Combined Petrological and Numerical Modeling Approach to Address Highly Crystalline Magma Remobilization Prior to Eruption at Volcán Llaima (Chile, 38.7°S)
Résumé
Volcán Llaima (Chile, 38.7° S) has an eruptive behavior that can be termed hyperactive Strombolian, with over 50 eruptions since 1640. It appears to have been erupting on similarly short repose time for at least 1000 years. If Llaima is locked into a highly repetitive eruptive mode because the processes operating in its sub-volcanic plumbing system are well established and control the eruptive behavior, it has the potential to be a predictable volcano. We aim at defining these processes by integrating petrologic insights with numerical modeling results. Llaima has produced fundamentally similar magmas in terms of whole-rock compositions (evolved basalt to basaltic andesite), mineralogy (plagioclase, olivine, and minor clinopyroxene), and crystallinity (30-55 vol% crystals) during the growth of the late Holocene edifice. Whole-rock compositions and matrix glasses span narrow ranges for a given eruption (51-54 wt% SiO2), whereas olivine crystals record wide ranges of core compositions (Fo84-67) indicative of multiple sources. An olivine-hosted melt inclusion study, focused on four historic tephra deposits, shows that magmas are stored in multiple batches, at shallow depths (<1.5 km according to calculated H2O-CO2-saturation pressures), in highly crystalline states. Magmas have mixed thoroughly enough to minimize variability at the whole-rock scale, and even at the matrix glass scale (rapid chemical diffusion in melts), but olivine phenocrysts did not have sufficient time to equilibrate before eruption. Time lags between recharge-related magma mixing and eruption were obtained by diffusion modeling of reversely and normally zoned rims on olivine crystals. These models yield incubation times on the order of a few months to a year. Rapid time scales linking magma recharge and eruption are in agreement with the high frequency of minor to intermediate scale eruptions at Llaima. A 2D, multiphase flow, finite volume model is used to explore in greater detail the dynamics of mixing and remobilization of crystal-rich mush bodies. We consider the injection of a slightly warmer and less crystalline magma into highly crystalline mush. Preliminary results suggest that the time scales of magma mixing and remobilization depend strongly on the vertical dimension of the mush body and the ratio of mush to recharged magma.