A periclase-dolomite-calcite carbonatite from the Oka complex, Quebec, and its calculated volatile composition

Abstract

The eutectic mineral assemblage calcite-dolomite-periclase-apatite-forsterite-magnesioferrite-pyrrhotite-alabandite in a carbonatite dike within the Oka complex, Quebec, buffers the fugacities (and partial pressures) of all gas species in C-O-H-S-F, assuming vapor saturation. At the inferred eutectic (640° C, 1 kbar), the most important gas species and their partial pressures (bars) were: H 2 O, 882; CO 2 , 110; H 2 , 4.6; H 2 S, 2.7; CO, 0.5; and CH 4 , 0.1. Oxygen fugacity was near the QFM buffer, log f (O 2 )=−18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, log f (S 2 )=−5.9. Fluorine fugacity was low, logf(F 2 )=−43.9, consistent with the absence of fluoride minerals other than apatite. Presence of a water-rich gas phase is consistent with experiments on synthetic carbonatite systems (e.g. Fanelli et al. 1981), although compositions of the gas phase in published experiments cannot be determined exactly.