ABSTRACT
Quartz in the veins at the Shenxigou section of Wenchuan earthquake fault zone was investigated by micro-Raman spectroscopic measurement, and the distribution of compressive stress in the fault zone was estimated by the frequency shifts of the 464 cm-1 vibrational mode of quartz grains in the veins. It was showed that the 464 cm-1 peak arising from the quartz grains in the veins near the fault plane shifts by 3. 29 cm-1 , and the corresponding compressive stress is 368. 63 MPa, which is significantly lower than the stress accumulation on both sides due to multi-stage events. Stress accumulation increased with moving away from the fault plane in the footwall with the offset of the 464 cm-1 peak arising from the quartz grains in the veins increasing, which can reach 494. 77 MPa at a distance of 21 m with a high offset of 4. 40 cm-1 of the 464 cm-1 peak. The compressive stress gets the maximum value of 519.87 MPa at a distance of 10 m from the fault plane in the hanging wall with the offset of the 464 cm-1 peak arising from the quartz grains in the veins being 4. 62 cm-1, followed by a sudden drop in stress accumulation, and it drops to 359. 59 MPa at a distance of 17 m. Because of moving away from the foult plane at the edge of the foult zone, the stress drops to 359. 59 MPa with a small value of 464 cm-1 peak offset 3. 21 cm-1 at a distance of 27 m from the fault plane in the hanging wall due to the little effect by the fault activity. Therefore, the stress of Wenchuan earthquake fault zone is partially released, but the rest of the stress distribution is uneven, and there is also a high stress accumulation in somewhere in the fault zone, which reflects that the mechanical properties of the rocks in the fault zone have a characteristic of unevenness in space.
ABSTRACT
The melting and recrystallizing experiments of alkali basalt powder and mixture of pure oxides mixed as stoichiometry were performed at 3 GPa and 1 200 degrees C. Electronic microprobe analysis and Raman spectra showed that the recrystallized products were omphacites, the FWHM (full width at half maximum) of the Raman peak was narrow and its shape was sharp, which is attributed to the stable Si-O tetrahedral structure and the high degree of order in omphacite. Based on the results of previous studies, the influencing factors of omphacite genesis and its primary magma were discussed. The results showed that the formation of omphacite could be affected by many factors, such as the composition of parent rocks, the concentration of fluid in the system and the conditions of pressure and temperature. This result could support some experimental evidences on the genesis studies of omphacite and eclogite.
ABSTRACT
Variation of crystal structure of oligoclase with pressure was investigated by the approach of diamond anvil cell (DAC) and in situ micro-Raman spectroscopic measurement at room temperature and under pressures from 1.0 to 4.4 GPa. At 2.9 GPa a new peak round 517 cm(-1) appeared, and a new phase was produced. Near 3.4 GPa a major discontinuity occurs in the pressure dependence of 288 cm(-1) peak arising from the stretching mode of M-O, and 517 cm(-1) peak disappeared, it implied that the oligoclase underwent triclinic to monoclinic phase transition completely at about 3.4 GPa. The peaks at 458 and 516 cm(-1) peaks arising from flexural vibrational mode of Si-O-Si shifted linearly with the increasing pressures, the pressure-related slopes are 1.667 cm(-1)/GPa and 3.560 cm(-1)/GPa, respectively, whereas, the flexural vibrational mode of Al-O-Al at 480 cm(-1) did not shifted linearly with the increasing pressures. The position of 288 cm(-1) peak did not change obviously in comparision with 458, 516 and 480 cm(-1) peaks, which shifted to lower frequency during decompression. The phase transition pressure of feldspar relates to the species of cation in the octatomic rings.
ABSTRACT
Variation of crystal structure of natural clinochlore with pressure was investigated by the approach of diamond anvil cell (DAC) and in situ micro-Raman spectroscopic measurement at 200 degrees C and up to 7.7 GPa. The 481 and 786 cm(-1) peaks shift towards high-frequency linearly with increasing pressure at 200 degrees C. The linear relations between Raman shift (N, cm(-1)) and pressure (P, GPa) for the two peaks are: N = 11.136P+482.6 (R2 = 0.987 4) and N = 5.055P+785.7 (R2 = 0.983 7), respectively. The 865 cm(-1) peak arising from the stretching mode of the Si-O(nb) shifts slightly because of the strong repulsion between T cations at the tetrahedral sites and M cations at the octahedral sites in the TOT layer. Raman shift of 481 cm(-1) and 786 means the shortening of the length of M-O(br) and Si-CO(br) bonds since the peaks are contributed by the stretching mode of the M-O(br) and Si-O(br) respectively. No phase transition of clinochlore under the experimental condition was found. The results indicate that chlorite minerals may be stable at least at a depth of 80-90 km in the cold subduction zones, and the fluid derived from chlorite dehydration may be an important fact for earthquake occurrence in the subduction zones.
