Compressional Behavior of Hydrous Orthoenstatite: Insight into the Nature of LVZ under Continental Plate
Compressional Behavior of Hydrous Orthoenstatite: Insight into the Nature of LVZ under Continental Plate
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DOI：
10.3390/min10010071
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发表时间：
2020-01
期刊：
影响因子：
2.5
通讯作者：
Florian Tian-Siang Hua;P. Dera;J. Kung
中科院分区：
文献类型：
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作者：
Florian Tian-Siang Hua;P. Dera;J. Kung
在5~7 Gpa和1100~1300°C条件下合成了一套含水正辉石晶体，与陆相区中上地幔条件相对应。合成的晶体呈透明的、无夹杂物的正面体形式，尺寸从100微米到几百微米不等。用拉曼光谱、红外光谱、电子显微镜和SIMS对晶体的铝含量和水分含量分别进行了表征，铝含量在2wt.%以下，水含量在~500~1000ppm之间。分析表明，这套含水正辉石的吸水能力与晶体结构中的铝含量有关。为了了解详细的晶体结构如何反映水和铝的影响，对这套含水正辉石进行了单晶X射线衍射分析。通过比较在常温下得到的结果，我们发现当Al含量较低时，Al~(3+)离子倾向于只占据一个四面体位置(Tb)。高压下X射线衍射结果分析表明，斜方辉石的弹性对低水和铝的存在不敏感。我们利用这一发现评估了大陆地区中上地幔的速度剖面，并与地震观测结果进行了比较。对比表明，陆相区低速带的形成源于地热剖面，而不是水对地幔相弹性的影响。
A suite of hydrous orthoenstatite crystals were synthesized at 5–7 GPa and 1100–1300 °C, corresponding to the mid upper mantle conditions in continental regions. The synthetic crystals presented a clear, inclusion-free, and euhedral form with a size range from 100 to a few hundred microns. The Al- and water content of crystals were less than 2 wt. % and ranging from ~500 ppm to 1000 ppm, respectively, characterized by Raman and IR spectroscopy, electron microscopy, and SIMS. The analysis shows that the capability of water incorporation for this suite of hydrous orthoenstatite is correlated to the Al-content in the crystal structure. To understand how the detailed crystal structure reflects the influence of water and Al, single crystal X ray diffraction analysis was performed for this suite of hydrous orthoenstatite. By comparing the results obtained at ambient conditions, we find that for low-level of Al-content, <2 wt. %, the Al3+ cation tends to occupy one of tetrahedral sites (TB) only. Analysis of the X ray diffraction results under high pressure indicates that the elasticity of orthoenstatites is insensitive to the presence of low-level water and Al. We use this finding to evaluate the velocity profile at the mid upper mantle of continental regions to compare with seismic observation. The comparison indicates that the cause of the low velocity zone in continental regions originates from the geotherm profile rather than the effect of water on the elasticity of mantle phases.