A petrological and conceptual model of Mayon volcano (Philippines) as an example of an open-vent volcano

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.

Determining the Pressure – Temperature – Composition (P-T-X) conditions of magma storage

“…At the opposite end of the spectra to determining pressures from fluid inclusions in CO 2-rich systems, many studies have used melt H 2 O contents to estimate pressures using a pure H 2 O solubility model, either through direct measurements in melt inclusions (e.g., Blundy andCashman, 2005, Rutherford et al, 1985) or calculated H 2 O contents from mineral and mineral-melt hygrometers (e.g., Amphibole Mg#s - Goltz et al, 2020, Krawczynski et al, 2012, Plag-Liq hygrometry -Ruth and Costa, 2021. For example, Ruth and Costa (2021) conduct thermobarometry on a variety of phases from Mayon Volcano (Fig. 25a).…”

A History of Andesite Production via Magma Mixing and Mingling Revealed Microscopically at Ngauruhoe Volcano

“…Some notable examples where homogeneous intermediate rocks have been frequently erupted but record microscopic evidence for open‐system processes are Arenal and Mayon volcanoes (Ruth & Costa, 2021; Streck et al., 2002). In these cases, the rapid and near‐continuous influx of mafic magmas and volatiles appear to have produced a steady‐state system.…”

“…In some cases magma mixing is evident from bulk‐rock compositional trends (e.g., Coombs & Jicha, 2021; Sisson et al., 2014). However, at some volcanoes where compositionally uniform magmas have been erupted, their origin via magma mixing can only be confirmed at microscopic (sub‐crystal) scales (e.g., Ruth & Costa, 2021; Streck et al., 2002; Tepley et al., 2000).…”

“…However, at some volcanoes where compositionally uniform magmas have been erupted, their origin via magma mixing can only be confirmed at microscopic (sub-crystal) scales (e.g., Ruth & Costa, 2021;Streck et al, 2002;Tepley et al, 2000).…”

Barometers behaving badly II: A critical evaluation of Cpx-only and Cpx-Liq thermobarometry in variably-hydrous arc magmas

“…Cpx is a stable phase over a very wide range of pressures, temperatures, melt compositions and oxygen fugacities (e.g., Costa, 2004;Nandedkar et al, 2014;Ulmer et al, 2018), meaning Cpx-based thermobarometry has broad utility and has been applied in a wide variety of tectonic settings. In this contribution we specifically focus on the use of Cpx-based thermobarometer to investigate storage conditions in magmas erupted in subduction zones (see Table . 1 for studies performing such calculations; Auer et al, 2013;Belousov et al, 2021;Cassidy et al, 2015;Caulfield et al, 2012;Cigolini et al, 2018;Dahren et al, 2012;Deegan et al, 2016;Freundt and Kutterolf, 2019;Geiger et al, 2018;Hollyday et al, 2020;Jeffery et al, 2013;Lai et al, 2018;Lormand et al, 2021;Moussallam et al, 2021Moussallam et al, , 2019Namur et al, 2020;Preece et al, 2014;Romero et al, 2022;Ruth and Costa, 2021;Sas et al, 2017;Scruggs and Putirka, 2018;Sheehan and Barclay, 2016).…”