Dihedral Angle of Carbonatite Melt and Olivine System at Low Temperature
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摘要: 碳酸盐熔体是地球内部重要的一种流体介质，具有较硅酸盐熔体更强的物理化学活性。上地幔中少量碳酸盐熔体的出现，会显著影响其诸多地球化学和地球物理学性质，如交代元素组成、高电导异常产生等。高温高压下的实验研究是认识熔体物理化学效应的强有力途径。熔体产生的很多物理化学影响与熔体在体系中的几何分布特征有密切关系，而熔体的二面角分布是相关表征中的一个重要参数。关于碳酸盐熔体二面角(和各种物理效应)的研究往往是在超过1 200 ℃的极高温条件下开展的，其潜在的问题是熔体与体系中固态介质间的复杂反应难以避免，且极端高温下的实验难度非常大。为了克服这些问题，选择一种特殊的低熔点碳酸盐混合物作为模拟介质，在不超过700 ℃的低温下对碳酸盐熔体-橄榄石体系的颗粒边界熔体分布进行了实验研究。相关工作使用活塞圆筒压机在1 GPa条件下进行，采用扫描电镜对产物进行了检测。结果表明，产物中熔体均匀分布，颗粒边界熔体的二面角主要分布在10°～40°，其平均值为24°～27°。这意味着这种熔体具有良好的颗粒边界湿润能力，与已有关于碳酸盐熔体二面角分布的极高温实验研究具有良好的一致性。这为探究碳酸盐熔体的地球物理效应提供了一种新的模拟介质。Abstract: As an important fluid medium in the upper mantle, carbonatite melt has stronger chemical and physical activity than silicate melt. The occurrence of a small amount of carbonatite melt in the upper mantle will significantly affect its many geophysical and geochemical properties, such as electrical conductivity and element composition. Experimental studies at elevated conditions are important approaches to understand the chemical and physical effects of carbonatite melt. The physical and chemical influences of carbonatite melt are closely related to its distribution and geometry in the system, and a key factor of them for the characterization is the dihedral angle. Available studies on the dihedral angle (and various physical effects) of carbonate melt are normally carried out at extremely high temperature exceeding about 1 200 ℃, and the potential problem is that the complex reactions between the melt and solid minerals is inevitable and the experiment is difficult under extreme high temperature. In this work, in order to overcome the problems, the reported dihedral angle distribution in the carbonatite-olivine system at low temperature not exceeding 700 ℃ was measured by a low melting pointing carbonatite mixture. The experiments were conducted at 1 GPa with an end-loaded piston cylinder apparatus, and the dihedral angle distribution in the recovered samples were carefully examined by scanning electron microscopy. The results demonstrate a homogeneous distribution of melt in the system, and the observed dihedral angles are mostly 10°−40°, with the average values of 24°−27°. Consequently, this carbonatite has greater ability in wetting grain boundaries, and provides a new analog for future studies on the behavior and geophysical properties of carbonatite melt inside the Earth.
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图 1 高温高压样品组装示意图
Figure 1. Sketch for sample-capsule design at high temperature and pressure
图 2 真实二面角的表征方法(a)和随机截面中
$\psi$ 与$ \theta$ 的关系(b)示意图Figure 2. Cartoon for the true dihedral angle (a) and the relationship between
$\psi$ and$\theta$ in a random profile (b)
图 5 低温和高温下碳酸盐熔体-橄榄石二面角的实验结果对比
Figure 5. Carbonatite-olivine dihedral angle at low and high temperature
表 1 橄榄石的化学成分
Table 1. Composition of starting and recovered olivine
% Sample SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Initial 40.35 0.01 0.02 0.01 9.26 0.15 48.26 0.07 0.02 0.01 0.42 98.56 Sample A 41.61 0.02 0.01 0.02 8.87 0.10 48.17 0.05 0.01 0.01 0.39 99.25 Sample B 40.80 0.05 0.02 0.02 8.73 0.12 48.98 0.06 0.01 0.01 0.35 99.14 
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