On the Controls of Oxygen Fugacity in the Generation and Crystallization of Peralkaline Melts

The formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation. The wide range in mineral assemblages, mineral compositions, and fluid compositions in peralkaline melts are, to a large extent, a function of the sodium/potassium content of these mantle-metasomatizing fluids, which also governs redox conditions. We propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene. In contrast, potassic fluids would oxidize the mantle assemblage by extracting Al2O3 from the garnet to make phlogopite. We suggest that during differentiation of peralkaline melts, once Fe–Ti oxides have been depleted from the assemblage, simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as $Na_{2}O + 2 FeO + 1 SiO_{2}+\frac{\text{1}}{\text{2}}O_{2} = 2 NaFeSi_{2}O_{6}$ Hence, the more persodic a melt is, the more aegirine will be crystallized and the more reduced this melt will be. This is because without a mineral donor for ferric iron, the crystal chemical forcing of the coupled crystallization of Na with Fe³⁺ drives oxidation of the dominant Fe²⁺ in the melt and thereby reduction of the C–H–S–O-bearing fluid and/or melt phase. This is in convincing agreement with observations from a variety of silica-undersaturated peralkaline complexes such as Ilimaussaq in Greenland, Mt. St. Hilaire in Canada, Lovozero and Khibina in Russia, and Tamazeght in Morocco, and explains the occurrence of magmatic methane in the most sodic of these complexes. In silica-oversaturated peralkaline melts such as comendites and pantellerites, however, this equilibrium appears to be less influential and these melts are accordingly less reduced. In perpotassic melts, in contrast, no K–Fe³⁺ pyroxene is stable and K can only be incorporated into dominantly Fe²⁺-bearing phases such as biotite or K-amphibole, if no K-feldspar is stable. Here, redox conditions in the coexisting fluid and/or melt are either unaffected or even driven to more oxidized values by the crystallization reactions, which is in accordance with observations on the ultrapotassic rocks of the Roman Province (Italy) or the kalsilite-bearing Murun complex in Siberia. We conclude that peralkalinity in magmatic rocks is tightly connected to oxygen fugacity, that both the generation and the evolution and crystallization of peralkaline melts are governed by redox equilibria, and that these depend strongly on the Na/K ratio as well as on the (Na + K)/Al ratio of the melt. The specific features of peralkaline rocks, such as the difference between agpaitic and miaskitic rocks, the unusual enrichment of high field strength elements and the occurrence of magmatic hydrocarbons in some of them, can all be explained by the oxygen fugacity-controlling model reactions discussed in this study.