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

合作研究：碳在新形成的裂隙表面的沉积及其对岩石变形和电性能的影响

• 批准号: 0337188
• 负责人: Edmond Mathez
• 金额: --
• 依托单位: American Museum Natural History
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-12-01 至 2007-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0337188&HistoricalAwards=false
• 关键词: Collaborative; Research; Deposition; Carbon; Newly

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.

在不含碳的结晶岩石中，裂纹表面通常包裹着一层含碳物质的薄膜。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导电性。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。在裂缝形成过程中，一层最初只有单层厚的吸附层在新的表面上迅速形成。这一层可能会随着时间的推移而增长，并可能潜在地改变物理性质。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，并影响了岩石的力学和电学性质。这些问题与理解岩石膨胀导致沿断层发生灾难性破坏时发生的情况特别相关，因此也与流体和新断裂表面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有关。在这个项目中，研究人员正在进行一项跨学科的研究，研究含碳流体和矿物表面在与地壳地震成核有关的条件下的相互作用。设计这些实验是为了回答以下问题。(1)浅层地壳条件下，在含碳气体作用下变形形成的新矿物断裂面上沉积的碳质层的性质和厚度如何？(2)碳质膜的沉积是否影响变形？(3)沉积是否影响强度随时间的演化？(4)沉积是否影响电导率？(5)沉积是否存在根本差异？不同流体成分对岩石物理性质的影响如何？该研究包括一系列渐进的实验，包括石英、斜长石和角闪石单晶在变应变率、螺旋驱动的三轴压缩装置中的变形。实验装置比以前的研究有了很大的改进，包括一个专门设计的电池来监测样品变形过程中的电阻率。样品在几种流体存在下会变形，例如：(1)纯二氧化碳以及石墨饱和的混合物；(2)二氧化碳和一氧化碳；(3)二氧化碳和甲烷。因此，实验确定了矿物裂缝上碳质膜形成和表征的最佳条件。在实验中，试样在恒定轴向加载速率下表现出初始阶段的弹性变形，随后是屈服和宏观破坏。通过宏观破坏曲线上各点的截尾实验，探讨了变形状态与电阻率的关系。此外，通过应力松弛实验，在低于导致宏观破坏的应力水平下进行应力松弛实验，探索了碳沉积与机械强度和电阻率随时间变化的相互作用。用电子显微镜测定了实验产物的微断裂分布，并用电子探针和飞行时间二次离子质谱对碳质膜进行了表征。实验后的样品在轴向切片上进行切割和抛光，并用电子探针绘制了这些表面上碳质相的分布。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。然后将碳和微断裂图与变形实验中获得的电阻率和应力测量相关联。