Carbon and oxygen isotope geochemistry of chlorite-zone rocks of the Waterville Limestone, Maine, U.S.A.

Analyses of 266 samples of calcite and dolomite from veins and wall rocks of the Waterville limestone from the chlorite zone have been made for δ¹⁸O and δ¹³C. The least altered samples of limestone have δ¹⁸O values of +19.5 to +20.5‰ (SMOW) and δ¹³C is − 1 to + 1‰ (PDB). The isotope values have been shifted by 3 to 7% in δ¹⁸O and 0 to 4‰ in δ¹³C relative to unaltered marine limestones of equivalent stratigraphic age. The shifts are similar to isotopic changes observed in unmetamorphosed but diagenetically altered limestones. The shifts are also consistent with the infiltration of H₂O-rich fluids during metamorphism. We cannot make a definite choice, at the present time, between the two explanations of changes in isotopic compositions, diagenetic or metamorphic.

The limestone in the chlorite zone is crosscut by four generations of veins and two sets of solution cleavages. Two older generations of veins and one of solution cleavage preceded metamorphism. The growth of metamorphic minerals was accompanied by solution of calcite along solution cleavage and its precipitation in synmetamorphic veins. During a postmetamorphic episode of vein formation, isotope alteration halos with depletions of 1 – 2‰ in δ¹⁸O and of 4‰ in δ¹³C were imposed on wall rocks. The Waterville limestone therefore has had a protracted history of fluid infiltration, involving both pervasive and fracture flow, that was not limited just to the peak of regional metamorphism.