Discovery of Ni-rich spinels in Upper Triassic chert of the Mino Terrane, central Japan

Abstract

The magnetic fraction of a lower Norian claystone layer (ca. 5 cm thick) in a radiolarian chert succession within the Mino Terrane, Inuyama area, Central Japan, contains a large number of small euhedral to subhedral crystals of oxidized Ni-rich spinels. The stratigraphic position of this concentration of Ni-rich spinels is clearly indicated by a sharp increase in magnetic susceptibility at the claystone layer. These spinels are distinguished from typical igneous spinels by their high Ni and Fe3+ contents, and show large variations in composition. The textures and chemical compositions of the spinel crystals are similar to those of Ni-rich spinels at the Cretaceous-Paleogene boundary. The discovery of Ni-rich spinels in a lower Norian claystone suggests an important sedimentary record of an extraterrestrial impact in the Late Triassic.