Origin of microspherules in Paleozoic-Mesozoic bedded chert estimated by PIXE analysis

Studies on the correlation between mass extinctions and asteroid impacts have been carried out over more than a decade, while interpretations of the extinction event at the end of the Cretaceous era have met with a number of difficulties. The authors analysed microspherules collected from Paleozoic-Mesozoic bedded chert in southwest Japan by means of PIXE. Our results are as follows: 1) The origin of microspherules as inferred from the Ti/Fe and Cr/Fe ratios is clearly extraterrestrial; and 2) their average chemical composition is quite similar to the magnetic components of carbonaceous chondrite. Additionally, other important characteristics are the following: 3) Most of the spherules are hollow and are smaller than those obtained from recent drill core samples; and 4) the number of the microspherules shows a sharp increase in the Triassic-Jurassic boundary. Consequently these possible evidences seem to support the molecular cloud hypothesis, i.e. that the Solar System encountered an interstellar molecular cloud in the past. (orig.)