P wave velocity of the pumice from the Okinawa Trough at high temperature and high pressure.

P wave velocity of the pumice sample from the middle Okinawa Trough and andesite sample from vicinity Yingdao volcanic island, Kyushu Japan were measured at temperature (from room temperature to 1500 C) and pressure (from room pressure to 2.4GPa) using a multi-anvil pressure apparatus called the YJ-3000 press. The measured data shows that at low temperature and low pressure (<1GPa, <800degreesC), the P wave velocity of pumice is lower than that of andesite, while at high temperature and high pressure (>1GPa, >800degreesC) the P wave velocity of pumice and andesite. becomes consistent (5.9km/s). The paper points out that 1GPa/800degreesC is the point of thermodynamic phase transformation Okinawa Trough pumice and vicinity andesite, and the point is deeper than 18km.

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 show that pumice magma was originally from the mantle, but had undergone 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 70,000 a ago and the latest eruption was about 10,000 a B.P. During this period, there were three volcanic eruption cycles which were respectively corresponding to the middle Late Pleistocene, the late Late Pleistocene and the Early Holocene.

Millennial-scale paleoceanography in Okinawa Trough during Late Quaternary period

Based on the analyses of foraminifer and accelerator mass spectrometer radiocarbon dating in DGKS9603 core from mid-Okinawa Trough close to bottom, oscillation curve, which expressed the relation between the surface water temperature and the depth, has been obtained by using foraminifer analysis and calculation of FP-12E transfer function. The whole core indicated seven cold phases and eight warm phases. Obvious expression of low temperature event during Middle and Late Holocene, YD,H1,H2,H3 and H4 events, as well as the short cold phase during the middle last glacial period, implied that short shifts since 50 kaBP would have been global significance. Sedimentation rate during cold phases is usually faster than that in warm stages, with the lowest rate in Holocene, which may be connected with rising sea level and principal axial of Kuroshio Current moving to west. Volcanic activities highly developed in Okinawa Trough during the Quaternary period, thus abundant volcanic glass and pumice were well preserved.

Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

Volcanic rocks from the northern and middle Okinawa Trough were dated by uranium-series dating method. Differential fractions using magnetic procedure were designed to separate samples. New report on the ages and isotopic data of rocks in the northern trough (especially black pumice) was discussed. Based on the uranium dates and Sr-Nd isotopic ratio, magmatic evolution process of the Okinawa Trough was noted. Firstly, there have been wide silicic volcanic activities in the Okinawa Trough from late Pleistocene to present, and the volcanic rocks can be divided into three subgroups. Secondly, magma generally came from PREMA source area under the Okinawa Trough. Magmatic evolution in the northern trough was similar to the middle, but different to the south. Finally, volcanic activities indicated that opening of the southern Okinawa Trough did not happen due to the collision between Luson Arc and Eurasian Plate until the early Pleistocene.