Collaborative Research: Deposition of Carbon on Newly-Formed Fracture Surfaces and Its Influence on Deformation and Electrical Properties of Rocks

无碳结晶岩石的裂纹表面通常覆盖着一层碳质材料薄膜。 详细的研究表明，至少在某些情况下，这些薄膜决定了大块岩石的电导率。 此外，实验表明，当岩石在含碳蒸汽存在下破裂时，其导电性增加。 在裂缝生成过程中，吸附层，最初只有单层厚，迅速形成新的表面。 这一层可能会随着时间的推移而增长，并可能改变物理特性。 这些观察结果提出了碳膜如何形成以及在什么条件下生长的问题，并影响岩石的机械和电气特性。 这些问题对于理解岩石膨胀导致断层沿着灾难性破坏时发生的情况特别相关，因此对于由流体和新断裂面的反应引起的导电性变化可能导致地震前兆电现象的可能性特别相关。研究人员正在进行一项跨学科的研究，在地壳中与地震成核有关的条件下，承载流体和矿物表面。 这些实验旨在回答以下问题。 (1)在地壳浅部条件下，在含碳气体存在的变形过程中形成的新矿物断裂面上沉积的碳质层的性质和厚度是什么？ (2)碳膜的沉积是否影响变形？ (3)沉积是否影响随时间变化的强度演化？ (4)沉积物是否影响导电性？ (5)对于不同的流体成分，沉积是否有根本的区别，以及它如何影响岩石的物理性质？这项研究包括一个渐进的一系列实验，涉及石英，斜长石，和角闪石的单晶在可变应变率，螺杆驱动，三轴压缩装置的变形。 的实验设置是大大改善了以前的研究，包括一个专门设计的细胞，以监测样品变形过程中的电阻率。 样品在几种类型的流体存在下变形，例如（1）纯二氧化碳以及（2）二氧化碳和一氧化碳的石墨饱和混合物，以及（3）二氧化碳和甲烷。 因此，实验确定了矿物裂缝上碳膜的形成和表征的最佳条件。 在实验期间，经受恒定轴向加载速率的样品表现出弹性变形的初始阶段，随后是屈服和宏观失效。 通过沿着宏观破坏曲线各点的截尾试验，探讨了变形状态与电阻率之间的关系。 此外，碳沉积和随时间变化的机械强度和电阻率之间的相互作用进行了探讨，通过应力松弛实验进行的应力水平低于那些导致宏观fault.The微观断裂分布在实验产品的电子显微镜和碳膜的特征在于电子探针和飞行时间二次离子质谱。 实验后的样品被切割和抛光的轴向截面和碳相在这些表面上的分布由电子探针映射。 飞行时间二次离子质谱用于分析表面的几个上层单层，以寻找微量的碳，因为。 然后将碳和微裂缝图与从变形实验获得的电阻率和应力测量值相关联。

Crack surfaces in otherwise carbon-free crystalline rocks are usually coated with thin films of carbonaceous material. Detailed studies have shown that these films determine, at least in some instances, bulk rock electrical conductivity. In addition, experiments indicate that as rocks fracture in the presence of carbonaceous vapor, their electrical conductivity increases. During fracture generation, an adsorbed layer, initially only monolayers thick, rapidly forms on new surfaces. This layer presumably grows with time and may potentially alter physical properties. These observations raise questions of how carbon films form and under what conditions they grow, and influence mechanical and electrical properties of rocks. These questions are particularly relevant to understanding what happens as rocks dilate in the time leading up to catastrophic failure along a fault, and thus to the possibility that changes in conductivity induced by reactions of fluids and new fracture surfaces could lead to earthquake precursory electrical phenomena.For this project, the investigators are conducting an interdisciplinary study of the interaction between carbon-bearing fluids and mineral surfaces under conditions relevant to earthquake nucleation in the crust. The experiments are designed to answer the following questions. (1) What is the nature and thickness of the carbonaceous layer deposited on new mineral fracture surfaces formed during deformation in the presence of carbonaceous gases under conditions of the shallow crust? (2) Does the deposition of carbonaceous films affect deformation? (3) Does the deposition influence time-dependent strength evolution? (4) Does the deposition affect electrical conductivity? (5) Is there a fundamental difference in deposition and how it influences rock physical properties for different fluid compositions?The study consists of a progressive series of experiments involving deformation of single crystals of quartz, plagioclase, and hornblende in a variable strain rate, screw-driven, triaxial compression apparatus. The experimental setup is substantially improved over those of previous studies, including a specially designed cell to monitor electrical resistivity during sample deformation. Samples are deformed in the presence of several types of fluids such as (1) pure carbon-dioxide as well as graphite-saturated mixtures of (2) carbon-dioxide and carbon-monoxide, and (3) carbon dioxide and methane. Thus, the experiments identify the optimal conditions for the formation and characterization of carbonaceous films on mineral fractures. During experiments, samples that are subjected to constant axial loading rates exhibit an initial phase of elastic deformation followed by yielding and macroscopic failure. The correlation between deformation state and resistivity is explored by curtailing experiments at various points along the macroscopic failure curve. Further, the interplay between carbon deposition and the time-dependent evolution of mechanical strength and resistivity is explored through stress-relaxation experiments performed at stress levels lower than those that lead to macroscopic failure.The microfracture distribution in the experimental products are determined by electron microscopy and the carbonaceous films characterized by electron microprobe and time-of-flight secondary ion mass spectroscopy. Post-experiment samples are cut and polished in axial sections and the distribution of carbonaceous phases on these surfaces are mapped by electron probe. The time-of-flight secondary ion mass spectroscopy is used to analyze the several upper monolayers of a surface to search for miniscule quantities of carbon because. The carbon and microfracture maps are then correlated to the resistivity and stress measurements obtained from the deformation experiments.