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

Marble occurs abundantly in a portion of the Grenville Province of the Cana dian Shield extending from the Adirondack Mountains, through Ontario, and i nto the Laurentian Highlands of Quebec, a distance of 300 km. In the Otter Lake area of western Quebec, major marble units, which are obviously metamo rphosed limestone, consist of combinations of calcite, dolomite, olivine, h umite-group minerals, Ca pyroxene, Ca amphibole, biotite, graphite, and ret rograde serpentine and brucite, whereas minor marble units. which are evide ntly metasomatic rocks, consist of combinations of (pink) calcite. Ca pyrox ene, Ca amphibole, biotite. garnet. K-feldspar, scapolite, and titanite. Th e oxygen and carbon isotopic composition of calcite in the major marble uni ts (delta O-18(VSMOW) in the range 28 to 17 parts per thousand; delta C-13( PDB-1) in the range +6 to -2 parts per thousand) is the same as in the Adir ondacks and in unmetamorphosed Proterozoic limestone only slight shifts (si milar to 1 parts per thousand) can be attributed directly to metamorphic re actions. In bodies of minor marble, mean concentrations of O-18 and C-13 ar e slightly lower (+14.5 and +0.23 parts per thousand respectively). Mean fr actionation-factors (alpha), expressed as 1000 in alpha, are as follows: O- 18, calcite/dolomite, +0.295 (n = 11); C-13, calcite/dolomite. -0.346 (n = 12): O-18 calcite/biotite, +3.49 (n = 5); C-13. calcite/graphite. +3.74 (n = 5), Additional (exploratory) results (n = 1) on the distribution of O-18 are as follows: calcite/olivine. +3.5: calcite/clinohumite. +5.1: calcite/g arnet, +2.4: caicite/titanite, +3.1 calcite/Ca pyroxene, +2.0; calcite/Ca a mphibole. +2.4. calcite/K-feldspar +1.8 calcite/scapolite, +2.0 calcite/ser pentine. +13.9: calcite/brucite, +19.7 a trend of increasing alpha is relat ed to increasing OH/(O + OH) in the non-carbonates and the preference of OH for O-16. For the first group of mineral pairs (n ! 5). a small variation in a across the study area (relative standard deviation similar to0.05%) is viewed as an expression of a close approach to isotopic exchange equilibri um, and for certain pairs from the second group (n = 1), a temperature esti mate close to 700 degreesC (the biotite-gurnet temperature of associated gn eisses) is also viewed as evidence for equilibrium. Attainment of isotopic exchange equilibrium was facilitated by dynamic recrystallization. which oc curred during the peak of metamorphism.