Terrestrial history of antarctic meteorites recorded in the cosmogenic nuclides.

By some mechanisms which have not been well explored yet, antarctic meteorites have naturally been concentrated in bare ice areas of Antarctica, and the quantities as well as the number of species of the fragments of the stone meteorites are impressively large. Based on the studies of cosmogenic nuclides in these samples, we may extend our discussions to the following problems: (1) The terrestrial ages are distributed thus far between 104 and 7·105y, or 2·105y on the average. The age seems to correspond to the mean life of the meteorites in Antarctica. (2) Their occurrences are indicating already that as a part of showers, each group of 10 to more than 100 small fragments which were recovered in small areas could be attributed to the same fall. In general, a pairing of fragments could be confirmed by the classification, the terrestrial age, the exposure age (21Ne and 40K contents), the contents of radiogenic and trapped components, etc. . On the other hand, many unpaired independent fragments have also been identified by the same measurements. (3) Although independent small meteorite falls may also be found, the majority seem to-be composed of fragments of ordinary size, 10-104 kg stone meteorites, as indicated by the 53Mn contents and the 22Ne/21Ne ratios. On the whole, local falls on the ablation area may be responsible for more than half of the total weight (depending on a material balance of ice). This amount is in the range of estimates from the world annual meteorite falls and the terrestrial age. The rest seems to be composed of many independent fragments which have been transported to the same ablation area by a local glacier. The fragments of many falls in a wide region of the accumulation area have been mixed together; and only a small fraction (10-2) has been exposed on the blue ice fields, giving an outstandingly enriched variety of meteorite species.