CALCITE-GRAPHITE ISOTOPE THERMOMETRY IN SOUTHEASTERN ADIRONDACK MARBLE, GRENVILLE PROVINCE, NEW YORK

We have undertaken an investigation of calcite-graphite carbon isotope thermometry in marble from the southeastern-most Grenville Province, New York State.  To date we have completed 68 analyses of 15 samples from 10 different outcrops.  Sample localities span 50 km from south of North Creek to Lake George and Whitehall, and two localities about 20 km north of Whitehall.  All samples are coarse-grained, white to light gray calcite marble with visible graphite.  Accessory minerals include subsets of diopside, tremolite, phlogopite, K-feldspar, plagioclase, titanite, quartz.  Marble is interlayered with granitic gneiss, charnockite, and less commonly, siliceous metasedimentary rocks.  The efficacy of this thermometer is being tested by analyzing two samples from each locality, and all samples are at least duplicated. The d¹³C values of analyzed graphite range from -3.53 to -1.61‰ and calcite from 0.10 to 2.65‰.  With the exception of two outliers, the Δ(Cal-Gr) values vary only between 3.0 and 3.8.  The calibration of Kitchen & Valley (1995) yields temperatures ranging from 695° to 815°C.  Reproducibility on standards is ±0.1 (2σ) for calcite (n = 11) and 0.18 for graphite (n = 7).  Uncertainty of 0.2‰ in Δ(Cal-Gr) corresponds to ± ~30°.  "Within sample" duplication is typically better than ± 30°.  "Within outcrop" duplication is ± 45° at worst, and in two cases, better than ± 6°.  Observed temperatures show no systematic gradient across this area, however one of the Δ(Cal-Gr) outliers is 4.31‰ at Whitehall, indicating T = 635°C, which suggests decreasing temperature near the easternmost Grenville exposures in this area.  The other outlier, about 20 km north of Whitehall, yields a Δ(Cal-Gr) value of 1.44‰ (triplicate analyses give ± 0.15) and indicates T = 1300°C.  This sample is exceedingly coarse-grained (Cal > 8 cm, Gr up to 1 cm) and must have experienced some modification of either calcite or graphite to produce this spurious result.  This study and previous thermometry of the SE Adirondacks using a variety of methods are all broadly consistent with granulite facies metamorphism ≈ 750 ±50 °C, but local variations appear to be present and discrepancies between different studies exist. No regional trend is apparent within this study area, although previous work has shown that thermal highs coincide with meta-anorthosite bodies.