| Summary: | 碩士 === 國立成功大學 === 地球科學系 === 107 === Crystallographic information plays an important role to interpret the physical properties of minerals in the upper mantle. The constituent minerals in the upper mantle, such as olivine, pyroxene, and garnet, are anhydrous in the crystal structure. However, OH-bond in the structure of NAMs were observed in natural samples from upper mantle. Water incorporated into minerals can affect their physical properties, such as elasticity and transport properties. The origin of the LVZ could be caused by reduction of seismic velocity induced by the change of physical property. Natural samples indicate that water content in pyroxenes is more twice or three-fold than those in olivine and garnet. Besides, water in OEn could change the phase boundary from OEn to HPCEn and may be the cause of the discontinuities in the upper mantle. Thus, the knowledge of compression behavior for the hydrous OEn shall shed light on the physical state of the upper mantle.
We synthesized the OEn from 5-7 GPa and 1100-1300℃. The pure OEn contains ~500 ppm water and Al-bearing OEn contains up to 1000 ppm, like the water content in natural pyroxene. The study shows the compression behaviors of hydrous orthoenstatite (OEn) at high pressure by single-crystal X-ray diffraction. The compression behavior of anhydrous OEn is similar to hydrous OEn which is about 500 ppm water content. However, there is a minor influence on the compression behavior along a-axis in Al-bearing OEn.
In this study, the results show that ~1000 ppm of water in the OEn does not have much influence on the compression behavior. It indicates that the influence of water on the P-wave velocity of OEn with the pressure in the upper mantle is negligible. On the other hand, the S wave velocity of the hydrous OEn has not been measured so far. Therefore, by combination with the S-wave velocity experimental data, it is expected that the impact of water on the velocity in OEn will be more fully understood in the future. Also, our estimation of shear velocities according to the transition of thermal gradient shows that it reduces insignificantly in the LVZ in continental regions. It implies that the origin of velocity reduction of the LVZ in the continental areas could be contributed by temperature rather than pressure.
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