| 其他摘要 | It is an important way for investigating the petrologic and petrophysical properties of lithosphere to compare the seismic data with the velocities of rocks measured under high temperature and high pressure. Geophysicists have paid more attention to the seismic low velocity zone (LVZ), because it usually is the source and the center of the geologic activity in earth’s interior. Scientists had proved it is an important reason for the LVZ that rocks are partially melted under high temperature and high pressure. In the previous works, the structure of crust and the LVZ were mostly investigated by measuring the compressional wave velocities (Vp) of rocks, but few studies were performed by analyzing the data of compressional and shear wave velocities (Vs). And few analyses were performed for the intermediate products during the experimental cycle. On the other hand, based on the special elastic properties of glass at high temperature and high pressure, some scientists presumed that the vitreous amorphization could account for the decrease in velocity in the LVZ. So the presumptions should be supported by the reliable experimental data. Here, author reports the studies on these issues above. In this work, all the measurements were carried out with the pulse reflection and the transmission-reflection method.
1. Wave velocities of the granite and the plagiogneiss, gathered in Sanjiang zone, China, were studied up to 1200 ℃ for Vp and up to 600 ℃ for Vs at high pressure, respectively. By analyzing the intermediate products during experimental cycle, elastic wave velocities of the rocks were discussed as functions of the dehydration, phase transition and partially melting. Some results were obtained as follows:
(1) The variation of wave velocities with pressure and temperature are similar between the granite and the plagiogneiss. At room temperature, wave velocities of the rocks increase with pressure. Both Vp and Vs are anisotropic and the anisotropies vary with pressure up to 0.5GPa.
(2) Under high pressure, the Vp of the rocks decreases slowly up to 750 ℃. As a result of the α-β transition of quartz, the Vp decreases quickly from 750℃ to 850℃, and then increases sharply from 850℃ to 950℃. Above 950℃, the Vp decreases quickly again because of the more melt in rocks.
(3) It is concluded that, based on the studies for both Vp and Vs, the granite is located in the upper crust in Sanjiang zone and the plagiogneiss is distributed from the upper crust to the middle crust, which is consistent with previous work based on studies for Vp.
(4) For the quartz-rich layer in Sanjiang zone, the α-β phase transition of quartz is the main reason for the LVZ. And it is also proved that partial melt in rock and the anisotropy of wave velocity can cause the LVZ in earth’s interior.
2. Investigations on seven types of glass, which were synthesized by quenching melted rocks range from basic to acidic in chemical compositions, were performed up to 1000 ℃ for Vp and 730 ℃ for Vs at 1.0GPa and 2.0GPa, respectively. The results are as follows:
(1) It is different from rock, in which wave velocity increases with pressure, that at room temperature and from 0.4 to 2.0GPa, the Vp of most of the glasses decrease with pressure except the basalt glass and the pyroxenite glass, and the Vs of all the glasses decrease with pressure. The increasingly more-anomalous behavior occurs with increase in SiO2 content.
(2) At high pressure, the velocities of glasses decrease slowly with temperature until the glass transition temperatures Tg, and then decrease at the faster rate.
(3) From this work, a new mode for LVZ has been proven, which hold that above 10vol% basalt glass in lherzolite would cause the LVZ in upper mantle and the intermediate and/or acidic glass will also result in LVZ in lower crust if which is above 20vol% in basic rocks. |
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