INCREASING URANIUM MIGRATION IN CHONDRITIC METEORITES WITH ANNEALING TEMPERATURE

Uranium in two new chondrites-Yanzhuang Meteorite and Sixiangkou Meteo rite in China-as well as in jilin Meteorite has been mapped by the fis sion track technique after different annealing temperatures from 700 t o 1300 degrees C for 1 hour. Uranium concentration and microscopic dis tribution have been obtained by counting tracks of induced fissions of U-235 in, the meteorites. It shows that: (1) Uranium concentration in chondrules decreases with increasing of annealing temperature while t he total uranium concentration in chondrules and their surrounding mat erials in each chondrite is nearly constant. This means that uranium m igrates from inside to outside of chondrules at rate which increases a s temperature increases. (2)Total Uranium concentration is about 20 pp b in Sixiangkou Meteorite and about 14 ppb in both Yanzhuang Meteorite and Jilin Meteorite. These results confirm that Uranium concentration in L group of meteorites is higher than that in H group. (3)It seems that the informations of Uranium concentration and its microscopic dis tribution can be used to study the processes of formation and evolutio n of chondrites.