Carbon isotope thermometry in marbles of the Adirondack Mountains, New York

Abstract

Abstract 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° 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° C, and indicate that regional metamorphic temperatures were up to 75° 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° C, in general agreement with cation and solvus thermometry.Data for amphibolite and granulite grade marbles show a 12%oo range in δ13Ccal and δ13Cgr. A strong correlation between carbon isotopic composition and the abundance of graphite (Cgr/Crock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ13C vs. Cgr/Crock and δ13Ccal vs. δ13Cgr 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 δ13C vs. Cgr/Crock indicates a biogenic origin for Adirondack graphites, even though low δ13C values are not preserved in marble.Delamination of 17 graphite flakes perpendicular to the c‐axis reveals isotopic zonation, with higher δ13C 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 >2km from anorthosite show a decrease of 0.5‐0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core‐to‐rim decreases of c.0.2%oo. 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° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(cal‐gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ13C(cal‐gr) = 3.56 ± 106T‐2 (K).