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: 0337747
• 负责人: Andreas Kronenberg
• 金额: $ 8.9万
• 依托单位: Texas A&M Research Foundation
• 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 2006-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0337747&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.

无碳结晶岩的裂隙表面通常覆盖着碳质材料的薄膜。详细的研究表明，至少在某些情况下，这些薄膜决定了岩石的电导率。此外，实验表明，当岩石在碳质蒸气存在下破裂时，其电导率增加。在裂缝形成过程中，最初只有单分子层厚的吸附层在新表面上迅速形成。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。这些观察提出了碳膜是如何形成的以及在什么条件下生长的问题，并影响了岩石的机械和电学性质。这些问题对于理解岩石在沿着断层发生灾难性破坏的过程中膨胀时会发生什么，以及流体和新的断裂面的反应引起的电导率变化可能导致地震前兆电学现象的可能性特别相关。在这个项目中，研究人员正在进行一项跨学科的研究，研究与地壳成核相关的条件下含碳流体和矿物表面之间的相互作用。这些实验旨在回答以下问题。(1)碳质层的性质和厚度是什么？(2)碳质膜的沉积是否影响变形？(3)碳质膜的沉积是否影响随时间变化的强度演化？(4)沉积是否影响电导率？(5)不同流体成分的沉积是否存在根本差异及其对岩石物理性质的影响？本研究包括一系列渐进的实验，包括在变应变率、螺杆驱动、三轴压缩装置中对石英、斜长石和角闪石单晶的变形。与以前的研究相比，实验装置有了很大的改进，包括一个专门设计的监测样品变形过程中的电阻率的池。样品在几种类型的流体存在的情况下变形，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，以及(3)二氧化碳和甲烷。因此，实验确定了在矿物裂隙上形成和表征碳膜的最佳条件。在实验过程中，承受恒定轴向加载速率的样品呈现出弹性变形的初始阶段，随后是屈服和宏观破坏。通过沿宏观破坏曲线不同点的截断实验，探讨了变形状态与电阻率之间的关系。此外，通过在低于导致宏观失效的应力水平下进行的应力松弛实验，探讨了碳沉积与机械强度和电阻率随时间的演化之间的相互作用。通过电子显微镜和电子探针和飞行时间二次离子质谱仪对实验产品中的微裂纹分布进行了表征。对实验后的样品进行了轴向切割和抛光，并用电子探针绘制了碳相在这些表面上的分布。飞行时间二次离子质谱仪被用来分析表面的几个上层单分子层，以寻找微量的碳。然后，将碳和微断裂图与变形实验获得的电阻率和应力测量结果相关联。