Evolution and dynamics of the plumbing system of Popocatépetl Volcano, Mexico

Abstract

Popocatépetl is an active arc volcano in the densely populated metropolitan area of Mexico City. Current mildly explosive activity is contrasted with at least ten voluminous effusive eruptions and six Plinian eruptions in the past ~23.5 ka. Previous work focuses on present-day and past explosive activity and neglects effusive eruptions, resulting in an incomplete understanding of Popocatépetl’s plumbing system. This thesis presents the first comprehensive stratigraphic, whole rock geochemical and petrological investigation of effusive and explosive eruptions in the last ~23.5 ka, with implications on the long-term evolution and dynamics of the plumbing system. Lavas and pumices are andesites – dacites with relatively homogeneous whole rock characteristics modified by crustal contamination. Parental melts feeding Popocatépetl volcano are olivine – chromite – augite bearing magmas derived from a depleted sub-arc mantle. The shallow plumbing system consists of a mafic reservoir crystallising orthopyroxene (Mg# >78), clinopyroxene (Mg# >82) and minor chromite at 1000 – 1115°C, and an evolved reservoir at ~960°C crystallising plagioclase (An50-28), orthopyroxene (Mg# <71), clinopyroxene (Mg# <79) and minor apatite, ilmenite and magnetite. Frequent injections of the mafic magma into the evolved reservoir produce transient mixed magmas at 960 – 1030°C, causing widespread textural and compositional disequilibria in both mafic and evolved phenocrysts. Such mafic injections are shown to occur weeks to months before explosive and effusive eruptions of the last ~2150 years. Furthermore, injection rates are higher in the years to decades before explosive eruptions compared to effusive activity, suggesting increased pressurisation of the plumbing system prior to explosive eruptions. Frequent mafic injections and magma hybridisation at Popocatépetl results in the eruption of hybrid lavas and pumices with narrow whole rock geochemical ranges containing multiple generations of mafic and evolved phenocrysts at different stages of re-equilibration. The repetitive nature of mafic injections dominates the activity, whole rock geochemistry and mineralogy of Popocatépetl volcano.