David S. O'Hanley

THE ORIGIN OF RODINGITES FROM CASSIAR, BRITISH COLUMBIA, AND THEIR USE TO ESTIMATE T AND P(H2O) DURING SERPENTINIZATION

Abstract The Cassiar serpentinite, located in north-central British Columbia, is a low-grade, alpine-type serpentinite within the Sylvester allochthon. Two stages of rodingitization have occurred at Cassiar and these correlate with two stages of serpentinization that have occurred in the ultramafic rock. The first stage is characterised by an increase in Ca ++ activity and decreases in Mg ++ and SiO 2 activities in the rodingite, culminating in the crystallization of grossular in the rodingite and by the formation of lizardite in the serpentinite. At a later time, a second event occurred, characterised by a decrease in Ca ++ activity and increases in Mg ++ and SiO 2 activities both in the rodingite and the serpentinite. This was marked by the crystallization of diopside, prehnite, and lizardite in the rodingite, and by the recrystallization of lizardite to chrysotile and antigorite in the serpentinite. The serpentine recrystallization and the development of the second-stage rodingite minerals indicate that the changes in rodingite and serpentine mineral assemblages were driven by similar changes in the bulk-rock compositions. This suggests that the second-stage event in both rodingite and serpentinite was caused by an externally derived fluid. Measurements on fluid inclusions in the rodingite minerals clinozoisite, grossular, and diopside of the second event indicate that the externally derived fluid was CO 2 -poor, but relatively saline (8 ± 1.5 equiv. wt% NaCl). By combining the pressure-dependant temperature measurements from fluid inclusions with the pressure independant temperature measurements on δ 18 O in coexisting serpentine-magnetite pairs the pressure of formation can be estimated. The second event of serpentine recrystallization and rodingitization was an isothermal process at 300° C ± 36° C , P ( H 2 O )