Abstract
Triassic and Jurassic bedded chert sequences in Japanese accretionary complexes are recognized as pelagic deep-sea deposits characterized by very low sedimentation rates (less than a few millimeters per thousand years) and the absence of coarse-grained terrigenous clastics. Owing to their low sedimentation rates and long-term continuous deposition, these bedded cherts may record the accretionary history of extraterrestrial material over geologic time. Here, we review studies of extraterrestrial material contained in bedded chert sequences from the Jurassic accretionary complexes, southwest Japan. Cosmic spherules, which are submillimeter-sized extraterrestrial particles that melt during atmospheric entry, have been found in Triassic and Jurassic bedded cherts from the Mino-Tanba and Chichibu belts. Two major spherule types have been identified, based mainly on their mineral and chemical compositions: iron-type and silicate-type. The extraterrestrial origin is indicated by the presence of metallic iron and nickel in the iron-type spherules, and by the chondritic composition and mineral assemblages of the silicate-type spherules, which contain olivine and low-Ca pyroxene. The detection of platinum group elements (PGE) and data on osmium (Os) isotope ratios (187Os/188Os) in the cherts provide evidence for crater-forming meteorite impact events. Anomalously high PGE concentrations and negative Os isotope-ratio excursions reported from Upper Triassic bedded cherts are also interpreted to indicate a large extraterrestrial impact event. Systematic studies of ancient cosmic spherules and extraterrestrial PGE signatures in bedded cherts could provide evidence for long-term changes in the composition and flux of incoming extraterrestrial matter.
