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

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: H₂O, 882; CO₂, 110; H₂, 4.6; H₂S, 2.7; CO, 0.5; and CH₄, 0.1. Oxygen fugacity was near the QFM buffer, logf(O₂)=–18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, logf(S₂)=–5.9. Fluorine fugacity was low, logf(F₂)=–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.Contribution no. 390 from the Mineralogical Laboratory, The University of Michigan