Abstract
Mayon is a basaltic andesitic, open-vent volcano characterized by persistent passive degassing from the summit at 2463 m above sea level. Mid-size (< 0.1 km3)="" and="" mildly="" explosive="" eruptions="" and="" occasional="" phreatic="" eruptions="" have="" occurred="" approximately="" every="" 10="" years="" for="" over="" a="" hundred="" years.="" mayon’s="" plumbing="" system="" structure,="" processes,="" and="" time="" scales="" driving="" its="" eruptions="" are="" still="" not="" well-known,="" despite="" being="" the="" most="" active="" volcano="" in="" the="" philippines.="" we="" investigated="" the="" petrology="" and="" geochemistry="" of="" its="" crystal-rich="" lavas="" (~ 50="" vol%="" phenocrysts)="" from="" nine="" historical="" eruptions="" between="" 1928="" and="" 2009="" and="" propose="" a="" conceptual="" model="" of="" the="" processes="" and="" magmatic="" architecture="" that="" led="" to="" the="" eruptions.="" the="" whole-rock="" geochemistry="" and="" mineral="" assemblage="" (plagioclase + orthopyroxene + clinopyroxene + fe-ti="" oxide ± olivine)="" of="" the="" lavas="" have="" remained="" remarkably="" homogenous="" (54="" wt%="" sio2, ~ 4="" wt%="" mgo)="" from="" 1928="" to="" 2009.="" however,="" electron="" microscope="" images="" and="" microprobe="" analyses="" of="" the="" phenocrysts="" and="" the="" existence="" of="" three="" types="" of="" glomerocrysts="" testify="" to="" a="" range="" of="" magmatic="" processes,="" including="" long-term="" magma="" residence,="" magma="" mixing,="" crystallization,="" volatile="" fluxing,="" and="" degassing.="" multiple="" mineral-melt="" geothermobarometers="" suggest="" a="" relatively="" thermally="" buffered="" system="" at="" 1050 ± 25="" °c,="" with="" several="" magma="" residence="" zones,="" ranging="" from="" close="" to="" the="" surface,="" through="" reservoirs="" at ~ 4–5="" km,="" and="" as="" deep="" as ~ 20="" km.="" diffusion="" chronometry="" on =""> 200 orthopyroxene crystals reveal magma mixing timescales that range from a few days to about 65 years, but the majority are shorter than the decadal inter-eruptive repose period. This implies that magma intrusion at Mayon has been nearly continuous over the studied time period, with limited crystal recycling from one eruption to the next. The variety of plagioclase textures and zoning patterns reflect fluxing of volatiles from depth to shallower melts through which they eventually reach the atmosphere through an open conduit. The crystal-rich nature of the erupted magmas may have developed during each inter-eruptive period. We propose that Mayon has behaved over almost 100 years as a steady state system, with limited variations in eruption frequency, degassing flux, magma composition, and crystal content that are mainly determined by the amount and composition of deep magma and volatile input in the system. We explore how Mayon volcano’s processes and working model can be related to other open-vent mafic and water-rich systems such as Etna, Stromboli, Villarrica, or Llaima. Finally, our understanding of open-vent, persistently active volcanoes is rooted in historical observations, but volcano behavior can evolve over longer time frames. We speculate that these volcanoes produce specific plagioclase textures that can be used to identify similar volcanic behavior in the geologic record.
Keywords
Basaltic andesite, Diffusion chronometry, Magma mixing, Magma mush, Mineral chemistry, Open-vent, Persistently active volcano