VARIATION IN OXYGEN FUGACITY DURING COOLING OF THE EAST HILL SUITE, MONT SAINT-HILAIRE, QUEBEC, AND IMPLICATIONS FOR CLINOPYROXENE COMPOSITIONAL TRENDS

The Mont Saint-Hilaire alkaline complex is a Cretaceous rift-related intrusion comprising two gabbroic suites and the East Hill suite, an assemblage of several distinct nepheline syenites and evolved syenitic rocks, rich in Mn, Zn, Zr, Y, and Ce. Whole-rock major and trace element geochemistry of the East Hill suite indicates that the syenites represent a continuous differentiation sequence.

The geochemistry of amphiboles and micas indicates generally decreasing oxygen fugacity with cooling, and phases such as titanite (present), kupletskite (present), and aenigmatite (absent) define thermobarometric boundaries in f_O2 – T space. These boundaries, combined with petrography and geothermometry, establish an estimated f_O2 – T curve for the East Hill suite magma, revealing that the oxygen fugacity ranged between approximately 10^‑10 bar at intrusion down to 10^‑25 bar subsolidus, and that, owing to non-linearity of oxygen buffers, the f_O2 – T path crossed below QFM early, returned above QFM near subsolidus temperatures, late in the crystallization history, and then above HM with cooling.

The estimated trend in oxygen fugacity contradicts earlier conclusions that the oxygen fugacity in the East Hill suite increased with cooling and that the observed aegirine enrichment trend in clinopyroxene was due to oxidation of ferrous iron in an increasingly oxygen-rich environment. Rather, elevated sodium itself drove the transition from hedenbergitic to aegirine-rich compositions. Iron compensated for electron-poor sodium by donating a total of three electrons to the silicate groups, converting to the ferric state to maintain charge balance. Therefore, the presence or dominance of ferric iron in clinopyroxene, and perhaps in ferromagnesian phases in general, may not be, by itself, indicative of elevated oxygen fugacity, and due consideration must be given to the alkalinity of the system, which may be the causative factor.