Magnetic microsperules in Permian and Triassic bedded chert from Southwest Japan

摘要

Magnetic microspherules collected from the Permian and Triassic bedded cherts in Southwest Japan were studied. The size range, as estimated from 801 microspherules, vary from 3 to 100μm, with a peak size distribution between 10-20μm. Three broad shapes are recognized : spherical-, drop- and rocket-shaped, all categories including hollow particles. The surface textures as studied from scanning electron microscope show five dominant varieties : random mosaic, dendritic mosaic, feather crystal, scaly and cracked, with intermediate textures between these types. Chemical analyses of representative microspherules by electron microprobe indicate that they are mostly composed of magnetite, resembling in composition with microspherules of cosmic origin from other regions and magnetite grains in carbonaceous chondrites. Consideration of the origin of the observed morphologic, textural and chemical parameters in these microspherules, along with the available data on experimental synthesis of ultramicro iron particles, preclude an origin by volcanogenic processes and strongly suggest a cosmic origin. Knowledge of the rate of chert sedimentation allows us to make an empirical calculation on the rate of microspherule sedimentation. We compute that the fall of microspherules on the earthu27s surface occurred at the rate of about 1 t/day during Permian, while it increased to about 3 t/day during the closing of the era and the Triassic.