New insights into the processes controlling compositional zoning in plagioclase

Abstract

Although plagioclase is the most abundant mineral in crustal rocks, the factors that control the magnitude of compositional zoning in plagioclase remain poorly constrained. The composition of magmatic plagioclase depends upon physical parameters such as temperature (T) and total pressure (P), as well as the full range of compositional controls, including the water content of the melt (wt.% H2O). The changes in these physical parameters can be quantified using available numerical phase equilibria models for various differentiation scenarios and the magnitude of these effects on plagioclase zoning can be calculated. The role of temperature, total pressure, and water content on plagioclase zoning has been experimentally investigated previously in a pure albite–anorthite (Ab–An) system. In this study, we use the MELTS algorithm to quantify the effects of P, T and water content of the melt on compositional zoning in plagioclase in a basaltic system. Our first two simulations are designed to evaluate the magnitude of compositional changes that may be attributable to specific system changes exclusive of assimilation and recharge. The first scenario involves a two-step crystallization process. In the first step, magma rises from a deeper to a shallower chamber while crystallizing plagioclase at successively lower pressures. In the second step, the magma pools in the shallower magma chamber and continues to crystallize isobarically. If these two steps are repeated, the result is the development of reverse, normal, and oscillatory zoning in plagioclase. The second scenario involves plagioclase zoning in a large, convecting magma chamber where plagioclase crystallizes as the magma convects, resulting in oscillatory zoning. The third scenario involves a two-step process where crystallization is interrupted by multiple recharge events. In this case, magma crystallizes isobarically as a new batch of primitive magma is injected periodically to the magma chamber and crystallization continues after mixing. During the repetition of these two steps, plagioclase develops normal and reverse zoning. 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.