What Goes into Making Volcanic Arc Magmas, and How Do We Know It?

One of the challenges in teaching undergraduate petrology students about the origins of subduction-related magmas and igneous activity at volcanic arcs is that this system is much more complex than that encountered at mid-ocean or intraplate settings: the sources of magmas are variable (i.e., the mantle, the downgoing plate, and/or the overriding crust may provide part or all of the magmatic source materials), and their histories from their point of origin to eruption are complex. An additional complication is that much of what we know about the material contributors to subduction-zone magmas has been learned through arcane trace element and isotopic studies of arc lavas and associated subduction zone metamorphic rocks, a body of literature (and geologic subdiscipline) with which undergraduates are likely to have little experience.

This activity is intended as a directed reading exercise for undergraduates in a junior- or senior-level petrology course aimed at leading them through the how and why of our current views on arc petrogenesis, while along the way familiarizing them with key aspects of trace element and isotope geochemistry: specifically, the concept of trace element solid/melt and solid/fluid partitioning, and its use in defining how a particular elemental signature in a rock can originate. The readings are key geochemical papers that track the development of our current views on the petrogenesis of arc lavas, and some associated controversies (i.e., melting of the subducting slab, and the role of subduction-related serpentinites).