Multi-stage chaotic magma mixing at Turrialba volcano

After a century of quiescence, Turrialba, a Quaternary stratovolcano in the Costa Rican Volcanic Front, started a new eruptive cycle in 2010 that is ongoing to this date (July 3rd, 2019). Violent eruptions began in 2014, producing columns of ash that reached 4 km above the summit of the volcano. Here we provide new major and trace element data from fresh, glassy fragments (basaltic to rhyolitic) and mineralogical data for feldspars, olivines, and pyroxenes from ash fallout collected from 2016 and 2017 eruptions. We investigate the composition of the magma replenishing the system as well as the different juvenile components involved in the latter phase of this renewed eruption cycle. We find a range of mafic to felsic compositions in every eruptive event sampled. Our new geochemical data, coupled with Monte Carlo binary mixing simulations, indicate that magmas at Turrialba volcano are influenced by the input of a back-arc-type, strongly high field strength element-enriched (20-60 ppm) component generated by decompression melting from subduction-driven corner-flow. Thermodynamic modeling using MELTS software suggests that fractional crystallization alone fails to explain the compositional range of Turrialba's eruptive products. We instead propose that a complex, multi-stage magma mixing process produced renewed activity at Turrialba based on the trace element variability, the dominant intermediate magmatic compositions observed, and the results of our multi-element mixing simulations. This mixing process occurs upon injection of new mantle-derived melts into Turrialba's fractionated magmatic reservoirs and is governed by chaotic dynamics induced by gradients in temperature and viscosity. (C) 2019 Published by Elsevier B.V.