Changing the paradigm - new views of magmatic systems provide new perspectives on volcanic processes

Abstract:

Igneous petrology is undergoing a major paradigm shift. For the past century, the paradigm of the magma chamber - a single, melt-dominated magma body - has informed models of magma evolution and underpinned studies of volcanic processes. Over the past decade, however, the community has converted to a model of complex magma storage regions composed primarily of crystal-rich “mush”, with crystal-poor melt partitioned into sill-like lenses that can be tapped, either sequentially or simultaneously, to feed volcanic eruptions. Why has this conceptual shift occurred? Evidence for complex magma storage regions derives from numerous sources, including (1) geophysical evidence for sill-like melt bodies, (2) petrologic studies, which provide strong evidence that many erupted magma contain crystals that have been incorporated from different parts of the magma storage region, and over different time scales, and (3) geochemical evidence that magmatic gases may accumulate and be released independently from the melt. These observations suggest that magma storage regions are not only complex, but also dynamic, in that the solid (crystal), liquid (melt) and gas (exsolved volatiles) components may move and interact at different rates. Critically, adopting a melt-mush model requires a re-evaluation of processes responsible for melt segregation, evolution and eruption. For example, melt segregation and evolution may depend more on processes related to reactive flow of melts and fluids through porous crystal networks than sinking or floating of crystals within a (dominantly liquid) body. Additionally, patterns of eruptive behaviour may be determined as much by the stability of, and interactions between, individual melt lenses as by the evolution of a single melt body. From this perspective, studies of volcanic processes can be linked directly to processes of magma generation and evolution. Forging these links provides an exciting challenge for the future.