Quantifying timescales of eruptible magma: Evidence from Sr and Mg diffusion in plagioclase at Cerro Galán caldera

Abstract:

The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of how many of the largest explosive eruptions in the geologic record are produced, yet these conditions are not fully understood. One key issue that has received attention in recent years is the duration that these magmas exist in an ‘eruptible state’ (e.g. at conditions suitable for producing significant volumes of eruptible magma), as it can provide context to volcano monitoring data for systems of all sizes. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to investigate the magmatic history of the 2.08 Ma, 630km³ Cerro Galán Ignimbrite (CGI) in NW Argentina in an effort to address these issues. Rhyolite MELTS modeling indicates that plagioclase crystallization occurs early on in the evolution of the CGI and continues until the system is completely crystallized, making it a useful recorder of the system’s long-term evolution. Diffusion of both Mg and Sr indicate that the majority of the Cerro Galán plagioclase spent <10² yrs time at or above temperatures (~750°C) where magma is rheologically mobile and thus eruptible. With published zircon data indicating timescales on the order of 10⁵ years for reservoir assembly, this potentially indicates that the magma from where these CGI grains resided spent an extended period during crustal storage in rheologically immobile state and was then rapidly remobilized prior to eruption. Conversely, some Sr profiles exhibit longer durations (10^3-4 yrs) at or above this temperature. These observations indicate that the Cerro Galán magmatic system also experienced heterogeneous thermal regimes on a reservoir scale. Occasional reverse zoning in plagioclase suggests that these heterogenous thermal regimes are produced from repeated injection of hotter, more mafic magma into the reservoir as the system grew.