Stable isotope study of coexisting metamorphic minerals from the Esplanade Range, British Columbia

Abstract

Hydrogen, carbon, and oxygen isotope analyses have been made of coexisting minerals from regionally metamorphosed rocks of the Esplanade Range, British Columbia. Regularities in oxygen and hydrogen isotope compositions imply attainment of isotopic equilibrium. Temperatures, determined on the basis of oxygen isotope fractionations between quartz and ilmenite (magnetite), range from 460°C (biotite-chloritoid zone) to 540°C (staurolite-biotite zone) and are in good agreement with temperatures estimated from comparison of experimental phase equilibria with the mineral assemblages. The isotope data are comparable to results obtained by other workers studying Barrovian-type metamorphism in different areas. The relatively narrow ranges of δO 18 values for quartz and δD values of biotite and hornblende from a wide range of protoliths suggest either that there was good oxygen and hydrogen isotope communication between these rocks or that they exchanged with a large reservoir having fixed isotope ratios. A single orthoamphibolite, however, has oxygen and hydrogen isotope compositions that show that it was not in communication with the other rocks. The carbonates in the Esplanade Range are unusually enriched in C 13 relative to similar metamorphic rocks in other localities. Values of δC 13 become more positive with increasing grade and range from −2.5 to +9.8.