Abstract
The temporal evolution of erupted magma compositions at Paricutin Volcano (Mexico) is often cited as a classic example of assimilation-fractional crystallization processes with significant progressive changes in major element, trace element, and isotopic compositions occurring over the relatively short 9 year lifespan of the volcano. In this study, major and trace element compositions of olivine- and orthopyroxene-hosted melt inclusions are integrated with new trace element analyses of the erupted lavas and data for entrained xenoliths and xenolith glasses to provide a more comprehensive evaluation of the evolution of Paricutin Volcano that questions this view. Melt inclusion compositions are bimodal with an undegassed, low-Si population (Type I) similar in composition to the whole-rock samples and a degassed, high-Si population (Type II) recording late-stage degassing and crystallization of the magma. Despite the rapid changes in lava composition, melt inclusions hosted in both olivine and orthopyroxene do not record any progressive contamination or mixing of magmas. Homogeneity of Type I melt inclusions within single lava samples implies significant contamination prior to crystallization and potentially a decoupling of assimilation-fractional crystallization processes. Pre-existing models of magma evolution at Paricutin Volcano are not consistent with the melt inclusion results or new trace element whole-rock data. Whole-rock and melt inclusion trace element analyses corroborate previous studies, which have suggested that the early erupted material (Phase 1; February-July 1943) was of a compositionally distinct magma compared with the bulk of the erupted material during Phase 2 (July 1943-1946). There is a second compositional transition between the Phase 2 and Phase 3 (1947-1952) lavas, marked by a sudden change in Zr/Nb despite similar MgO values, that is consistent with the arrival of a new magma batch. This transition occurs prior to the major compositional change from basaltic andesite to andesite magmas in the waning stages of the eruption that is consistent with progressive crustal assimilation within this latest magma batch. These data demonstrate that the petrogenetic evolution of magmas at Paricutin is more complex than simple progressive assimilation and fractional crystallization and requires the presence of three compositionally distinct magma batches at shallow levels.