Cosmic Spherules from Triassic Deep-sea Sediments in Japan

Introduction: Cosmic spherules are subspherical to spherical particles of <1 mm diameter which are produced by melting of interplanetary dust and large objects during atmospheric entry. Although cosmic spherules are known principally from the ice sheets of polar regions and Cenozoic deep-sea sediments [1-3], a few spherules older than Mesozoic were recovered from the sedimentary rocks of pelagic limestone, radiolarian chert, and halite [4, 5]. Here we report newly-discovered spherules from the Triassic radiolarian chert of southwest Japan, which is considered to have accumulated in a mid-oceanic basin of the ancient Pacific Ocean (Panthalassa). Spherules from the Triassic chert are of great significance for understanding the composition of earthcrossing materials as well as their flux rate during Triassic time. Samples and methods: Spherules were discovered from the Triassic radiolarian chert succession of the Chichibu terrane, defined as a Jurassic subduction-generated accretionary complex in Japan. Radiolarian chert of the Chichibu terrane is considered to be long-lived Panthalassic remnants that represent a far-traveled proto-Pacific plate assemblage [6]. Our study section of the Chichibu terrane is located on Ajiro Island (GPS coordinates:33 ̊4.12N/131 ̊55.11E) in eastern Kyushu, southwest Japan. Radiolarian biostratigraphic investigation reveals that the chert succession (19 m thick) on Ajiro Island is correlated with the Anisian (ca. 245-237 Ma) to lower Ladinian. The average sedimentation rate of the chert, estimated from measured thickness and the time interval, is approximately 2 mm/kyr. Spherules were extracted from samples of thin siliceous shale partings intercalated within 2-6 cm thick chert beds. Samples were mechanically crushed and their fragments were passed through a 250-!m mesh sieve. A grain size of less than 250 !m were selected until the total value of them became greater than 3 g. After sieving, the magnetic components were separated using the method for liquidsuspended particles [7]. Spherules were handpicked from the magnetic components under a binocular microscope and analyzed for detailed textural features using scanning electron microscope (SEM). The surface texture of the spherules was observe first, and Fig. 1. Secondary (a, b) and backscatter (c, d) electron micrographs of cosmic spherules from Middle Triassic (Anisian) radiolarian chert. a, c) I-type spherule; b, d) porphyritic S-type spherule.