STABLE-ISOTOPE AND MAJOR-ELEMENT COMPOSITIONS OF FLUID INCLUSIONS IN DEVONIAN AND CAMBRIAN DOLOMITE CEMENTS, WESTERN CANADA

Homogenization and melting temperatures, major element and hydrogen and oxygen isotope data for fluid inclusions, and carbon and oxygen isotope data for dolomite from the Manetoe Facies (Devonian) and Cathedral Formation (Cambrian) of western Canada indicate that the dolomite cements formed from alteration of the host limestone by heated mixtures of evaporated seawater or residual evaporite brines and meteoric water. The measured delta(18)O values for fluid inclusion waters are much more negative (about 8 parts per thousand) than those calculated for waters in isotope equilibrium with dolomite at the moderately high temperatures of formation. Fluid inclusion waters appear to have exchanged oxygen isotopes (reequilibrated) with the host dolomite as they cooled. The data may provide low temperature isotope exchange equilibrium fractionation factors between dolomite and water. Since no hydrogen-bearing compounds other than water were found in these samples using Quadrupole Mass Spectrometry, the inclusion fluids likely retained their initial hydrogen isotope compositions. Therefore, their delta D values (-81 to -42 parts per thousand) are more useful for determining the origin of the dolomitizing fluids.