. The hollow crystalline spherules may be the first Phanerozoic examples yet found of some Archean age impact spherules. The impact spherules in our collection have a wide variety of colors, compositions and morphologies. This variety suggests that impact spherules might serve as a stratigraphic tool. Where biostratigraphy is available, the spherules with the same morphology and compositions have the same biostratigraphic age. However, impact spherules are preserved in water depths at which microfossil preservation is poor to non-existent. Despite their high K content, the spherules are so small that it is difficult to assemble enough spherules from one core to produce a reliable 40Ar/39Ar isochron. Consequently, varying spherule morphology, color, and composition in different impact layers may constitute important tools for stratigraphy in deep-sea cores.