Vertically extensive and unstable magmatic systems: A unified view of igneous processes

Making magma chambers from mush Shallow magma chambers either erupt as volcanoes or solidify as intrusive magma bodies. These magma bodies are traditionally considered to be long-lived and dominated by melt. Cashman et al. review the evidence that shallow magma chambers are actually assembled quickly from much larger, crystal-rich transcrustal magmatic systems. This paradigm helps explain many geophysical and geochemical features of volcanic systems. It also presents challenges for understanding the evolution of magma and provides insight into how and why volcanoes erupt. Science, this issue p. eaag3055 Shallow magma chambers are ephemeral expressions of larger transcrustal magmatic systems. BACKGROUND For more than 100 years, the melt-dominated magma chamber has been central to conceptual models of volcanism and igneous processes. Over the past few decades, however, emerging evidence is proving enigmatic, puzzling, or inconsistent with this paradigm. At the same time, physical models of igneous processes suggest that magma chambers are surprisingly difficult to form and maintain. The consequence has been an increasing emphasis on igneous processes involving a magma chamber that occupies only the top of a much larger magmatic system; this system extends through the crust, is largely crystalline, and comprises melt, crystals, and exsolved volatiles that are heterogeneously distributed in space and time. ADVANCES New conceptual models of transcrustal magmatic systems raise new questions about the development and stability of these complex systems and the processes that control their chemical and physical evolution. Over long times, magmatic processes are modulated by slow (plate rate) melt generation and transfer to the lower crust; these slow processes contrast with volcanic eruptions, where large amounts of magma are erupted quickly. The end results of these slow and fast processes are a diverse suite of igneous rocks and a wide spectrum of eruptive behaviors observed in the world’s volcanoes. We infer that igneous processes differ substantially in the lower and upper crust. Thermal and rheological conditions in the hot lower crust allow extensive and rapid melt segregation and fractionation. At shallower levels in the crust, by contrast, high rates of melt transfer are required to maintain melt-dominated magma bodies, but long-lived cooler, noneruptible crystal mushes can easily develop. Percolative melt transfer is a major mechanism for creating melt-rich regions and for the chemical evolution of the magmatic system. Upward-moving melt is chemically reactive, causing geochemical variations that are not captured by mathematical descriptions of end-member fractional or equilibrium crystallization. As a consequence, the tools used to interpret geochemical variations in igneous rocks may be ill suited to understanding the processes that created them. Important for the physical evolution of magmatic systems are threshold behaviors that control transitions from slow melt accumulation to rapid melt transfer. Melt-rich segregations, in particular, are inherently unstable and can move through regions of mush. This process may be rapid if melt is redistributed from multiple vertically stacked lenses within a mush into a single magma chamber. OUTLOOK These new views of magmatic systems have great explanatory power, but require new conceptual models for addressing questions related to magma evolution and the behavior of volcanic systems. Connecting processes throughout the entire transcrustal magmatic system should allow us to (i) relate physical processes of crystal-melt-fluid segregation to magmatic differentiation, (ii) anticipate interactions between melts and fluids generated and stored at different depths, and (iii) determine the physical mechanisms by which magma and associated fluids move through the crust, accumulate in shallow chambers, and then erupt. Major explosive eruption at Cordon Calle, Chile, on 4 June 2011. The underlying causes of violent volcanic eruptions after long periods of dormancy remain a fundamental question. Photo credit: D. Basualto, Southern Andes Volcano Observatory (OVDAS), Sernageomin, Chile Volcanoes are an expression of their underlying magmatic systems. Over the past three decades, the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust. A transcrustal perspective must balance slow (plate tectonic) rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions. Reconciling these observations is engendering active debate about the physical state, spatial distribution, and longevity of melt in the crust. Here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt-rich layers, focusing particularly on conditions that cause them to destabilize, ascend, and accumulate in voluminous but ephemeral shallow magma chambers.