Effect of water and stress on the lattice-preferred orientation of olivine
Effect of water and stress on the lattice-preferred orientation of olivine
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DOI：
10.1016/j.tecto.2006.02.011
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发表时间：
2006-07
期刊：
影响因子：
2.9
通讯作者：
H. Jung;I. Katayama;Zhenting Jiang;T. Hiraga;S. Karato
中科院分区：
文献类型：
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作者：
H. Jung;I. Katayama;Zhenting Jiang;T. Hiraga;S. Karato
通过高温高压（P=0.5- 2.1GPa，T=1470-1570 K）下的大应变剪切变形实验，研究了贫水和富水条件下水分和应力对橄榄石聚集体晶格择优取向（LPO）的影响。这些样品是热压合成的橄榄石集合体或橄榄石单晶。通过滑石和水镁石的混合物的分解向样品提供水。在Griggs装置上进行了γ（剪切应变）≤ 1.6的变形实验，其中水逸度在2GPa压力下达到1.13GPa。在压力为0.5- 2GPa时，水在橄榄石中的溶解度为400-1200 ppmH/Si。橄榄石中的几种新类型的LPO被发现取决于含水量和应力。在贫水条件下变形的样品显示橄榄石的常规LPO，其中橄榄石[100]轴与剪切方向近似平行，（010）面与剪切面近似平行（A型）。然而，我们确定了三种新的类型（B型，C型和E型）的LPO的橄榄石取决于水含量和应力。在相对高应力和/或中高含水量条件下发现的橄榄石B型LPO的特征是橄榄石[001]轴与剪切方向近似平行，（010）面与剪切面近似平行。在低应力和富水条件下发现的C型LPO的特征是橄榄石[001]轴与剪切方向近似平行，（100）面与剪切面近似平行。在低应力和中等含水量条件下发现的E型LPO，其特征是橄榄石[100]轴与剪切方向近似平行，（001）面与剪切面近似平行。透射电子显微镜（TEM）和扫描电子显微镜（SEM）观察表明，贫水样品（A型）中的位错是弯曲的，并且B= [100]和B=[001]位错具有相似的布居。在背散射电子图像中，在贫水样品中可以看到许多亚晶粒。相比之下，富水样品（B型和C型）主要包含B=[001]位错，并且位错是直的，与贫水样品相比，亚晶界很少。这些观察结果表明：（1）橄榄石中的主要滑移系随水逸度（和应力）而变化;（2）水的存在增强了晶界迁移。富水条件下组构对应的地震各向异性与贫水条件下组构对应的地震各向异性有显著差异。因此，在富水区，地震各向异性和流动几何形状之间的关系预计将不同于A型组构占主导地位的贫水区（即，岩石圈）。讨论了包括俯冲带和上地幔深部各向异性在内的几种情况。
The influence of water and stress on the lattice-preferred orientation (LPO) of olivine aggregates was investigated through large strain, shear deformation experiments at high pressures and temperatures (P=0.5–2.1 GPa, T=1470–1570 K) under both water-poor and water-rich conditions. The specimens are hot-pressed synthetic olivine aggregates or single crystals of olivine. Water was supplied to the sample by decomposition of a mixture of talc and brucite. Deformation experiments were conducted up to γ (shear strain)∼6 using the Griggs apparatus where water fugacity was up to ∼ 13 GPa at the pressure of 2 GPa. The water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at P=0.5–2 GPa was ∼ 400–1200 ppm H/Si. Several new types of LPO in olivine are found depending on water content and stress. Samples deformed in water-poor conditions show a conventional LPO of olivine where the olivine [100] axis is subparallel to the shear direction, the (010) plane subparallel to the shear plane (type-A). However, we identified three new types (type-B, C, and E) of LPO of olivine depending on the water content and stress. The type-B LPO of olivine which was found at relatively high stress and/or under moderate to high water content conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (010) plane subparallel to the shear plane. The type-C LPO which was found at low stress and under water-rich conditions is characterized by the olivine [001] axis subparallel to the shear direction, the (100) plane subparallel to the shear plane. The type-E LPO which was found under low stress and moderate water content is characterized by the olivine [100] axis subparallel to the shear direction, the (001) plane subparallel to the shear plane. Observations by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) show that the dislocations in water-poor samples (type-A) are curved and both b= [100] and b=[001] dislocations have a similar population. Numerous subgrains are seen in water-poor samples in backscattered electron images. In contrast, water-rich samples (both type-B and type-C) contain mostly b=[001] dislocations and dislocations are straight and sub-grain boundaries are rare compared to those in water-poor samples. These observations suggest that (1) dominant slip systems in olivine change with water fugacity (and stress) and (2) grain boundary migration is enhanced in the presence of water. Seismic anisotropy corresponding to the fabrics under water-rich condition is significantly different from that under water-poor condition. Consequently, the relationship between seismic anisotropy and flow geometry in water-rich regions is expected to be different from that in water-poor regions in which type-A fabric dominates (i.e., the lithosphere). A few cases are discussed including anisotropy in the subduction zone and in the deep upper mantle.