Carbon and Conductivity in Rocks

Carbon in rocks and its effect on electrical conductivity

Nearly all crystalline rocks contain at least small quantities of graphitic carbon and hydrocarboncompounds. They are present both as discrete particulates in, and as films on the surfaces of microcracks. Such forms of carbon have been observed in peridotite xenoliths from basalts, in mafic intrusive rocks, and in deep crustal amphibolites. The origin of the carbon in microcracks is not obvious. A working hypothesis, first proposed for xenoliths, is that the films form instantaneously upon formation of a microcrack by reaction of whatever carbon-bearing fluid is present and the new, chemically active surface. The carbonaceous films could well be relegated to a status of mere curiosities except for two reasons. First, it is now clear that the films can influence the electrical conductivity of rocks, at least at the scale of centimeters, as demonstrated by the work of Shankland et al. (1996). This emphasizes the possibility that microstructure may influence the gross transport properties of parts of the crust. Second is the possibility that the terrestrial subsurface may harbor a biological community, and the carbon films, at least in near-surface rocks, may affect the sustenance of this community or perhaps even be products of it. 

Our work to date is mainly concerned with the chemical characterization of the carbons, the determination of their distribution in rocks, and the relation of these variables to the rock's electrical conductivity. Two new studies are in progress. One is an experimental study with Al Duba, Jeff Roberts (Lawrence Livermore National Laboratory), Tom Shankland (Los Alamos National Laboratory) and Ed Mathez in which rocks are fractured at high temperature (440 C) and pressure (1 kb) in a C-H-O atmosphere while electrical conductivity is monitored as the rock fails. We observe decreases in electrical conductivity during the formation of microfractures and are in the process of trying to interpret these results. In a second study, Dave Mogk (Montana State University) and Ed Mathez are developing the application of time-of flight secondary ion mass spectroscopy to the characterization of carbon in rocks. This technique holds great promise not only for the in situ characterization of carbon films, but also for identifying biogenic materials, and for studies that require detection and identification of contaminants in microcracks in rocks. 

See Ed Mathez's research site for references and more information.