Promotion of graphite formation by tectonic stress - a laboratory experiment

Abstract

S U M M A R YGraphitization of less-ordered hexagonal carbon was studied under in-situ pressure and temperature conditions on anthracite, black shale and a synthetic calcite/anthracite mixture at upper greenschist facies conditions. Anthracite exhibited a continuous loss of volatiles in the temperature range from 100 • C up to 850 • C (9.9 weight per cent at 450 • C) as detected by Differential-Thermal-Analysis (DTA) and Thermo-Gravimetry (TG). Energy dispersive X-ray diffraction (EDX) revealed a broad amorphous 002 graphite reflection while after p, T-treatment nearly perfect crystallized graphitic carbon was detected. The electrical conductivity was measured at the same time in the frequency range from 0.7 up to 100 kHz. As a function of time the bulk resistivity was decreased by about three orders in magnitude at constant pressure and temperature conditions (0.7 GPa, 450 • C), while the complex response exhibited a continuous decrease of the imaginary part of the impedance. 'Quasi-metallic' conduction now dominated the charge transport. Application of pressure, strain, temperature and time caused an increase in ordering and the degree of interconnection of the formerly randomly oriented carbon sheets. The experimental results are an approach towards the explanation of the abundant occurrence of crystalline graphite observed in overthrusts and nappe structures, which are distinguished by high-conductivity structures.

Tracing of paleo-shear zones by self-potential data inversion: case studies from the KTB, Rittsteig, and Grossensees graphite-bearing fault planes

“…On the basis of these two boreholes, it is evident that the graphite is associated with a number of steeply inclined shear planes (ELEKTB Group 1997;Nover et al 2005;Stoll et al 1995). As indicated in the "Background" section, this graphite forms an interconnected coating along shear planes over a distance of hundreds or thousands of meters, and it is locally concentrated in millimeter-thick layers that constitute good electrical conductors (Emmermann and Lauterjung 1997).…”

“…The shearing process is the reason beyond the interconnection of the graphite on the shear planes (ELEKTB Group 1997). Nover et al (2005) carried out a laboratory experiment to understand better the mechanisms that lead to the abundant formation of graphite in overthrust structures. They concluded that graphite is found on shear planes and attributed this graphite to shear movement, shear strain, and strain energy (Bustin et al 1995;Mathez et al 1995).…”

The Whitehill Formation a high conductivity marker horizon in the Karoo Basin

“…However, "graphitic particles" may also be produced by increased amounts of shear stress within this horizon (e.g. Nover et al, 2005). Graphite can also be from the reduction of CH 4 and CO 2 released from thermal alteration and cracking of both kerogen and bitumen which (depending on the temperature, pressure, and oxygen fugacity) can be oxidized or reduced to form graphite (Kribek et al, 1994).…”

Crustal fluid pathways imaged using magnetotellurics - implications for the South Australian heat flow anomaly

“…Formation of highly conductive graphite films has been found to occur in the presence of CO/CO 2 fluid during fracturing (Roberts et al 1999;Glover & Ádám 2008). Laboratory results by Nover et al (2005) have shown graphite crystals can form at temperatures and pressures simulating continental crustal conditions. During these trials, formations of graphite lead to an increase in conductivity of the host rock of up to three orders on magnitude (Nover et al 2005;Glover & Ádám 2008).…”

“…Laboratory results by Nover et al (2005) have shown graphite crystals can form at temperatures and pressures simulating continental crustal conditions. During these trials, formations of graphite lead to an increase in conductivity of the host rock of up to three orders on magnitude (Nover et al 2005;Glover & Ádám 2008). These results are consistent with the modelled conductive anomaly under Paralana, suggesting that it is a large graphite-rich body formed during the uplift of the MPD during the formation of the Flinders Ranges.…”