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 an d oxygen isotope data for fluid inclusions, and carbon and oxygen isot ope data for dolomite from the Manetoe Facies (Devonian) and Cathedral Formation (Cambrian) of western Canada indicate that the dolomite cem ents formed from alteration of the host limestone by heated mixtures o f evaporated seawater or residual evaporite brines and meteoric water. The measured delta(18)O values for fluid inclusion waters are much mo re negative (about 8 parts per thousand) than those calculated for wat ers in isotope equilibrium with dolomite at the moderately high temper atures of formation. Fluid inclusion waters appear to have exchanged o xygen isotopes (reequilibrated) with the host dolomite as they cooled. The data may provide low temperature isotope exchange equilibrium fra ctionation factors between dolomite and water. Since no hydrogen-beari ng compounds other than water were found in these samples using Quadru pole Mass Spectrometry, the inclusion fluids likely retained their ini tial hydrogen isotope compositions. Therefore, their delta D values (- 81 to -42 parts per thousand) are more useful for determining the orig in of the dolomitizing fluids.