Coptopaxi Volcano, Ecuador, provides an unparalleled opportunity to constrain the mechanisms and timescales of magma differentiation at an active arc volcano. A detailed and complete tephra record at the volcano, comprising widespread layers of pumice and ash, has been painstakingly elucidated by our colleagues and collaborators at the Instituto Geofisico in Quito. From this, we can conclude that Cotopaxi has erupted regularly over the past few thousand years, with an average periodicity of around 120 years. The erupted products vary in composition leading to different styles of eruption. The cycles are punctuated by occasional explosive eruptions of rhyolite. The methodology will involve comprehensive petrological and geochemical analyses of the rocks - with a particular emphasis on obtaining information from the constituent phases such as minerals and melt inclusions. By comparing stratigraphically constrained erupted products we will be able to effectively take "snapshots" of the state of the magma reservoir(s) through time and address a number of important questions - including; 1) What are the principle mechanisms of magmatic differentiation (how does a given magma relate to or evolve from one that erupted a hundred years or so earlier)? 2) What are the timescales over which magmatic diversity (how long does it take to generate extreme compositions such as rhyolites)? 3) How do (1) and (2) relate to variations in the volcano's style of eruption?
