Carbonaceous material in metamorphosed carbonate rocks from the Waits River Formation, NE Vermont, and its effect on electrical conductiviy

A. Leger, E.A. Mathez, A. Duba, F. Pineau, S. Ginsberg
Dept. of Earth and Planetary Sciences American Museum of Natural History Central Park West at 79th Street New York, NY 10024-5192

Abstract:

The relations among electrical conductivity and graphite content, metamorphic grade, and fluid:rock interaction are investigated for a suite of regionally metamorphosed graphitic carbonate rocks from the Waits River Formation, NE Vermont. Graphitization was complete by the lowest grade of metamorphism attained in the area (450 degrees C and 450 MPa). In low-grade rocks, graphite occurs as inclusions within calcite and along grain boundaries, and in medium-grade rocks, it concentrates near porphyroblasts. In neither suite does graphite form interconnected networks. Average reduced carbon abundances are 4400 and 2800 ppm for the low- and medium-grade rocks, respectively. High-grade rocks are almost void of graphite, except for one specimen. The 13C values were determined for graphite and carbonates and 18O values were determined for the carbonates and silicate residues. There is no change in isotopic values between low- and medium-grade rocks. In contrast, high-grade rocks show marked decreases in 13C and 18O values of about 4 per mil compared to low and medium-grade rocks. Both isotopic analyses and graphite depletion in high-grade rocks was caused by influx of large quantities of magmatic water during peak metamorphism. Electrical conductivity was measured in the laboratory on representative specimens. Rocks with high carbon content (>7000 ppm) from both the low- and high-grade zones display almost an order of magnitude higher electrical conductivity than expected from their fluid content alone. Graphite does not form an interconnected network in these rocks, yet it combines with the saline fluids to significantly increase electrical conductivities.

AGU Index Terms: 5109 Magnetic and electrical properties; 3660 Metamorphic petrology; 3645
Keywords/Free Terms: Electrical properties, metamorphic petrology, isotope composition.

JGR-Solid Earth 96JB01757