Petrology of the 1998–2000 products of Volcán de Colima, México

The 1998–2000 activity of Volcán de Colima generated a series of lava flows and block-and-ash flows of andesitic composition (59–61% SiO2). Juvenile clasts from both lava flows and pyroclastic deposits contain phenocrysts of plagioclase>two pyroxenes>Fe–Ti oxides and rare amphibole set in a groundmass of the same minerals and rhyolitic glass (74–77% SiO2). Prior to the eruption, the andesitic magma was stored at a temperature of ∼900°C based on titanomagnetite–ilmenite equilibrium. The magma had an oxygen fugacity of 10−11.1, corresponding to 0.8 log units above the NNO oxygen buffer, and water contents in the rhyolitic melt of ∼2 wt%, suggesting that the magma was water-undersaturated at depth. The presence of amphibole with clear signs of disequilibrium, quartz xenocrysts, and plagioclase (An<50) as inclusions in pyroxenes and phenocryst cores suggests that the magma chamber was first occupied by an andesitic magma at temperatures <900°C and probably as low as 840°C. The presence of plagioclase phenocrysts with reverse zoning (rims, An>50) and microphenocrysts (An>50) indicate the injection of a new hotter and more mafic andesite into the magma chamber. This process was the main trigger of the 1998–2000 activity.