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

Texas A&M Research Foundation, College Station TX

Investigators

Abstract

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.