ISOTOPIC CHARACTERISTICS AND AGE DATING OF THE PUMICE IN OKINAWA TROUGH

The data on the isotope compositions of rubidium, strontium and oxygen in the pumice of Okinawa Trough are reported for the first time. The ages of the pumice were successfully dated with the method of U-series disequilibrium. Then, the material source, crystallization evolution of magma and activity cycles of volcanos are explored. Isotopic data s how that pumice magma was originally from the mantle, but had undergon e a full crystallization differentiation and had been contaminated to a fair extent by crust-derived materials before the magma was erupted out of the sea floor. According to the dating results available so far , the earliest volcanic eruption in Okinawa Trough occurred about 70,0 00 a ago and the latest eruption was about 10,000 a B.P. During this p eriod, there were three volcanic eruption cycles which were respective ly corresponding to the middle Late Pleistocene, the late Late Pleisto cene and the Early Holocene.