Reconstruction of the magmatic processes and eruption dynamics influencing the 1870-5 CE eruption of Ceboruco Volcano, Mexico
Gustavo Adolfo Béjar López
May 20, 2020
Department or Program
Ceboruco is a stratovolcano located in the western Trans-Mexican Volcanic Belt with an eruptive history that includes two well-studied caldera-forming events and multiple effusive episodes. The volcano represents a hazard for the ~55,000 inhabitants of the valleys surrounding its main edifice. Ceboruco alarmed its neighboring villages in 1870 CE when a 7.5 km long lava flow moved down the western flank of the volcano. Later studies associated Ceboruco’s Niyé and El Escarabajo trachytic domes to this same eruption, though the sequence of the eruptive phases remained undetermined. This project interpreted that eruptive sequence and constrained the magmatic processes occurring prior to the 1870-5 CE eruption. The high vesicularity and disequilibrium textures of minerals in the lava flow are linked to magma degassing at the beginning of the eruption. In contrast, the lavas of El Escarabajo dome are less vesicular with high Ba concentration suggesting a degassed melt with higher plagioclase crystallinity which in turn implies that El Escarabajo dome emplacement occurred later in the eruption because subsurface magma devolatilizes most nearest the surface. Tapped deeper magmas tend toward lower volatile concentrations. This study proposes that Niyé dome formed last, after subsidence of the vent for the lava flow. The subsidence initiated from the evacuation of the supportive magma from the shallow reservoir below the vent. As determined by MELTS modeling, a closed-system fractional crystallization model (at 600 bar pressure and 2 wt.% H₂O) explains the evolutionary trends of Ceboruco pre-1870 lava compositions, it cannot describe the high concentration of some of the components seen in the 1870-trachydacites. This holistic project infers that partial assimilation of the granitic crust modified the Ceborus melts to create the higher Si, Al, and alkali element contents seen in the 1870-5 magma. The lavas also show evidence for two magmatic domains (likely due to the stratification of the magma chamber): one with Mg# between 0.27 and 0.42 and a more evolved magma with Mg#＜0.25, as observed in the orthopyroxene disequilibrium with the melt. This set of results delivers a preliminary image of the processes leading to Ceboruco’s 1870-5 CE event and allows us to understand the spatio-temporal sequence of the eruption.