Effects of Crystalline Anisotropy on Fluid Distribution in Ultramafic Partial Melts
Effects of Crystalline Anisotropy on Fluid Distribution in Ultramafic Partial Melts
复制标题
晶体各向异性对超镁铁质部分熔体中流体分布的影响
DOI：
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
U. Faul
中科院分区：
文献类型：
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作者：
H. Waff;U. Faul
实验生产的超镁铁质部分熔体的织构与各向同性平衡理论预测的形态一致且显著偏离。在这些体系中，平面晶界普遍存在，它们与各向同性理论预测的光滑弯曲的边界共存。在压力为1.0至2.0 Gpa、温度为1350°C至1400°C的橄榄石-玄武岩混合物上进行了长期实验运行评估。体积百分比大于或等于2.5%的熔体的扫描电子显微镜图像显示，至少有20%的可观察到的晶界被熔体润湿。此外，在这个样本中，大约60%沿着三个结点出现的熔化小管被发现至少有一个平坦的界面，这一影响增加了渗透率与孔隙率的比率。实验证据和理论考虑都表明，在这些部分熔体中，平面是稳定的平衡或稳态特征，并且它们是由晶体控制的。晶体熔体形态受多晶聚集体约束下的结晶平衡习性(通过最小化单个晶体的表面能而获得)的影响。在这些运行中观察到熔体分布方式与熔体分数的依赖关系。当熔体分数很低时(小于1体积%)，织构主要由熔体填充的三重结和大部分干燥的晶界主导，而当熔体分数较高(但低于5体积%)时，沿晶界出现更多的熔体口袋和熔体薄膜。对观察到的织构进行了解释，将已建立的晶体生长和界面理论应用于稳态部分熔融体系。在部分熔体中广泛出现扁平或刻面晶面，需要对其渗透率以及块体弹性、滞弹性和电学性质的建模从现有的熔体分布模型进行重大改变。在预计橄榄石晶格择优取向的上地幔区域(例如，在大洋中脊附近)，小平面的存在和熔体分布的相关变化将产生各向异性渗透率和地震衰减的变化。
The textures of experimentally produced ultramafic partial melts show consistent and significant deviations from the morphology predicted by isotropic equilibrium theory. Flat crystalline interfaces are pervasive in these systems and they coexist with smoothly curved boundaries which are predicted by the isotropic theory. Long-duration experimental runs on olivine-basalt mixtures held at pressures between 1.0 and 2.0 GPa and temperatures from 1350°C to 1400°C were evaluated. Scanning electron microscope images of samples with 2.5 or more volume percent melt showed at least 20% of observable grain boundaries to be wetted by the melt. In addition, approximately 60% of the melt tubules occurring along triple junctions in this sample were found to have at least one flat interface, an effect which increases the ratio of permeability to porosity. Both the experimental evidence and theoretical considerations indicate that the flat faces are stable equilibrium or steady state features in these partial melts, and that they are crystallographically controlled. The crystal-melt morphology is influenced by the crystalline equilibrium habit (obtained from minimization of surface energies of individual crystals) under the constraints of polycrystalline aggregates. A dependence of the style of melt distribution on melt fraction was observed in these runs. At very low melt fractions (less than 1 vol %) the texture is dominated by melt-filled triple junctions and mostly dry grain boundaries, whereas at higher melt fractions (but below 5 vol %) more melt pockets and melt films along grain boundaries appear. An interpretation of the observed texture is made, applying established crystal growth and interface theories to steady state partially molten systems. The extensive occurrence of flat or faceted crystallographic faces in partial melts requires major changes in the modeling of their permeabilities, as well as bulk elastic, anelastic, and electrical properties, from existing models of melt distribution. In regions of the upper mantle where olivine lattice preferred orientation is expected (e.g., in the vicinity of mid-ocean ridges) the presence of faceted faces and associated changes in melt distribution will produce anisotropic permeabilities and changes in seismic attenuation.