Melt inclusions are not reliable proxies for magmatic liquid composition: evidence from crystal-poor andesites and dacites in the Tequila volcanic field, Mexico

A compositional study of >200 melt inclusions in plagioclase and orthopyroxene phenocrysts from six crystal-poor (2-5 vol%) andesite and dacite lavas (60-68 wt% SiO2) from the Tequila volcanic field in the Mexico arc is used to evaluate whether melt inclusions in phenocrysts accurately record magmatic liquid compositions. The crystal-poor andesites and dacites were erupted contemporaneously with crystal-poor rhyolites, and there is a continuum in the SiO2 concentration of the erupted magmas. The liquid line of descent defined by the whole-rock compositions ranges from andesite to rhyolite (60-77 wt% SiO2), as illustrated on Harker diagrams. The crystal-poor andesites and dacites are multiply saturated with five to seven mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), most of which crystallized via degassing during magma ascent (Frey and Lange, 2009). By comparison with phase equilibrium experiments from the literature, it is shown that the vast majority of crystals are phenocrysts and not xenocrysts. Textural evidence of rapid crystal growth includes skeletal, hopper, and swallow-tail morphologies and abundant melt inclusions. The inclusions range in size from a few microns to > 50 μm and occur as isolated pockets and extensive channels that mimic the crystal morphology. Inclusions are typically brown glass, with occasional microphenocrysts of titanomagnetite and/or apatite within or adjacent to the melt inclusions. The compositions of the melt inclusions in the plagioclase and orthopyroxene phenocrysts, when plotted on Harker diagrams, vary systematically from one another and from the liquid line of descent defined by the whole rock compositions of erupted magmas. For example, melt inclusions in plagioclase are systematically depleted in Al2O3 relative to the whole rock samples, whereas those in coexisting orthopyroxenes are systematically enriched in Al2O3. The opposite trend is found for FeO, where it is depleted in orthopyroxene melt inclusions and enriched in plagioclase melt inclusions. In most cases, K2O is enriched in the melt inclusions by up to a factor of two compared to the whole-rock compositions. SiO2 is also systematically enriched in the melt inclusions by up to 3-9 wt% compared to the bulk composition of each crystal-poor lava. The combined evidence clearly indicates that melt inclusions in plagioclase and orthopyroxene in andesite and dacite do not accurately record the composition of the magmatic liquid from which the phenocrysts were crystallized. Instead, the melt inclusions show the effects of boundary layer enrichments and depletions during rapid crystal growth, followed by diffusion of different elements at different rates. However, because water solubility is weakly dependent on melt composition, and because the H2O component diffuses relatively rapidly, it is likely that analyses of water in melt inclusions reliably record minimum values for the host magmatic liquid. But to assume a dacite liquid composition in a crystal-rich sample with a bulk composition of andesite, on the basis of elevated SiO2 in plagioclase melt inclusions, is not a reliable conclusion.