New insights into the processes controlling compositional zoning in plagioclase

Based on our results, we interpret zoning in plagioclase in terms of the partitioning of Na2O and CaO between the melt and plagioclase and the changes in the total volume during dissolution reactions of albite (ΔVAbdissolution) and anorthite (ΔVAndissolution) components. For the first two scenarios, our calculations show that as pressure decreases ΔVAbdissolution and ΔVAndissolution decrease while as pressure increases both of them increase. ΔVAbdissolution always being larger than ΔVAndissolution controls the zoning in plagioclase by compensating the larger volume change in the melt either by albite component dissolving into its oxides or by oxides reacting to produce albite component. Therefore, at a given temperature with decreasing pressure, Na2O partitions increasingly into the melt compared to CaO, making the coexisting plagioclase more An-rich. Thus, ascent of magma from a deeper to a shallower chamber isentropically, or convection of a magma in a large chamber polybaricaly, results in the development of reverse zoning in plagioclase. The magnitude of the effects is on the order of 3 mol% An per kbar for both decompression driven crystallization and isothermal/polybaric convection.