Unsteady magma discharge during the “El Retiro” subplinian eruption (Turrialba volcano, Costa Rica). Insights from textural and petrological analyses

Turrialba is an active volcano of the Cordillera Volcánica Central in Costa Rica, which is currently experiencing a renewal of the eruptive activity after ~150 years of eruptive quiescence. Because of its explosive eruptive records in the recent geological history and the proximity with the metropolitan area of San José, this volcano poses serious threats to the population and economy of the area. In this study, we investigated the andesitic products of an explosive event (VEI 4) that occurred at Turrialba volcano ~2 ka, which we referred to as “El Retiro” eruption. Results fromfield observations, textural and chemical analyses indicate that the El Retiro eruptionwas characterized by two main pulses separated by a low energy activity. The eruption was fed by a homogeneous andesitic magma batch that crystallized along a P-T path of 0.1–400 MPa and 1050–1145 °C. The vesicle number density of pumiceous clasts ranges between 107 and 108 cm−3, in agreement with previous data from subplinian eruptions. The magma batch underwent decompression rates and pre-fragmentation velocities of 0.40–0.95 MPa s−1 and 23–37 m s−1, respectively. A marked textural variability within each stratigraphic layer has been also observed. This potentially suggests a horizontal zonation of the magma column, a condition that may have induced velocity gradients and high viscosity contrasts in the conduit leading tomagmafragmentation. The observed pulsating behavior translates into mass eruption rate fluctuations that can be related to changes in magma volatile contents. Further explosive eruptions with characteristics similar to those observed for El Retiro, cannot be excluded at Turrialba volcano. This denotes the importance of investigating eruption dynamics at the conduit, providing key information to rigorously assess the volcanic risk in populated areas