Storage conditions of the ~29 ka rhyolitic Guangoche White Pumice Sequence, Los Azufres Volcanic Field, Central Mexico

Rhyolitic magmas in occasions can be related to explosive eruptions in active volcanoes that may not exhibit precursory signals, making them very dangerous for the surrounding communities. To understand the pre-eruptive conditions of such magmas it is important to decipher the mechanisms capable to produce these explosive events. In this study we analyzed the pre-eruptive conditions of Guangoche stratovolcano, located to the southwest of the Los Azufres Volcanic Field, in the central part of the Trans-Mexican Volcanic Belt. During the late Pleistocene Guangoche generated three Plinian-subPlinian eruptions and some effusive eruptions, all of them involving high-silica rhyolitic magmas. One of these Plinian events (VEI 5) occurred ~29 ka and deposited the White Pyroclastic Sequence (WPS). This eruption ejected ~0.7 km3 (DRE volume) of rhyolitic magma and deposited a pumice fallout followed by three pumice-rich pyroclastic density currents. Petrography, combined with mineral composition, melt inclusions, and water-saturated phase equilibria experiments were used to determine the evolution of the magma storage of the WPS event. Prior to the eruption, the magma was a crystal-poor (~14 vol%) rhyolite, containing the following mineral assemblage: biotite, Fe-Ti oxides, amphibole, sanidine and quartz, set in a rhyolitic matrix (77.3 wt% SiO2), with low-Ba (121 ± 16.7 ppm), Eu (0.15 ± 0.02 ppm) and Sr (37 ± 7 ppm). Based on Fe-Ti oxythermometers and phase equilibria experiments, we determine that the magma was stored between ~75 and 100 MPa (3-4 km depth), at a temperature of 769 ± 8 °C. Moreover, H2O-CO2 dissolved in quartz melt inclusions and the textural features of the mineral assemblage of the WPS products, provided insights of the presence of a quartzo-feldspathic crystal-mush, located at minimum depths between 4.3 and 9.5 km (98-218 MPa). Large sanidine, quartz, plagioclase, and amphibole phenocrysts and mineral clots in the WPS rocks represent assimilated parts from this mush by a melt extraction process probably triggered by the arrival of a relatively hotter magma at the base of the crystal-mush, which in turn caused reheating and partial melting of the quartzo-feldspathic crystal-mush, generating the 29 ka rhyolitic WPS magma.