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

The eutectic mineral assemblage calcite-dolomitepericlase-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: H20, 882; CO2, 110; H z, 4.6; H2S, 2.7; CO, 0.5; and CH,, 0.1. Oxygen fugacity was near the QFM buffer, l o d ( O 2 ) = 18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, logf(S2)=-5 .9 . Fluorine fugacity was low, l o g f ( 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.