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