IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1997
Formation of Carbon Films on New Fracture Surfaces: Implications for
Continental-Collision-Zone Conductivity Anomalies
Dr. Al Duba
working group on EM induction Geomagnetics IAGA
E. Mathez, S. Karner, A. Kronenberg, J. Roberts
Electrical resistivity of dense crystalline quartzite is reduced by carbon films deposited on fractures
during failure experiments performed at T=400 C in the presence of carbon- bearing fluids. Hollow
cylinders of Sioux quartzite, jacketed by silver, were hydrostatically loaded to failure by applying
pressurized argon gas at the outer diameter (reaching ~290 MPa at a rate of 0.1 MPa/s) while
maintaining a constant pore pressure at the inner diameter. Pore fluids consisted of CO, CO2, CH4, a
1:1 mixture of CO2 and CH4 (each with pore pressures of 2.0 to 4.1 MPa) and air (at atmospheric
pressure). Biaxial-stress states are calculated using elastic-stress solutions that account for the applied
pressures and hollow-cylinder dimensions. For the inner wall of the cylinders, effective radial stress (sr)
is zero and calculated effective differential stresses (sq- sr) reach 1225 MPa. Failure of hollow Sioux
quartzite cylinders occurred by the formation of mode II shear fractures that transect the cylinder wall.
The distribution of carbon in the run products was mapped by electron probe. Samples deformed in CO2
and air contained little or no carbon above the small amount that exists in the undeformed rock.
Samples deformed in CO contain ubiquitous carbon films on the fracture surfaces that formed during
deformation. Because carbon is absent on other free quartz surfaces that existed during the
experiments, we conclude that the carbon films formed preferentially on the fractures as they formed.
The radial resistivity of dry, undeformed Sioux quartzite cylinders is extremely large in the ambient
laboratory atmosphere (>23 MW-m). The radial resistivity of Sioux quartzite cylinders that failed in pore
fluids that promote carbon deposition are lower (2.8 to 4.6 MW-m for CO tests; 15.2 to 18.4 MW-m for
CO2:CH4 tests). The results of this study help to isolate the role of carbon deposition on fresh fracture
surfaces in altering the electrical properties of rocks with little initial porosity from that of carbon
deposition on fractures and preexisting equant voids of porous rocks. Taken together, our results and
those of Roberts et al. (1999) indicate that electrical conductivity in rocks may be enhanced due to
carbon deposition on grain and/or fracture surfaces. Our results are important for studies linking
variations of crustal electrical properties to seismogenesis, as well as to illuminate physico-chemical
mechanisms that may be exploited to monitor injection sites for carbon sequestration. Furthermore, this
work offers an explanation of the genesis of shallow, linear conductivity anomalies observed in
continental-collision zones in Europe, in the Andes, in the Himalayas, and in South Africa.
Keywords: conductivity anomaly, continental collision zones, carbon sequestration