Petrology of the 1877 eruption of Cotopaxi Volcano, Ecuador: Insight on magma evolution and storage

Cotopaxi is a stratovolcano in the Northern Andes Volcanic Zone, and has a history of bimodal volcanism, alternating between rhyolite and andesite. With Cotopaxi reawakening in 2015 after 138 years of quiescence, the question of what is occurring beneath the surface becomes especially poignant. To contribute to this question, it is productive to look to the volcano's recent past. This work characterizes the mineralogy and geochemistry of the recent eruptive products of Cotopaxi, with emphasis on the two pulses of the 1877 eruption. Additionally, pressures and temperatures are estimated for magmas prior to all eruptions. This will allow a better understanding of the magma plumbing system and its evolution over time. Over the past 500 years Cotopaxi has had five major eruptive events (VEI 3-4), which occurred in 1532, 1742, 1744, 1768, and 1877, and included pyroclastic surges, scoria flows, and lahars. After the initial pulse of the 1877 eruption and the subsequent lahars, a second pulse of magma produced a pyroclastic density current, containing scoria clasts up to 1 meter in diameter. All samples collected from these eruptions are basaltic-andesite to andesite (56-59 wt. % SiO2), with a mineral assemblage of pl + opx + cpx + mag ± ol. Plagioclase from all samples range from An47 to An78 and show both normal and reverse zoning. Normally zoned crystals exhibit greater compositional variation between cores and rims than reversely zoned crystals (median Ancore-Anrim 8% vs 4%, respectively). The presence of disequilibrium textures such as zoned plagioclase and mingled magma indicate that magma mixing plays a role in magma genesis in addition to crystal fractionation. Pyroxenes occur as augite and enstatite and do not exhibit significant zoning. The very similar petrologic character of these deposits suggests that they were sourced from a relatively long-lived magma system that underwent differentiation and replenishment between eruptions. Thermobarometric data indicate that magma storage occurred at temperatures of 1000-1150°C and pressures ranging from 0.20 GPa (during the 1877 eruption) to 0.43 GPa (during the 1532 eruption), which is equivalent to depths of 7 and 15 km (± 9 km), respectively. Data from geodetic and seismic studies indicate that magma was injected at a depth of 4 to 5 km which caused the ground deformation and seismic unrest from 2001 to 2002. While thermobarometry is poorly constrained, these results suggest that magma storage has become progressively shallower over time.