Influence of Carbon on the Electrical Properties of Crustal Rocks Page: 4 of 4
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An interdisciplinary workshop was convened that included forty petrologists and geophysicists
from Europe and North America. The workshop was held at the American Museum of Natural
History, New York, on April 18 and 19, 2001. Although general consensus was reached on the
importance of carbon in producing upper crustal electrical conductivity anomalies, the situation for
the lower crust was much less clear. One problem is simply that some lower crustal features are
unseeable because uplift, by whatever mechanism, modifies microstructure and generally modifies
geochemistry, even subtly, in fresh rocks. Although geophysical methods lack spatial and temporal
resolution and laboratory deformation experiments are poor matches for in-situ crustal conditions,
geophysical researchers were more confident of carbon as an explanation of conductivity anomalies
than were most petrologists present. For instance, there was no consensus on what constitutes
viable mechanisms for mobilizing and precipitating carbon at pressures and temperatures of the
deep crust. Further, there is the problem that thermodynamic equilibrium may not prevail on
microcracks because of unknown kinetic and shear effects and because the carbon may not be
purely graphitic. However, veins of graphite described from New Hampshire demonstrate the
mobility of carbon, presumably transported as CO2, with or without graphitized wall rocks in
sillimanite-grade schists, and gneisses. The location of the graphite veins appears to be tectonically
controlled, with deposition near the top and possibly much deeper in the lower crust. Issues that
need further attention include: measurement of electrical properties of hydrous minerals that may
be stable in the crust; how the interconnectivity of carbon can be dynamically maintained at lower
crustal to upper-mantle conditions where equilibrium conditions would tend to produce isolated
graphite crystals; and experimental observations of mechanisms for destruction of interconnected
carbon films during laboratory simulation of near-surface crustal conditions.
TOPICAL KEYWORDS:
Properties of Earth Materials
Resource Definition and Utilization
Hydrocarbon Formation
SCIENTIFIC KEYWORDS:
Electrical Conductivity
Hydrocarbons
Retrograde Metamorphism
Carbon
Mathez, E.A., and D.M. Mogk, 1998, Characterization of carbon compounds on a pyroxene surface
from a gabbro xenolith in basalt by time-of-flight secondary ion mass spectrometry. Am.
Mineral. 83, 918-924.
Mogk, D.W., and E.A. Mathez, 1998, Carbonaceous films in a garnet amphibolite from the KTB
deep borhole measured by time-of-flight-SIMS. Geol. Soc. Am., Abs. Programs, A-186.
Mogk, D.W., and E.A. Mathez, 2000, Carbonaceous films in mid-crustal rocks from the KTB
borehole, Germany, as characterized by time-of-flight secondary ion mass spectrometry. EOS 81,
S34.
Mogk, D.W., and E.A. Mathez, 2000, Carbonaceous films in mid-crustal rocks from the KTB
borehole, Germany, as characterized by time-of-flight secondary ion mass spectrometry.
Geochemistry Geophysics Geosystems 1, paper 2000GC000081.
Duba, A.G., E.A. Mathez, and T.J. Shankland, 2001, Workshop addresses crustal carbon and its
effect on electrical conductivity. EOS 82, 456.
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Mathez, E. A. Influence of Carbon on the Electrical Properties of Crustal Rocks, report, November 19, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc888659/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.