Insight on magma evolution and storage through the recent eruptive history of Cotopaxi Volcano, Ecuador

Cotopaxi is a stratovolcano in the Northern Volcanic Zone of the Andes, and has a history of bimodal volcanism, alternating between rhyolite and andesite. With Cotopaxi reawakening in 2015 after 138 years of no major eruptions (> VEI 1-2), the question of what is occurring beneath the surface becomes especially poignant in the 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. 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 regional tephra fallout, pyroclastic scoria flows and surges, as well as lahars. After the initial pulse of the 1877 eruption and the subsequent lahars, a second pulse of magma produced pyroclastic density currents, containing scoria clasts up to 1 m in diameter. All samples collected from these eruptions are basaltic-andesite to andesite (56-59 wt % SiO2), with a mineral assemblage composed of pl + opx + cpx + mag +/- ol. Plagioclase from all samples range from An(47) to An(78) and show both normal and reverse zoning. Pyroxenes occur as augite and enstatite and do not exhibit significant zoning. The presence of disequilibrium textures such as zoned plagioclase and enclaves with fluidal margins with host indicate that magma mingling/mixing plays a role in magma genesis. The very similar petrologic characteristics of these deposits suggests that they were formed in a relatively long-lived magma system that experienced moderate differentiation and magma replenishment between eruptions. Fractional crystallization modeling shows that crystallization of plagioclase (11.7%) + pyroxene (3.4%) + amphibole (4.6%) + magnetite (1.1%) + apatite (0.2%) can produce the observed compositional variation. However, some of the variation can be explained by mixing, thus we conclude that both differentiation processes occurred. Thermobarometric data indicate that magma storage occurred at temperatures of similar to 1017 degrees C and pressures ranging from 0.215 GPa (during the 1877 eruption) to 0.619 GPa (during the 1532 eruption), which is equivalent to depths of 7 and 21 km (+/- 9 km), respectively.