Effects of melt density on magma mixing in calc-alkaline series lavas

Abstract

We discuss here the melt density variations expected to occur during differentiation in calc-alkaline magma systems, and show how these density changes can influence the generation and chemical composition of mixed magmas. We assume that fractional crystallization of basaltic melt is the dominant process in generating the association high alumina basalt (HAB), basaltic andesite (BA), andesite, dacite and rhyolite¹. During differentiation of a basaltic parent there is an overall decrease in the density of derivative calc-alkaline liquids^2–4, but associated with the decline in density are density increases and related minima. At Medicine Lake Highland, petrological studies^5–7 have identified mixed BA, mixed andesite and mixed dacite lavas. We show how liquid density variations may explain the origin of the mixed basaltic andesites and andesites at Medicine Lake. We emphasize that our mechanism may be limited to aphyric calc-alkaline suites similar to those found at Medicine Lake volcano, and we do not propose magma mixing as a general model for the origin of all andesites. Densities of residual liquids produced by differentiation of primary basaltic magmas at elevated pressures pass through a minimum, and many of the basalts and basaltic andesites in island arcs have compositions that coincide with this minimum. Fractionation at crustal pressures may create a density filter that prohibits the eruption of denser primary magmas.