OXYGEN AND CARBON ISOTOPIC COMPOSITION OF GRENVILLE MARBLE, AND AN APPRAISAL OF EQUILIBRIUM IN THE DISTRIBUTION OF ISOTOPES BETWEEN CALCITE AND ASSOCIATED MINERALS, OTTER LAKE AREA, QUEBEC, CANADA

Abstract

Marble occurs abundantly in a portion of the Grenville Province of the Canadian Shield extending from the Adirondack Mountains, through Ontario, and into the Laurentian Highlands of Quebec, a distance of 300 km. In the Otter Lake area of western Quebec, major marble units, which are obviously metamorphosed limestone, consist of combinations of calcite, dolomite, olivine, humite-group minerals, Ca pyroxene, Ca amphibole, biotite, graphite, and retrograde serpentine and brucite, whereas minor marble units, which are evidently metasomatic rocks, consist of combinations of (pink) calcite, Ca pyroxene, Ca amphibole, biotite, garnet, K-feldspar, scapolite, and titanite. The oxygen and carbon isotopic composition of calcite in the major marble units (δ18OVSMOW in the range 28 to 17‰; δ13CPDB–1 in the range +6 to −2‰) is the same as in the Adirondacks and in unmetamorphosed Proterozoic limestone; only slight shifts (~1‰) can be attributed directly to metamorphic reactions. In bodies of minor marble, mean concentrations of 18O and 13C are slightly lower (+14.5 and +0.23‰, respectively). Mean fractionation-factors (α, expressed as 1000 ln α, are as follows: 18O, calcite/dolomite, +0.295 ( n = 11); 13C, calcite/dolomite, −0.346 ( n = 12); 18O, calcite/biotite, +3.49 ( n = 5); 13C, calcite/graphite, +3.74 ( n = 5). Additional (exploratory) results ( n = 1) on the distribution of 18O are as follows: calcite/olivine, +3.5; calcite/clinohumite, +5.1; calcite/garnet, +2.4; calcite/titanite, +3.1; calcite/Ca pyroxene, +2.0; calcite/Ca amphibole, +2.4, calcite/K-feldspar +1.8; calcite/scapolite, +2.0; calcite/serpentine, +13.9; calcite/brucite, +19.7; a trend of increasing α is related to increasing OH/(O + OH) in the non-carbonates and the preference of OH for 16O. For the first group of mineral pairs ( n ≥ a small variation in α across the study area (relative standard deviation ~0.05%) is viewed as an expression of a close approach to isotopic exchange equilibrium, and for certain pairs from the second group ( n = 1), a temperature estimate close to 700° (the biotite–garnet temperature of associated gneisses) is also viewed as evidence for equilibrium. Attainment of isotopic exchange equilibrium was facilitated by dynamic recrystallization, which occurred during the peak of metamorphism.