Experimental deformation of mica at high temperature under hydrothermal conditions

自然地球动力学过程包括造山、沉积盆地的发育和板块边缘的过程，这些过程可能导致地震或断层上缓慢、稳定的滑动。所有这些岩石都变形了。我们理解自然岩石变形过程的能力取决于能够在实验室中在高压和高温下进行变形（断裂和流动）实验，模拟地球深处的自然条件。从过去50年的这些研究中，可以用岩石圈结构的简单模型来预测岩石强度随深度的变化，从而提高了对地球动力学过程的理解。在这类模型中，通常作出一种简化的假设，即地壳外层的强度由石英决定，下地壳由长石决定，上地幔由橄榄石决定。云母组矿物的作用很少得到考虑，尽管这种矿物在泥岩及其等效变质岩中很常见，并且在许多主要断裂带中含量丰富。最近在这个实验室和其他地方进行的研究表明，在地壳中深度（约15公里）及以下，云母矿物可能比以前怀疑的要弱得多。这是由于激活了一种以前未预料到的与矿物的天然薄片结构平行的粘性滑动过程。如果这一过程能够被描述和证实，它将对我们理解大陆地壳的强度和所有依赖于岩石变形的地质过程产生重大影响。在这里，我们提出了一种新的高压/高温实验方案，旨在充分表征这种类型的粘性流动。剪切实验将在温度范围内进行，温度范围达到云母矿物通过去羟基化和最终熔化的化学分解的开始，使用高压将分解抑制到温度范围的上限（400至800摄氏度）。这将使我们可以使用的温度最大化，因为岩石变形过程在更高的温度下进行得更快。然后，我们可以从高温下相对较短的持续时间（最多几周）的实验室实验推断出较低温度环境但较长时间尺度的自然岩石变形。我们将使用合成的多晶云母“岩石”，但也使用单云母晶体，这应该最好地显示粘性蠕变过程。我们将确定云母试样的变形速率如何取决于施加的应力、温度、围压（埋藏深度）和孔隙水压力。对于实验变形的样品，我们将使用扫描和透射电子显微镜加上微化学技术来确定变形的机制和伴随它的任何化学变化。我们还将对自然变形的云母岩进行补充研究，帮助验证我们实验结果的适用性，并将我们的实验数据外推到地质变形条件，推断云母变形对自然地球动力学过程的影响。

Summary Natural geodynamic processes include mountain building, the development of sedimentary basins, and processes at plate margins that can lead either to earthquakes or to slow, steady slippage on faults. In all of these rocks become deformed. Our ability to understand processes of natural rock deformation depends on being able to carry out deformation (fracture and flow) experiments in the laboratory under high pressures and temperatures that simulate natural conditions at depth in the Earth. From such studies over the past 50 years it has become possible to predict variation of rock strength with depth for simple models of lithospheric structure, and this has led to improved understanding of geodynamic processes. In such models the simplifying assumption has usually been made that the strength of the outer part of the Earth's crust is determined by quartz, the lower crust by feldspar and the upper mantle by olivine. The role of minerals of the mica group has received little consideration, despite such minerals being common in mudstones and their equivalent metamorphic rocks and being abundant in many major fault zones. Recent studies in this laboratory and elsewhere suggest that mica minerals may become much weaker than previously suspected at mid-crustal depths (about 15 km) and below. This is due to activation of a previously unsuspected process of viscous sliding parallel to the natural sheet structure of the mineral. If this process can be characterized and verified it will have substantial implications for our understanding of the strength of the continental crust and of all geological processes that depend upon rock deformation. Here we propose a new program of high pressure/temperature experiments aimed at fully characterizing this type of viscous flow. Shearing experiments will be done at temperatures ranging up to the onset of chemical breakdown of mica minerals by dehydroxylation and ultimately melting, using high water pressures to suppress the breakdown to the upper end of the temperature range (400 to 800 degC). This will maximize the temperatures that we can use, for rock deformation processes operate faster at higher temperatures. We can then extrapolate from relatively short duration (up to a few weeks) laboratory experiments at high temperatures to the lower temperature environment but longer timescale of natural rock deformation. We will use both synthetic, polycrystalline mica 'rocks', but also single mica crystals, which should best display the viscous creep process. We will determine how the rate of deformation of the mica specimens depends on applied stress, temperature, confining pressure (depth of burial) and pore water pressure. For experimentally deformed samples we will use scanning and transmission electron microscopy plus microchemical techniques to determine mechanisms of deformation and any chemical changes that accompany it. We will also carry out complementary study of naturally deformed micaceous rocks to help verify the applicability of our experimental results, and carry out extrapolation of our experimental data to geological deformation conditions to deduce the impact of mica deformation for natural geodynamic processes.