CARBON-ISOTOPE THERMOMETRY IN MARBLES OF THE ADIRONDACK MOUNTAINS, NEW-YORK

Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670 degrees C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675 degrees C, and indicate that regional metamorphic temperatures were up to 75 degrees C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670-780 degrees C, in general agreement with cation and solvus thermometry. Data for amphibolite and granulite grade marbles show a 12 parts per thousand range in delta(13)C(cal) and delta(13)C(gr). A strong correlation between carbon isotopic composition and the abundance of graphite (C-gr/C-rock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in delta(13)C vs. C-gr/C-rock and delta(13)C(cal) vs. delta(13)C(gr) could not have been preserved if significant amounts of CO2-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of delta(13)C vs. C-gr/C-rock indicates a biogenic origin for Adirondack graphites, even though low delta(13)C values are not preserved in marble. Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher delta(13)C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located < 2 km from anorthosite show a decrease of 0.5-0.8 parts per thousand in the outer 100 mu m of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c. 0.2 parts per thousand. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by Later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860-890 degrees C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Delta((cal-gr)) < 3.2 parts per thousand) are not included, then the remaining 38 granulite facies samples define the relation Delta(13)C((cal-gr)) = 3.56 X 10(6)T(-2) (K).