Correlation of Tephra Layers in Antarctic Ice by Trace Element Abundances and Refractive Indices of Glass Shards

Abstract

A large number of meteorites have been discovered in the bare ice areas of the Yamato Mountains and the Allan Hills, Antarctica. There are many dirt layers in these areas. It was confirmed recently that the dirt layer consists of tephra layers. As one tephra layer is considered to have been produced by asingle volcanic event, the age of ice can be estimated from the age of the tephra layers. Hence, a tephra layer can serve as a possible time marker of the ice. Therefore, it is possible to indirectly detemine the age of meteorite fall from tephra layers. One dirt layer is composed of ice with dispersed ash grains and its thickness is a few cm～10cm. From this thickness of ice, it is implied that tephra was supplied continuously over 1 year from a source volcano. Because a large scale eruption of a volcano usually does not continue for such a long period, a thick dirt layer may be a composite of tephra layers from multiple eruptions. In order to examine whether the tephras present in ice are products of a single eruption or multiple eruptions, the chemical abundances of more than 20 elements and the refractive indices of glass shards in the thin slabs of a dirt layer were determined by instrumental neutron activation analysis (INAA) and the phase-contrast technique, respectively. Those of glass shards separated magnetically from a dirt layer were also determined. INAA of the glass shards separated from 3 dirt layers in the Yamato Mountains area and 5 dirt layers in the Allan Hills area were carried out to test the correlation techniques for tephra layers in the Antarctic ice. The refractive indices of them were also determined. The results of these measurements show that most of dirt layers consist of a single fall unit of tephra, except one dirt layer in the Allan Hills area. The tephra layers in the Yamato Mountains area were divided into 2, and the tephra layers in the Allan Hills area were divided into 4. The source volcanoes of the tephras in the Yamato Momtains area, which is composed of island arc tholeiite, should lie on the South Sandwich Islands. The sources of those in the Allan Hills area which is composed of alkalic rocks, should be in the McMurdo Sound area. Trace element abundances and refractive indices of glass shards are useful tools for correlation of tephras in the Antarctic ice.