Abstract: To understand the mechanism of lower-crust earthquake and slow slips, it is necessary to study the frictional properties of mafic rocks and their major rock-forming minerals. Previous studies have performed a series of experimental researches on gabbro, basalt and their major constituents.
According to the results of previous experiments, frictional sliding of plagioclase under hydrothermal conditions(100~600℃)shows a property of velocity weakening, and the experimental results show that both the direct rate effect parameter(a)and the healing effect parameter(b)increase with temperature, a typical feature for thermally-activated processes. Velocity weakening means property of a shear band that has a stronger friction healing effect than the direct rate effect in the rate and state friction constitutive framework, and the healing effect(b value)in constitutive relation mainly reflects the increase in contact area with time under hydrothermal conditions, with some minor effect of structural changes. Since the microphysical mechanism of feldspar minerals at the contacts is mainly brittle cataclasis for temperatures below 600℃, the significant frictional healing effect in this case can only be explained by the mechanism of pressure solution. In order to determine if the dissolution process of plagioclase actually occurs on the laboratory time scale, we conducted hydrostatic experiments on plagioclase powder samples under hydrothermal conditions whereby frequent contact switch between particles seen in frictional sliding experiments can be avoided, making the observation on the dissolution sites possible.
Experimental temperatures were 400℃ and 500℃, with confining pressure of 90~150MPa, pore pressure of 30MPa, with 2mm initial thickness of fault gouge. The mechanical data show that a creep process occurred in the plagioclase fault gouge in the experimental temperature and pressure range; and the microstructures of the experiment show that precipitation of new grains is prevalent as the product of pressure solution process between plagioclase particles. At the same time, it is observed that the contact points have an appearance similar to fused, fuzzy structure as signatures of dissolution. The results of our experiments provide a definite experimental evidence for the healing mechanism in friction of plagioclase and for the theoretical relation between unstable slip and the pressure solution process.
The results of the experiments are summarized as follows:
(1)Drainage rate of pore water in plagioclase gouge was high in the first few hours of experiment, but gradually decreases over time for both temperature and pressure series of experiments slowing down to a steady state. This feature indicates that there is a creep process that evolves inside the plagioclase gouge.
In the temperature-series experiments, the drainage rate of the pore water in the plagioclase gouge at 400℃ is relatively low than the cases for higher temperatures. Thus, the applied temperature is positively correlated with the creep of plagioclase gouge.
(2)Scanning electron microscopy(SEM)observations of the experimentally deformed samples were performed on thin sections cut along the sample axis. Firstly, from the images of microstructure, it was found that the degree of particle fracture became more significant at a higher effective pressure, with smaller pore volume between particles. In the temperature-series experiments it was found that the degree of compaction of plagioclase gouge increased with increasing temperature. Precipitation of plagioclase grains in layered structures was generally observed in high-magnification images, indicating the presence of pressure solution processes. Contact points were also found to be in a state of ambiguity that seems to be a fused morphology, but the details of the structure remain to be determined by further observations.
The above results indicate that the pressure solution process of plagioclase particles can occur on a typical laboratory time scale, and the results of this study provide robust experimental evidences for the theory that links between pressure solution and the mechanism of frictional healing and unstable slips for plagioclase.

Key words: plagioclase, frictional healing effect, pressure solution, precipitation, isostatic experiment

摘要： 斜长石在水热条件下（100~600℃）的摩擦滑动实验表现为速度弱化，且实验结果表明其速率直接效应参数（a）和愈合效应参数（b）均随温度的增加而增加。在速率与状态摩擦的本构理论中，速度弱化是摩擦愈合效应比直接效应更强的结果，而速率状态摩擦本构关系中的b在水热条件下主要反映了实际接触面积随时间的增长。由于长石类矿物在600℃以下的接触点微观变形机制以脆性破裂为主，因此在该条件下其明显的摩擦愈合效应只能用压溶机制来解释。为了确定斜长石的压溶过程是否可以在实验室的时间尺度下发生，文中在水热条件下对斜长石粉末样品进行了均匀静压实验研究，由此可避免摩擦滑动实验过程中因颗粒之间接触的持续时间短而无法观察压溶点的问题。实验的温度条件为400℃和500℃，围压为90~150MPa，孔隙压力为30MPa，断层泥厚2mm。实验力学数据表明斜长石断层泥在实验温度和压力范围内均发生了流变过程；实验微观结构显示斜长石颗粒析出现象普遍存在，为斜长石颗粒之间压溶过程的产物，同时也观察到接触点有类似融合的模糊结构，为压溶留下的痕迹。文中的实验结果为斜长石在摩擦滑动中的愈合机制及不稳定滑动与压溶过程关联的理论推断提供了确切的实验证据。

关键词: 斜长石, 摩擦愈合效应, 压溶析出, 均匀静压实验