Jordan Lubbers , Shan de Silva, Adam J Kent

The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of the timing, frequency, and intensity of volcanic eruptions, and provides important context for interpreting volcano monitoring data. Despite this, however, individual magmatic systems may exhibit a range of time-temperature paths, or thermal histories, that are the result of many complex and, in some cases competing processes. This complexity contributes to an incomplete understanding of the long-term thermal evolution of magma stored within the Earth’s crust. Of recent interest to the volcanology community is the length of time large volumes of rheologically eruptible and geophysically detectable magma exist within the crust prior to their eruption. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to quantify the long-term thermal evolution of the 2.08 Ma, 630km3 Cerro Galán Ignimbrite (CGI) in NW Argentina; one of the largest explosive volcanic eruptions in the recent geologic record. We find that diffusion of both Mg and Sr in plagioclase indicate that erupted magmatic material only spent decades to centuries at or above temperatures (~750°C) required to maintain significant volumes of stored eruptible magma. Calculated plagioclase equilibrium compositions reveal an array of liquids that is controlled overall by fractionation of plagioclase + biotite + sanidine, although high-resolution trace element transects record a diversity of fractionation pathways. Overall, we suggest that there is compelling evidence that the magma erupted from the CGI magmatic system spent most of its upper crustal residence in a largely uneruptible state and was rapidly remobilized shortly before eruption.